The Jovian Auroral Distributions Experiment (JADE) on the Juno Mission to Jupiter

1.2.3.1 Jovian Trapped Radiation Belts and Detector Background

One of the most difficult aspects of exploring Jupiter is effectively meeting the challenge of Jupiter’s large and intense trapped-radiation belts. Two key aspects of the Juno design are used to mitigate the radiation issue. The first is the selection of a polar orbit for the mission design, which largely avoids the harshest regions of the radiation belts until late in the mission. The second key to the design is the incorporation of a radiation vault architecture for the spacecraft design. Rather than shield electronics boxes individually, the Juno mission provides a large titanium vault in which almost all of the electronics are housed. The JADE suite comprises four sensors that are outside the vault on the perimeter of the spacecraft, in order to measure the Jovian plasma with relatively unobstructed fields of view (FOVs). The minimum amount of front-end electronics required to perform the initial signal processing are located within the four sensors in order to minimize shielding mass. The majority of the signal processing electronics is contained with the JADE Electronics Box (Ebox), which is located within the Juno Radiation Vault. Cables from the JADE Ebox are passed to the sensors through bulkhead connections for low-voltage signals. To increase reliability, the high-voltage cables are passed through “mouse holes” (specifically designed pass throughs incorporating EMI and radiation shielding to ensure the continuity of the radiation-limiting design) in the vault wall to minimize the number of high-voltage connections required.

While much of the work at the mission level for radiation mitigation is focused on electronics parts, science instruments such as JADE, with sensitive microchannel plate (MCP) detectors must also solve the issue of detector background caused by penetrating radiation. The JADE team used the best currently available knowledge and models of Jupiter’s magnetosphere to estimate the upper and lower bounds of the signals JADE measures. These calculations and JADE’s design response to the challenging penetrating particle environment is detailed in Sect. 2.5.

1.2.3.2 EMI/EMC Requirements

The EMI/EMC requirements for Juno conformed to the MIL-STD-461D test, with most of the Juno-specific tailoring involving a set of deep notches for the MWR instrument’s sensitive receivers in the microwave range.

The JADE-E sensors are the near field of the MWR antenna. A detailed analysis was performed by the Juno project to determine the coupling between the JADE-E sensors and the MWR antennae. Figure 2 provides an example of one of the MWR notches. In the frequency range of interest for the MWR instrument, the notch is 50 dB below the standard level. To mitigate the risk that JADE could corrupt the MWR observations, extensive EMI testing was done during both the engineering model (EM) and flight model (FM) development phases. During the EM EMI testing we found that special shielding and bulk-head closeout pieces were required to contain EMI and produce the notches. The JADE sensors subsequently met all MWR EMI requirements during FM qualification testing.

Open image in new window

1.2.3.3 Magnetic Cleanliness Requirements

Juno’s magnetic cleanliness requirements are similar to most deep space science missions that carry magnetometers. JADE minimizes the use of magnetic or magnetizable materials. Acceptance testing at the spacecraft contractor’s facility found that JADE had only very small residual magnetic moments. The JADE electronics unit had a magnitude difference of 30 mA m² between its magnetized and demagnetized values. This equates to only a 0.01 nT delta to the spacecraft AC magnetic signature, which is below the spacecraft’s allocation for JADE.

1.2.3.4 Spacecraft Charging

The Juno spacecraft must function properly in Jupiter’s strong magnetic and radiation environments. Furthermore, in order to minimize the level of spacecraft charging, all surfaces on Juno had to be ESD dissipative with a resistance <10⁹ ohms per square. All exposed surfaces on JADE are grounded and the MLI, radiator surfaces, and tapes used are electrically dissipative.

Additionally, Internal Electrostatic Discharge (IESD) was determined to be a significant threat owing to the high fluxes of penetrating radiation. In order to minimize the risk of damage to circuits from IESD, the JADE Intra-instrument harness is wrapped with a dissipative kapton tape. In essence, the mission IESD requirements are basically in conflict with the instrument design requirements needed for high voltages in JADE and for spacing between parts at various voltages. In particular, ceramic parts inside JADE posed a significant potential issue leading to considerable collaborative work with the EMI working group to get JADE materials and designs approved.

1.2.3.5 Thermal Environment

The Juno spacecraft mission trajectory includes heliocentric distances from 0.87 AU to 5.6 AU, which produce over a factor of 40 difference in solar illumination. Thus, the thermal environment for the JADE instrument is strongly driven by these extreme thermal cases. The JADE limits are shown in Table 2. The sensors are located on the periphery of the spacecraft, have large apertures and therefore have larger temperature extremes. The Ebox is located within the Juno Radiation Vault and therefore it has a narrower environment which tends to be biased warm.

Table 2

JADE temperature limits (AFT = Allowable Flight Temperature, FA = Flight Acceptance, PF = Proto-Flight)

Open image in new window

Owing to the large thermal range of environments, there was extensive thermal testing at multiple times during the development Through this testing, we developed and employed numerous strategies to limit the highest temperatures for JADE, including using different coatings on the tops and bottoms of each deflector of the ion sensor and striving to thermally isolate the deflectors on the ion and electron sensors from the rest of each sensor body. None-the less, shortly after launch, we found that the JADE-I sensor temperatures were ∼20^∘ above those predicted by the pre-launch thermal model.

JADE-I neared, but did not exceed, non-operational limits shortly after launch. A tiger team, including representatives from JADE and the Juno mission (systems, spacecraft, thermal, etc.), was formed to address the issue. An updated JADE-I thermal model was created to better match the as-built configuration and this new model has shown good agreement with flight data. The current baseline plan anticipates JADE-I exceeding planned flight and protoflight temperatures at perihelion. A series of thermal and solar illumination tests on the JADE-I engineering model is being carried out prior to perihelion to determine the effect of the thermal environment on JADE-I. After JADE temperatures fall below operational limits post-perihelion, the instrument will be turned on again with additional precautions.

1.2.3.6 Contamination

1.2.3.7 Vibration

The Juno launch environment is similar to other missions launched on an Atlas V launch vehicle. However, the JADE sensors carry radiation shielding that is much heavier than typical spaceflight instruments, which are not operated in such harsh radiation environments. Thus, we developed a program of vibration testing starting at the EM level that provided insights into this unique development case and allowed us to find and solve vibration induced structural issues with the heavy radiation shields early in the program. Figure 3 and Table 4 provide the random vibration environment requirements for JADE.

Open image in new window

Table 4

Random vibration spectrum and overall GRMS

Freq (Hz)	Protoflight (g2/Hz)	Acceptance (g2/Hz)
20	0.02	0.01
40	0.30	0.15
180	0.30	0.15
2000	0.015	0.0075
GRMS	11.9	8.4

2.2.1.1 Electro-Optics Design

The azimuth angle (the angle in the imaging plane of the analyzer—see Fig. 6), is determined by measuring the position where electrons impact the detector with a position-sensitive anode. The elevation angle, which defines the incoming direction of electrons, is measured perpendicular to the imaging plane, with positive angles defined when the electron comes from above the imaging plane. Each sensor has an FOV of 120° and all three are mounted such that they cover 360° in the imaging plane (Fig. 5). The spacecraft X axis is defined by azimuth and elevation of (0^∘, 0^∘), spacecraft Y by (90^∘, 0^∘), and spacecraft Z by (any azimuth, 90^∘).

The deflectors (upper, DFL-UP, and lower, DFL-DN) provide an energy-dependent deflection of electron trajectories up to about ±35° before the electrons enter the energy analyzer. The deflectors are biased alternately with positive voltages up to +10 kV. The inner ESA electrode (Fig. 6, blue) is also biased up to +10 kV. There is a high transmission (∼90 %), electroformed nickel grid (not shown) on top of the MCP stack (purple). The MCP detector is biased at a voltage that ensures saturated amplification in the MCPS (can be set up to +3.8 kV maximum power supply). All the other parts are at ground potential.

Our design strategy started with an ESA radius of curvature of ∼50 mm and a gap of 2.5 mm. Then we used an automated computer optimization system (optimizer) to refine the design for maximum performance within the constraints listed above. The optimizer created and evaluated different geometries using the electro-optics simulation software SIMION^® (Dahl 2000). SIMION simulates a design then returns a certain value (determined by a function that can be the throughput, for example) to the optimizer that compares the performance with previous designs. For JADE-E, the optimizer worked within a parameter space defined by the user and explored the effects of varying the parameters. Using a simplex algorithm, the optimizer adjusted the parameters until it found a local minimum. We changed the initial conditions and modified the parameter space and repeated many optimizations until we found a design that gave excellent results. The criteria for the ESA optimization were to maximize the throughput and angular resolution in the imaging plane (i.e., these parameters were used in the function for the optimization) while keeping the energy resolution (ΔE/E FWHM) around 10 % of the central energy of the ESA passband. Next, we used the optimizer on the deflectors to redirect 40 keV electrons up to ∼45^∘ with +10 kV maximum. The criteria for optimizing the deflectors were to keep the FWHM of the angular distribution to better than 5° and to maximize the throughput. The electric field constraint led us to round all edges.

All of the optical surfaces inside the top-hat analyzer (except the deflectors) were blackened using the Ebonol-C process to significantly reduce the UV transmission over bare aluminum (Al) surfaces (e.g., Zurbuchen et al. 1995).

A schematic view of the detector and the anodes is shown in Fig. 7. The detector consists of (1) a secondary electron suppressor grid (SE Sup); (2) an MCP stack (MCPs and resistor divider); and (3) sixteen 7.5^∘ (total of 120°) adjacent discrete imaging anodes and one 7.5^∘ background anode. The secondary electron suppressor grid (at −12 V) faces the ESA and repels low energy (∼2 eV) secondary electrons that are emitted in this region. The lowest energy (∼100 eV) electrons transmitted through the ESA can easily overcome this potential barrier and reach the MCP below the grid.

Open image in new window

We used Photonis MCPs in a Chevron configuration in series with resistors to provide an accelerating potential (∼50 to 100 V) between the secondary electron suppressor grid and the top of the MCPs, and between the bottom of the MCPs and the anodes. The MCPs have a low resistance (∼25 to 30MΩ per stack at room temperature), an extended dynamic range, 25 μm pore diameter, and a 60:1 ratio of length to diameter of pore. Since the anodes are at high voltage, the signal is decoupled to ground with high-voltage capacitors.

Each discrete anode has a dedicated pre-amplifier (A121 from Amptek, threshold set at ∼3×10⁶ electrons) to register the events. The background anode is identical to the imaging anodes (MCP, anode, decoupling capacitor, pre-amplifier), but its access from the ESA is blocked by a mask built in the secondary electron suppression grid. No electron from the ESA side can reach the top of the MCP stack. Therefore, only penetrating radiation and internal MCP noise can generate a signal on the background anode.

2.2.1.2 Influence of a Strong Magnetic Field on the Response

In principle, measurement of the flux as a function of energy and angle to the magnetic field vector is all that is required to determine the pitch-angle distributions of electrons. In high-rate science and burst mode (Sect. 4.2) JADE-E sets the deflector voltage to measure electrons that travel along the magnetic field line. Those electrons fall onto one particular anode in one of the JADE-E sensors. The neighboring anodes measure the flux away from the magnetic field vector allowing reconstruction of the pitch-angle distribution.

The ESAs sweep voltages in steps to measure electrons over the JADE-E energy range. For each ESA energy step, the deflection voltage is calculated so that the JADE-E look direction always tracks the magnetic field vector direction. The magnetic field vector components are measured and broadcast to JADE (and other instruments onboard) nominally once per second, with a resolution of 1 nT. The vector orientation is calculated and a table of deflection voltages is created for the next energy sweep. If the deflection angle falls outside the ±35^∘ JADE-E FOV, a limit is set to 35^∘ and the voltages are calculated accordingly. Note that over the Juno mission the spacecraft spin axis orientation and orbit does not require JADE-E to deflect more than ±30^∘ to track the magnetic field direction. If the voltage required to track the field direction exceeds 10 kV, e.g. at the upper end of the energy range and for large deflections, then the voltage is set at 10 kV. If the magnetic field vector information is missing or comes late for the next DFL angle calculation, then JADE software extrapolates (propagates) the next orientation using the latest value received. JADE extrapolates up to a configurable number of seconds (currently 25 s), and if the information is still missing, JADE-E stops deflecting until a new value is received.

At any time, the magnetic field azimuth is aligned with two anodes of the JADE-E sensors. If the elevation is positive for one of these, then it must be negative for the other, i.e. the first sensor deflects for a positive elevation, and the other for an equally negative elevation. The third sensor does not deflect. For a constant magnetic field vector direction, the configuration of deflecting and non-deflecting sensors changes every sixth of a spacecraft rotation, i.e. every 60^∘ or 5 s.

Figure 8 shows histograms of the estimated magnetic field strength integrated over Juno’s baseline trajectory. We used the VIP4 Magnetic Field model that also includes a current sheet model (Connerney et al. 1981, 1998) to calculate the magnetic field strength at the expected location of the spacecraft at 1-min intervals. We then used the same model to trace these field lines to the magnetic equator, here defined as the L-shell or the point at which the radial component of the magnetic field vanishes (i.e., B_r=0). We identified a total of 3257 minutes during the mission when the spacecraft is expected to be inside the auroral regions, which we define as regions when R<3R_J and L>15. Note that L=15 corresponds to the L-shell of Ganymede, which has an auroral spot equatorward of the main oval. The peak field calculated in the auroral region is 7.73 G. Overall, Juno is expected to be in regions where B<1 G for 63.4 % of the time, B<3 G for 95.6 % of the time, and B<5 G for 99.2 % of the time.

Open image in new window

With the aid of detailed SIMION simulations, we characterized and quantified the response of the JADE-E sensor from 200 eV to 40 keV electrons in a wide variety of magnetic field strengths and orientations. Figure 9 shows simulations of pitch-angle deflections that 1 keV and 5 keV electrons undergo after passing through a SIMION model of the JADE-E sensor in an external magnetic field strength of 3 G (which Juno is expected to experience only toward the end of its prime mission). In the upper panel, the crosses represent the pitch-angle mapping done with the JADE-E anodes, and the white line indicates where the crosses would be expected to fall if the magnetic field had no influence on the electron trajectories inside the ESA; the bottom panel shows the difference. It is clear that the degree of pitch-angle distortion depends on the electron energy. For instance, the pitch angles for 1 keV electrons shift by less than ∼10^∘, while pitch angles for 5 keV electrons shift by less than ∼5^∘.

Open image in new window

Pitch angle is not the only quantity altered in the JADE-E measurements. The energy measurement and the geometric factor (G) are also affected by the magnetic field. To evaluate these other effects of the magnetic field on the JADE-E measurements, we used electro-optics simulations with constant magnetic field. As expected, the strongest effects are seen when the field is strongest and the energy the lowest. In our continuing efforts in preparation for the JADE data analysis phase, we are in the process of simulating a large number of cases of different magnetic field strengths and orientations, electron energies, and look directions to construct a simulation-based forward model for JADE-E. This model will be validated using laboratory measurements with a high-fidelity flight-like unit. The model inputs are the magnetic field vector (i.e., strength and direction) and the electron energy and angle distributions; the output is count rate as a function of anode and energy.

Finally, we note that early in the development of JADE, we also investigated the possibility of using magnetic shielding (mu-metal) around the sensor in order to mitigate the effects of the magnetic field on the electron trajectories. The main result of that study is that although the field inside the sensor is nearly canceled, the field near the aperture is concentrated, creating a strong lens effect on the electrons entering the sensor. Consequently, the response is very difficult to characterize and too complicated to deconvolve from the measurement signal. Therefore, we did not use magnetic shielding.

2.4.2.1 JADE-E Anode

The JADE-E Anode (Fig. 15) is a ceramic board that collects charge from the MCPs. It is divided into seventeen 7.5^∘ segments. Sixteen anodes provide 120^∘ of imaging for the particles collected through the ESA. The seventeenth anode bin is “dark” and measures the background from penetrating radiation. The ceramic anode also provides a seal that separates the ultra-high purity detector region, which is limited to glass, metal, and ceramic materials, from the standard electronics boards, which outgas at higher rates and contain polymer materials. The MCPs are stacked into the JADE-E Anode and a pickup ring is provided to make electrical connection to the MCP output face. The anode segments and the output face of the MCP are at high voltage. The ceramic anode has walls that surround the MCPs and provide the high dielectric strength necessary to prevent high-voltage breakdown to the tantalum shielding that is in close proximity to the MCPs.

Open image in new window

2.4.2.2 Capacitor Board

The Capacitor Board (Fig. 16) decouples the high-voltage signals from the anodes down to ground potential where they can be processed by the analog electronics. 1000 pF surface-mount high voltage (6 kV) ceramic capacitors are stood vertically on the board with a solder connection on only the board end in a “tomb-stone” array. A beryllium copper spring is soldered to the top end-cap of the capacitor. When stacked together the springs on the Capacitor Board make electrical contact with the back of the Anode Board to provide the signal path for each of the anodes. The decoupled signals are then transmitted through the board using an axial resistor that is mounted with one wire connected to the top side and one connected to the bottom side of the Capacitor Board. On the bottom side of the board, the signal from the resistor is limited using back-to-back diodes that form a “zap trap”. The axial leaded resistor is mounted through the board to minimize board space and eliminate any arc paths around the zap-traps using the circuit board itself as a barrier.

Open image in new window

2.4.2.3 Charge Amplifier Board

The Charge Amplifier Board (Fig. 17) uses 17 Amptek A121 charge amplifiers to process the MCP pulses and transform them from charge signals into digital signal. Pogo pins are used to bring the signals down from the Capacitor Board to the Charge Amplifier Board. Each channel’s processing electronics is isolated from the other channels by a pocketed aluminum housing cover (Fig. 18). Resistors are used to set the various adjustable parameters of the A121. The threshold is fixed at 3×10⁶ electrons. The pulse width is set to 90 ns and the dead-time at 180 ns.

Open image in new window

Open image in new window

2.4.2.4 Digital Board

The digital board (Fig. 19) registers digital pulses from all seventeen of the A121 outputs. It uses passive termination to translate the 5 V level outputs from the A121 to the 3.3 V serializer input levels. The serializer chip is employed to minimize wiring from the sensor electronics to the JADE Ebox; the signals are serialized into a frame and transmitted through high-speed, low-voltage differential signaling (LVDS) lines to the Sensor Interface Board in the Ebox. The sampling clock frequency, the A121 pulse width, and the A121 dead-time were selected to assure that at least one high-state and one low-state is transmitted serially for each of the inputs so that the JADE Sensor Interface Board (JSIB) can correctly count all pulses of all A121 channels at a periodic rate of 5.55 MHz for each pixel. The serializer chip samples all of the inputs every 60 ns and, using a phase-locked loop, transmits the samples to the JSIB over three pair of high speed LVDS serial lines. The LVDS signal levels allow us to transmit at high bandwidth while maintaining the low EMI emissions required for compatibility with the MWR instrument. Figure 20 shows the JADE-E digital board timing.

Open image in new window

Open image in new window

2.4.2.5 High Voltage Distribution Board

The High Voltage Distribution Board (HVDB) (Fig. 21) provides shielded, mechanical termination of the doubly-shielded high voltage coaxial cables, filters the detector bias signals, and provides dropping resistors to set the proper optical voltages in the detector. For the optical high voltages within the JADE-E sensor, this HVDB provides a convenient way to make connections between the high voltage wiring within the sensor and the pig-tailed high voltage wiring that comes from the various high voltage power supplies (HVPSs) in the Ebox. The HVDB provides local zap-traps to provide a chassis return path as close as possible to the sensor, should any discharges occur there. The HVDB also provides a 1-pole filter that attenuates the ripple on the MCP supply below 0.1 Vpp peak-to-peak.

Open image in new window

2.4.3.1 JADE-I Anode

The JADE-I Anode (Fig. 24) is a ceramic board that collects charge from the MCPs. It is divided into a start anode in the central portion, which measures signals from the portion of the MCP that collects start-electrons from the carbon foils. The remaining outer annulus is divided into thirteen 22.5 degree anode bins. Twelve of the anode bins are used to measure the elevation angle of the incoming ion as well as provide a stop signal for the TOF electronics. The remaining 22.5 degree anode bin is “dark” and used to collect background rates for penetrating radiation. Similar to JADE-E, the JADE-I ceramic anode also separates the ultra-high purity detector region from the standard cleanliness in the volume that contains the front-end electronics.

Open image in new window

2.4.3.2 Time-of-Flight (TOF) Board

The TOF Board (Fig. 25) takes signals from the Start Anode and the sum of all of the anode signals from the Charge Amplifier Board and performs constant-fraction discrimination to convert these analog signals into digital start and stop pulses, which are transmitted to the JSIB in the Ebox over dedicated LVDS signal pairs. The TOF Board includes lower- and upper-level discriminators. The dynamic range is from 1×10⁶ to 5×10⁷ electrons. When an in-range signal is processed, the lower-level discriminator opens up a comparison window once a pedestal has been established; then a delayed, scaled, and inverted signal is compared against the original, and the TOF event is marked when the Zero Crossing discriminator fires. Over the valid range of input signals, the TOF “walk” is less than 1 ns. The minimum start-stop pulse pair resolution is 1.45 ns. The longest valid TOF is 330 ns. The maximum repetitive start rate is 8.3 MHz.

Open image in new window

2.4.3.3 Charge Amplifier Board

The Charge Amp Board (Fig. 26) uses thirteen A121 charge amplifiers to process the MCP Stop Events and determine the elevation angle of the measured particle. A zap-trap protected voltage follower amplifier process the charge input from each Stop Anode and converts it to a voltage waveform. That waveform is sent in two directions. It is sent to a summing amplifier and then on to the TOF board in order to represent the Stop Timing Pulse. Simultaneously, the voltage waveform is sent to the A121 for each Stop anode. Figure 26 shows a schematic of one channel. The A121 is set with a 3×10⁶ electron threshold. The pulse width is set to 90 ns and the dead-time to 180 ns. The output of the A121 is sent to the Digital Board, which is integrally connected to the Charge Amplifier Board through a common flexible circuit layer. Once on the Digital Board, the Stop anode signals are serialized and transmitted to the JSIB via two high-speed, LVDS serial transmission lines.

Open image in new window

2.4.3.4 Digital Board

The Digital Board is permanently connected to the Charge Amplifier Board through shared flex-circuit layers. The Digital Board registers the digital pulses from all thirteen of the A121 outputs and uses passive termination to translate the 5 V level outputs from the A121 to the 3.3 V serializer input levels. The serializer chip is employed to minimize wiring from the sensor electronics to the JADE Ebox; the signals are serialized into a frame and transmitted to the through high-speed LVDS lines to the Sensor Interface Board in the Ebox. The sampling clock frequency, the A121 pulse width, and the A121 dead-time were selected to ensure that at least one high-state and one low-state is transmitted serially for each of the inputs so that the JSIB can correctly count all pulses of all A121 channels at a periodic rate of 5.55 MHz for each pixel. The serializer chip samples all of the inputs every 60 ns, and then, using a phase-locked loop, transmits the samples down to the JSIB via two pairs of high speed LVDS serial lines. The LVDS signal levels allow high bandwidth transmission while maintaining the low EMI emissions required for compatibility with the MWR instrument.

2.4.3.5 JADE-I High Voltage Distribution Board

The JADE-I HVDB (Fig. 27) provides shielded, mechanical termination of the doubly-shielded, high voltage coaxial cables; filters the detector bias signals; and provides dropping resistors to set the proper optical voltages in the detector. The distribution of voltages is shown in Fig. 28.

Open image in new window

Open image in new window

2.4.4.1 Instrument Processor Board

The Instrument Processor Board (IPB, Fig. 30) is the computing platform on which the JADE Flight Software runs. Figure 31 shows a block diagram of the IPB. The IPB provides an Atmel AT697E SPARC8, RISC processor, capable of operating at 86 Dhrystone Million Instructions per Second (MIPS) or 23 Whetstone Million Floating Point Operations per Second (MFLOPS). The IPB also functions as a v2.2 PCI compliant cPCI bus master to communicate with the JSIB across the JADE backplane. The IPB responds to spacecraft commands, transmits low-rate Science and Housekeeping telemetry, and transmits high-rate, synchronous Burst Mode Science data. Through the JSIB, the IPB controls the sensors and reads back telemetry data collected. The data is then formatted into packets and transmitted to the spacecraft.

Open image in new window

Open image in new window

The spacecraft synchronizes communication using the one half pulse-per-second (1/2PPS) signal. Every two seconds the spacecraft asserts a 1/2PPS signal, which is used in concert with a spacecraft command to precisely update JADE’s 1 μs resolution system timer. Spacecraft commands are received and buffered on the IPB over a Universal Asynchronous Receive and Transmit (UART) interface. Low Speed Telemetry is written by the software into a buffer and transmitted to the Spacecraft over the UART interface. During Burst Mode, JADE transmits high speed burst data, which is written into a buffer and transmitted at a higher rate across a dedicated high-speed synchronous serial interface.

The IPB is responsible for system resets of JADE for operational and fault tolerant purposes. The JADE instrument can be reset by power cycling, spacecraft command, or JADE software reset. Watchdog resets for both the processor and the FPGA ensure a software fault does not cause an instrument failure.

The IPB has the following memory: 128 KB PROM for the boot code; 512 KB EEPROM to hold copies of the Flight Software and Flight Look-Up Tables; and 4 MB SRAM used for Code and Data when the software is executing. All memory uses a 7-bit Hamming Code for double bit error detection and single bit error correction, providing additional hardware fault tolerance.

The logic control of the IPB is contained within an ACTEL RTAX2000S-1 FPGA. All logic is Triple-Module-Redundant to decrease susceptibility to Single Event Upsets (SEUs). The FPGA drives the CPU clock and allows on-the-fly adjustment from 48 MHz to 1 MHz. This allows the software to have access to substantial computing power for time-critical tasks, and then to throttle back to a very low processing speed to conserve overall power.

2.4.4.2 Low Voltage Power Supply

The Low Voltage Power Supply (LVPS, Fig. 32) uses 28 V Spacecraft Primary power to generate 3.3 V, ±5 V, and ±12 V secondary power for the JADE instrument. JADE can be powered by redundant A- or B-side of the Juno Spacecraft inputs. These two power lines are diode OR-ed together. Inrush current is limited to reduce part stresses. EMI Filtering is performed to prevent spacecraft noise from affecting JADE and noise from JADE being reflected back to the spacecraft bus.

Open image in new window

Separate sets of power rails provide different types of loads with their own sets of power planes in order to keep noisy loads from affecting quiet loads. JADE provides Digital Returns, Analog Returns, and High Voltage Returns. The returns are referenced to chassis through a resistor so that, under all test conditions, the power supply returns are chassis referenced. However, the Analog Grounds are tied to chassis at the sensors to maximize the noise performance of the Front End Analog Electronics in the sensors. The resistor that references ground to chassis prevents ground loops and keeps return currents on the paths provided in the wiring harnesses. Elements of the LVPS system are shown in Fig. 33.

Open image in new window

2.4.4.3 JADE Sensor Interface Board

The JADE Sensor Interface Board (JSIB) is the heart of the Ebox (see JSIB block diagram in Fig. 34). The JSIB commands all of the high voltage power supplies to their required levels, it opens an acquisition window, processes raw count data from all sensors, and determines coincidence in the JADE-I sensor while measuring the time-of-flight of the incoming particle. All of the control operations are governed by sweep tables that are loaded into memory by the IPB. Once the JSIB is enabled it begins processing events from the sensors and builds up double-buffered histograms that are turned into science telemetry products by the IPB.

Open image in new window

The JSIB contains a high speed Digital-to-Analog Converter to generate the specific voltages required by the sweep tables. Once the level is generated it is captured by one of many Sample-and-Hold Amplifiers which directly drive the analog command level, across the backplane, to each of the HVPS inputs.

Raw anode counts from each of the sensors are de-serialized by the JSIB and recorded in the appropriate histogram or real-time count-rate register. The JSIB measures the JADE-I TOF by using a Time-to-Digital Converter (TDC) that consists of a coarse counter that is generated off of the 62.5 MHz clock and a circuit that also captures, using delay lines, the position of the start and stop pulses with a resolution approximately ten times finer than the 62.5 MHz clock pulses themselves (Fig. 35). Coincidence logic determines what ion event has occurred. Using the measured TOF as well as the commanded ESA and DFL settings, the event is classified by ion species and is recorded in histograms using table-driven logic.

Open image in new window

Test pulsers to all of the sensors can be generated by the JSIB so that the amplifiers in each of the sensors can be checked at any time. Various rates and combinations can be selected. In the case of JADE-I various TOFs are also selectable.

The JSIB measures 80 different housekeeping analog values and stores them in a buffer that can be read out by the IPB and sent to the ground to monitor the health and safety of the JADE instrument. The JSIB communicates over PCI backplane to the IPB. The JSIB is a 6U board (Fig. 36) and uses the spare connector pins to communicate analog commands, analog monitors, and various enable and safe signals to the HVPS over the backplane.

Open image in new window

2.4.4.4 High Voltage Power Supply

On the HVPS Board 1, there is a +10.5 kV bulk power supply for each JADE-E sensor (see HVPS Board 1 block diagram in Fig. 37). Three linear regulator supplies use this bulk supply to produce the +10 V to +10 kV stepping supplies for the DFL-UP, DFL-DN, and ESA optical voltages which set the acceptance angle and the acceptance energy of each sensor. Each of these supplies can be commanded to be in low-range or high-range mode. If they are in high range, then the analog command level is interpreted to control the stepping supply across the full +10 kV range. If they are in low-range, the analog command level is interpreted to control the stepping supply across the range 10 V to 316 V.

Open image in new window

On HVPS Board 2 there are four programmable MCP power supplies (see HVPS Board 2 block diagram in Fig. 38). Three +3.8 kV supplies are for the JADE-E MCPs. The remaining supplies on Board 2 are used for the JADE-I sensor. One −3.8 kV supply is for the JADE-I MCP. There is also one −10 kV programmable TOF power supply. To supply the DFL-UP and DFL-DN voltages, there is a +10.5 kV bulk power supply that supplies the voltage for the two deflector linearly regulated stepping power supplies. To supply the ESA voltage, there is a −10.5 kV bulk power supply that supplies the voltage for the ESA linearly regulated stepping power supply.

Open image in new window

The power supplies were designed to be disabled, limited, or armed by means of a Safe/Arm Plug on the Ebox. No voltage is generated when the HVPS is disabled. When the HVPS is limited the supply cannot be commanded to full voltage and instead is limited to non-vacuum safe levels. When no plug is installed, as is the case for flight, then the HVPS supply is armed and software can enable and control the HPVS to full voltage levels.

2.4.4.5 Backplane

The backplane provides the interconnections between all of the boards in the Ebox. All of the connectors are custom pinouts on standard 96-pin VME connectors with the exception of the connection between the IPB and the JSIB, which are interconnected using a standard PCI interface. This provided the high speed communication path between the processor board and the sensor interface board. The ground planes in the backplane are systematically segmented to provide well-separated, low-impedance power and ground planes for the each of the types of signal and load: digital, analog, and high voltage. This maintains the signal integrity of each subsystems and helps to minimize noise and cross-talk.

2.4.4.6 Intra-instrument Harness

All of the signals for the four sensors are contained in an intra-instrument harness that connects them to the Ebox in the titanium radiation vault. There is a custom radiation vault plate that these signals pass through and the low voltage signals have standard, bulkhead D-connectors in this plate. To avoid the addition of any high voltage connectors, all sensor high voltage cables are pigtailed from the HVPS and pass through mouse-holes in the radiation vault plate.

2.5.3.1 JADE-I

Using the model electron fluxes throughout the auroral regions (e.g., from Fig. 41), we can estimate the fraction of time that the JADE-I background count rate is below 6 kHz for each orbit using a graded-Z shield with a stopping power of 5 g/cm² and 5 mm total thickness. Note that the peak background rate due to transmitted photons and electrons through a 5 g/cm² shield, corresponding to the peak auroral flux expected during orbit 32, is ∼12 kHz (Fig. 42).

For a shield thickness of 5 g/cm², Fig. 43 shows that (1) the JADE-I count rate will remain below ∼6 kHz throughout the southern hemisphere passes, (2) the fraction of time that the JADE-I background count rate will exceed 6 kHz increases with increasing orbit number, (3) the mean (μ) and median (m) background rate for JADE-I is ∼2.9 and 2.2 kHz, respectively, and (4) the JADE-I background count rate will remain below ∼6 kHz for ∼92 % of the time during the northern and southern hemisphere passes. Owing to its time-of-flight coincidence system, JADE-I can tolerate a significantly higher noise rate, up to 17 kHz/cm².

Open image in new window

2.5.3.2 JADE-E

Figure 44 shows the results of a similar analysis for JADE-E, where we estimate the fraction of time that the JADE-E MCP background count rate will remain below 2 kHz for each orbit if the sensor is shielded with a graded-Z shield with a stopping power of 8.5 g/cm² and 9 mm total thickness. Note that the peak background rate due to transmitted photons and electrons through this shield will result in a peak noise rate of 7 kHz corresponding to the peak auroral flux of ∼2.2×10⁶ at 1 MeV. The results are similar to those for JADE-I, and the third panel shows that the JADE-E background count rate will remain below ∼2 kHz ∼75 % of the time during the northern and southern hemisphere passes.

Open image in new window

3.2.1.1 Analyzer Constant

In principle, this ratio is a constant for a given ESA and for non-relativistic energies. At the upper end of the JADE-E energy range, the non-relativistic assumption is no longer valid. The JADE-E analyzer constant deviates from Eq. (3.1). Figure 48 shows the measured analyzer constant from JADE-E pre-calibration, calibration, and simulations. A correction due to relativistic effects is necessary to account for the dependence on the energy of the analyzer constant. The analyzer constant can be related to the equation of motion in the analyzer.

$$ k = \frac{E}{V} = \frac{\frac{1}{2}m_{e}v^{2}}{V} \cong\frac{r}{2\Delta r} $$

(3.2)

where m_e is the electron mass, v is its speed, r and Δr are the mean radius and the gap between the electrodes of the analyzer, respectively. The relativistic kinetic energy, E_r, can be written as a function of the non-relativistic energy (Keski-Rahkonen and Krause 1978)

$$ E_{r} = E - m_{e}c^{2} + \sqrt{E^{2} + \bigl( m_{e}c^{2} \bigr)^{2}} $$

(3.3)

where c is the speed of light. After expansion into a power series we obtain

$$ E_{r} = E \biggl[ 1 + \frac{1}{2}\frac{E}{m_{e}c^{2}} + O \biggl( \frac {E^{3}}{m_{e}^{3}c^{5}} \biggr) \biggr] $$

(3.4)

Open image in new window

The differences between pre-calibration and calibration are 1–2 % at the lowest energies and become less than 1 % above 1 keV. The pre-cal data deviate from the curves at low energies because the residual magnetic field changed between pre-calibration and calibration. We used only calibration data for the fits at the lowest energies.

Analyzer Constant Uniformity

The gap between the ESA plates is critical for the analyzer constant. Since the mechanical alignment is not perfect, we expect differences across the FOV. We measured the analyzer constant at 10 keV for at least 6 anodes for each sensor. Figure 49 shows the variations of k (E=10 keV) across the azimuthal FOV.

Open image in new window

3.2.1.2 Energy Resolution

We define the energy resolution of the analyzer as the FWHM divided by the most probable energy of a Gaussian fit to the energy passband. Figure 50 compares the energy resolution from pre-calibration and calibration measurements with simulations as a function of energy. The measurements were taken on anode 9 for each sensor. The energy resolution increases with energy for both the measurements and the simulations, but more rapidly for the measurements. Electrons reflect off surfaces inside the sensor (Sect. 3.2.1.5), and the higher the energy, the higher the reflectivity. Thus, electrons reflecting off the analyzer surfaces contribute to the tails of the energy distribution, which increase the FWHM. Here we use a linear fit to the pre-calibration and calibration data.

$$ \Delta E / E = a_{0} + a_{1}E $$

(3.10)

where E is given in keV and the fit coefficients are given in Table 11.

Open image in new window

Table 11

Energy resolution fit coefficients and estimates

Sensor	a0	a1 [1/keV]	ΔE/E estimates from the fits
			at 0.2 keV	at 40 keV	at 100 keV
E060	0.104	0.000474	0.104	0.123	0.151
E180	0.105	0.000673	0.106	0.132	0.172
E300	0.104	0.000345	0.104	0.118	0.139

The minimum (10.4 %) and maximum (13.2 %) resolutions are well within the requirements (5 %<ΔE/E<15 % for energies between 0.2 and 40 keV). We measured the energy resolution at 10 keV for the other anodes and found it to be relatively constant across the FOV, confirming the good alignment of the ESA electrodes. We found variations of only 0.31 % for E060, 0.25 % for E180, and 0.23 % for E300.

3.2.1.3 Geometric Factor

It would have been convenient to separate the detector efficiency, ε, which may vary over time, from the geometric factor, which is a property of the sensor and does not change over time. However, it is very difficult to obtain an absolute calibration of the detector efficiency. Instead, we measured the product of the geometric factor with the detector efficiency as shown in Fig. 51 for all three sensors as a function of energy. The product, G⋅ε, reaches a maximum around 1 keV and decreases slowly by a factor of ∼2 at 30 keV. The variations in G⋅ε as a function of energy are mostly due to the energy dependent electron detection efficiency. Our simulations show a ∼4 % increase of G from 0.1 to 100 keV.

Open image in new window

Because the FOV is slightly narrower when using the deflectors, the geometric factor decreases accordingly, as illustrated in Fig. 52 for six deflections ranging from −29^∘ to +33^∘ at 10 keV. The difference between the minimum and the maximum is ∼30 % to ∼33 % for all sensors at 0^∘ deflection.

Open image in new window

3.2.1.4 Angular Resolution

We considered three criteria in defining the anode number: (1) each JADE-E sensor has anodes from 1 to 16, (2) all three JADE-E sensors are seen as a “single sensor” that has anodes from 1 to 48 where E060 has anodes from 1 to 16, E180 from 17 to 32, and E300 from 33 to 48, and (3) same as (2) with anode 1 having the same look direction as the magnetic field vector. The first definition allows easy comparison from sensor to sensor, but the JADE-E sensor name has to be used in order to uniquely identify a look direction on the spacecraft; for example anode 1 in E060 has a look direction 120^∘ away from anode 1 in E180 or E300. Note that the center of the azimuthal FOV is at 60^∘ for E060, 180^∘ for E180, and 300^∘ for E300 in spacecraft coordinates. The second definition has the advantage of a unique anode number for a particular direction but is fixed with respect to the spacecraft coordinates. The third definition is mostly used to collapse the burst data (full angle and energy resolution) into the high rate science (HRS) data format.

Azimuthal Angle

We measured the azimuthal response by scanning angles in the imaging plane in small (0.75^∘) steps across the FOV. Adjustments to the elevation angle and ESA voltages were required to account for the slight misalignments due to the tolerances of the mounting fixture and ESA plates. The count rate for a constant beam as a function of azimuth is shown in Fig. 53 for all three sensors. In general, the response is uniform to within 20 % for all anodes. Anodes 1 and 16 are at the edge of the FOV, which is why the curves have different shapes and the integral of the counts is slightly lower compared to other anodes. A reduction in transmission at the edge of the FOV is expected. The azimuthal angle resolution is ∼7.5° (average) for all three sensors, matching our goal and fully half of the maximum requirement of better than 15^∘ FWHM.

Open image in new window

Elevation Angle

We measured the elevation angle response when the deflectors are biased at a given voltage by scanning elevation in 0.5° steps. We also repeated the elevation scans for different voltages applied to the upper and lower deflectors. An example of the deflector response is shown in Fig. 54. The colors correspond to various deflector voltage settings (indicated at the top). This particular set of measurements was taken with a 10 keV beam on anode 9. The deflection range is roughly −35° to +35°. We integrated the total number of counts (per unit of time) for each of the distributions shown in the figure and plotted this number (red diamonds) according to the scale on the right. The total counts per deflection setting is roughly constant (within ∼10 % of the maximum) with a small decrease (∼15 to 35 %) beyond +30°.

Open image in new window

Taking the most probable values of Gaussian fits to the distributions in Fig. 54, we obtain the deflection angles and plot them versus deflector voltage divided by the energy in Fig. 55. The relationship is very nearly linear, but is slightly better represented with a 3rd order polynomial fit.

Open image in new window

At most, this is ∼1^∘ for a deflection close to the maximum. This effect has been calibrated, to be accounted for in the data analysis.

The maximum deflection is obtained when the deflector voltage is at its highest value (+10 kV). For 100 keV electrons the maximum deflection is ∼16^∘, and 30° is achievable for <50 keV electrons.

The beam from our electron source can be approximated by a point source, which implies that the response is different from that obtained with a parallel beam. This affects the elevation angle distribution, and particularly its width. We compared the calibration measurements with simulation results, taking into account the exact geometry of the calibration (source distance to JADE-E aperture, orientation and position of JADE-E with respect to the beam, etc.). Figure 56 shows this comparison for 10 keV electrons at ∼0^∘ elevation. These two distributions, as well as others taken at larger deflection angles, agree very well thus validating our electro-optics model for deflections. From the simulations, we have ∼3 to 6^∘ FWHM angular resolution for a parallel beam of electrons. The resolution is significantly better than the 15^∘ requirement.

Open image in new window

3.2.1.5 Electron Reflections

Electrons easily scatter off internal surfaces, which modify the electron trajectories and generates spurious counts because some electrons hit electrodes outside the ESA’s energy-angle acceptance. We used mitigation strategies, including an Ebonol-C coating on the ESA electrodes, which also significantly attenuates UV photons from reflecting deep into the sensor. Nevertheless, we observed some electron reflection as shown in Fig. 57. In that figure, the ratio of the signal for a beam centered on anode 9 to the signal on all anodes is plotted versus electron incident energy. Ideally, the ratio is 1, that is, all the electrons fall on anode 9. A value of 0.9 means that about 90 % of the total signal is measured on anode 9 and ∼10 % on neighboring anodes. The percentage of reflected electrons is ∼5 % below 0.5 keV and increases with energy up to ∼20 % at 30 keV. This effect is most pronounced in the center of the azimuthal FOV and decreases at the edges. For example, at 10 keV on anode 9 the percentage reflected is roughly 13 to 17 %, and it is only ∼6 to 9 % when the beam is directed toward anode 1 or 16. Once quantified as we have done here, the reflected signal can be accounted for and removed in the data analysis.

Open image in new window

3.3.1.1 Analyzer Constant as a Function of Energy and Pixel

The analyzer constants for JADE-I are determined similarly to those of JADE-E (Sect. 3.2.1.1). However, many of the perturbations do not need to be accounted for in the ion sensor due to the much larger mass of ions (m_p/m_e=1836) and, for example, relativistic and magnetic effects are not included for JADE-I.

A more significant effect on the ESA response is the TOF pre-acceleration described in Sect. 2.3. Modeling shows that the analyzer constant, energy resolution, and azimuthal angular response all have some energy dependence. Figure 11 shows how the focal point of the ESA changes as the ESA voltage increases. The analyzer constant (Eq. (3.1)) also has a weak energy dependence. Over the full energy range of the instrument (10 eV to 45 keV), modeling shows the analyzer constant varies from 4.41 at 10 eV to 4.36 at 40 keV, with most of the variation occurring between 10 and 200 eV (see top panel of Fig. 59). Field penetration into the ESA effectively “shortens” the ESA. Particles receive an additional pull from the TOF, changing their trajectories and allowing those that would have impacted the end of the ESA to pass through to the TOF section.

Additional effects of the ESA shortening include energy-dependent energy resolution, azimuthal angular resolution, and offset of the non-deflected azimuthal zero. SIMION modeling predicts that the energy resolution of the analyzer, ΔE/E, varies from 28 % at 10 eV to 19 % at 40 keV (Fig. 59). The azimuthal angular resolution varies by nearly 1.5^∘ and the offset of the FOV by 0.3^∘ over the full energy range of the sensor.

The calibration data agrees well with the model results. In Fig. 60 the measured values for the flight unit for 10 and 40 keV are plotted for three species: He⁺ (stars), \(\mathrm{N}_{2}^{+}\) (diamonds), and Ar⁺ (crosses). The measured values differ by ∼5 % from the modeled values.

Open image in new window

Angular resolution is determined by the intrinsic response of the electro-optics, angular scattering of particles as they pass through the carbon foils, and the anode pad size, which corresponds to 22.5^∘. As the size of the electron pulse from the MCPs increases, the charge deposited on a given anode can also capacitively couple to its nearest neighbors. This is seen in the increased number of simultaneous adjacent and non-adjacent anodes. This effect helps define an optimal gain for the MCPs to be large enough to be sensitive to incident events, but not so large as to couple to the nearest neighbors.

The constant post-ESA acceleration has a varying effect on the different energy ions. When the ions exit the ESA they are accelerated toward the TOF in a direction normal to the carbon foils. For the lowest energy ions, the energy gained during the post acceleration is significantly larger than their incident energy, and they consequently impact the foil normal to its surface. As the energy increases, the post acceleration has a smaller overall effect on the ions’ trajectories. The higher energy ions can impact the foils at larger angles depending on how they exit the ESA.

Without scattering from grid or foil interactions, the highest elevation angular resolution is at the lowest energies. Figure 61 shows how the idealized angular resolution varies with energy in the absence of any scattering. The distribution of events is normalized for each energy step. The dashed lines show the anode spacing, and the solid white line shows the location of the FWHM. For this SIMION modeling run, all ion grids and foils are assumed to have 100 % transmission. It is also assumed that the carbon foil does not cause any angular scattering or energy straggling for the ions, and all ions exit the carbon foils with a neutral charge. Including scattering and other effects broadens the angular width so the result presented in Fig. 61 is a limiting case of the angular resolution.

Open image in new window

The carbon foils add a mass and energy dependent scattering to the system. For higher masses and lower energies, the magnitude of the scattering increases (Funsten et al. 1993). The 10 keV post acceleration reduces the amount of scattering for the lowest incident energy ions. Without post acceleration, the lowest energy ions (<100 s eV) cannot even pass through the foil. Those low energy ions able to pass through have significant scattering (∼10 s of degrees). The measured angular resolution in the elevation direction (plotted in Fig. 62) includes all of the effects discussed above.

Open image in new window

The JADE-I deflector plates scan ±45^∘ for energies up to 24 keV, and up to ±25^∘ at 40 keV. The linearity of the deflectors is shown in Fig. 63 for an 8 keV proton beam. The voltage required to deflect an angle θ (in degrees) and energy E (in eV) is

Open image in new window

Open image in new window

The factor δ is the offset angle of the FOV center and is plotted in Fig. 59. Since a positive potential is applied to the deflector plates, the up deflector is used to look downward, and vice versa.

The geometric factor for JADE-I was determined through particle ray tracing based on SIMION modeling where 2.6×10⁶ ions were launched into the JADE-I electro-optics for each of the 32 nominal ESA voltages settings; it was also measured directly in calibration using 10 and 40 keV ions of 3 different species. Figure 64 shows geometric factor assuming 100 % foil and grid transmission and detection efficiency.

Open image in new window

During flight calibration, we measured the total effective geometric factor, including actual transmission through all grids and structures, and the measurement efficiency of the MCPs. The incident flux of particles was measured using the ABM. Figure 65 shows the measured geometric factor for STARTS and STOPS as a function of elevation angle for three species and two energies. The sensor has a mechanical structure between each anode. The inner=0 data is taken when the beam is incident between two anodes and partially obstructed by these mechanical structures. The geometric factor for events with valid TOF is ∼4×10⁻⁴ cm² sr eV/eV for the full FOV and ∼4×10⁻⁵ cm² sr eV/eV per pixel.

Open image in new window

The MCPs were burned in and fully characterized prior to installation in the flight sensor. In addition to the burn in process, an MCP gain curve was determined in calibration. In this test, the MCP voltage was stepped from 1700 to 2400 V while being illuminated with a 10 keV proton beam. Figure 66 shows the results of this test. The top panel plots the log of All Starts (green) and All Stops (red). The MCPs start being active at 1700 V and are starting to reach saturation by ∼1900 V.

Open image in new window

The lower panel of Fig. 66 shows how the anode imaging responds to the MCP gain. Plotted are the fraction of direct events that are seen in only one anode (dark blue), 2 adjacent anodes (light blue), non-adjacent anodes (yellow), any combination of more than one anode (red), and events where no anode could be identified (green). For cases where 2 adjacent anodes are simultaneously identified, the event is mapped to the lower anode number in addition to incrementing a counter of the number of adjacent events. Non-adjacent anode events are defined as any event that is not a single anode or 2 anodes that are not adjacent.

JADE nominal operations include periodic calibration runs where the MCPs are swept to verify that the detectors continue to operate at their proper levels. At the time of instrument turn on in space, JADE-I operated at 2100 V, an optimal setting of the MCP gain for good anode imaging. Calibration runs are performed once every other orbit. The MCP voltages are adjusted as needed to maintain optimal performance of the sensor.

4.1.1.1 Burst During Auroral Field Line Crossings

The most critical JADE measurements occur on the auroral field-line crossings. Active field lines are small—less than 1,000 km wide according to HST observations. Juno crosses these field lines at ∼50 km/s, and the critical data occur in ∼20 s intervals, with the exact timing of the crossings unknown.

Burst mode is designed to capture these auroral crossings. The highest possible resolution data is captured and prioritized by how ‘interesting’ the data is, as defined by the Waves quality factor (see Sect. 4.4.2.6 for Burst details). Waves flight software sets the quality flag based on (1) the number of solitary structures in the Waves wideband data, (2) the amplitude of waves near the electron cyclotron frequency, and (3) sudden changes in broadcast magnetic field direction or magnitude (Kurth et al. 2013, this issue). Waves broadcasts a Burst trigger when the quality flag rating is above a threshold. When a Waves trigger is broadcast, the spacecraft continues collecting data for 30 s; data ±30 s from the trigger are stored with the associated quality flag. There is telemetry to transmit ∼7 60-s bursts during each perijove, and more are added to the plan as bandwidth allows.

JADE can only store burst data if it is in Burst mode when the Waves trigger is sent. Burst mode recording sessions are defined by time-tagged command loads uploaded 2 orbits in advance of execution. These recording sessions are planned during the auroral crossings using trajectory and field-line model predictions.

4.1.7.1 Post-Perihelion Initial Ramp Up

JADE-I temperatures were ∼20^∘ above those predicted by the pre-launch thermal model. JADE-I neared, but did not exceed, non-operational limits shortly after launch and fell to acceptable values for the initial high voltage turn on. Initial science observations from that turn-on are shown in Sect. 5. After perihelion, when JADE temperatures fall below post-perihelion operational limits, the instrument will be turned on again with additional precautions.

4.1.7.2 Periodic Instrument Maintenance

Juno has 3 designated week-long periods for Periodic Instrument Maintenance (PIM). JADE will not participate in Earth flyby or the first PIM due to thermal constraints. However, JADE will participate in the second PIM in 2014 and the final PIM in 2015. During these periodic maintenances, JADE starts with a gain test early in the activity, spends time in each mode including coordinated Burst mode periods with Waves, and concludes with a gain curve at the end of the activity.

4.1.7.3 Cruise Science

When bandwidth allows, and additional science can be obtained without increasing mission risk or compromising long-term objectives, JADE participates in cruise science activities using LRS mode. If bandwidth allows the standard 600 s accumulation rate will be decreased to 300, 60 or 30 s and an additional ion species may also be transmitted. Cruise science activities are conducted using standard command packages to turn on, set the initial configuration, and then dwell in that mode. Minimal changes are made to keep operations and bookkeeping as simple as possible.

4.1.7.4 Approach Science

JADE participates in approach science activities as bandwidth and power allow, on a non-interference basis with primary mission objectives. Approach science consists primarily of LRS, which can be augmented with additional activities based on consensus among the MWG and project.

4.1.7.5 Capture Orbit

JADE takes LRS data during the capture orbit, as bandwidth and power allow, on a non-interference basis with primary mission objectives. JADE is planned to operate until just a few days before JOI and then turn off for JOI. JADE will turn back on as soon after JOI as possible and resume LRS operations. A model prediction shows tens of magnetosheath crossings within the capture orbit (Bagenal et al. 2013, this issue), providing intriguing secondary science opportunities. Additionally, JADE will work with the MWG on any coordinated instrument efforts during the capture orbit.

4.2.1.1 JADE-E Data Resolution by Mode

JADE-E has an inherent resolution of 64 energies×48 directions×1 s. Sweeping and data collapsing are dependent on mode.

In HRS and Burst the JSIB sweeps all 64 ESA/DFL pairs in 1 s. In Burst there is no collapse of energy, direction or time resolution. In HRS the data is collapsed to 32 energies×24 directions×1 s resolution, with directions collapsed according to pitch angle. Pitch angles within 0–60^∘, and 120–180^∘ from the magnetic field direction are uncollapsed in direction. The remaining bins are collapsed from 4 adjacent fine-pitch angle bins to 1 course pitch-angle bin as shown in Fig. 71.

Open image in new window

In LRS and Cal, the data is not collapsed, and retains 64 energies. There is no directional energy collapse based on pitch angle because all data are taken in a plane. The deflectors are swept, but not to magnetic field alignment. Rather than 1 s resolution, the data are binned into histograms with typically 5 or 10 minute resolution for Low Rate LRS, and 30 or 60 s for Intermediate Rate LRS and Cal.

4.2.1.2 JADE-I Ion Data Resolution by Mode

Sweeping, species, and data collapsing are dependent on mode. In HRS and Burst modes, JADE-I’s inherent resolution is 32 energies×8 elevations×12 azimuths×256 TOF×4 s. Based on LUTs, ranges of TOF for each energy step are defined as species. Species 0 and 1 are predefined as being any events measured on any stop anode and any event with a valid TOF, respectively. Species 2 is for mass per charge of 1 amu/q, species 3 is for light components of the plasma (mass per charge between 2 and 5 amu/q), and species 4 is for heavy components of the plasma (mass per charge >5 amu/q). Up to 3 more species can be identified, and the values for species 2–7 can be redefined by upload of a table at any time.

In Burst there is no data collapse, and up to 6 species are stored configurable by command. In HRS the data is only collapsed in elevation by summing adjacent elevations; the data stored is 32 energies×4 elevations×12 azimuths for each transmitted species (default 2 species in HRS, configurable by command). An additional ‘optional’ species can be transmitted while in HRS. Our default configuration will be to transmit species 2 and 4, and have species 3 as the optional species while in HRS.

In LRS and Cal, the data is 32 energies×56 directions×1 species. Rather than 4 s resolution, the data are binned into histograms with 5 or 10 minute resolution for Low Rate LRS and Cal, and 30 or 60 s for Intermediate Rate LRS. JADE calculates the spin phase at each observation cycle boundary based on the time stamp of the spin phase provided by the spacecraft. JADE-I bins translate to 56 near-constant solid angle sectors. In LRS the data is mapped per spin, such that the counts in each anode are mapped over approximately equal solid angles. Since anodes 4 and 11 look nearly along the spin axis, in half a spin they sweep out about the same amount of solid angle as anodes 7 and 8 sweep out in about 15^∘ of rotation, therefore in LRS, the count rates for each anode are mapped to different sized bins in spin phase (see Fig. 72). Anodes 0–3 are not used in LRS for the species data since they view the same region of space as anodes 4–7 out of phase by 180^∘.

Open image in new window

4.2.1.3 JADE-I TOF Resolution by Mode

JADE-I inherent TOF resolution is 32 energies×256 TOF×1 s over all available directions. In Burst science the TOF data is collapsed such that elements 0–83 are mapped one to one, and for TOF bins 84–255 the 4 consecutive bins are collapsed (i.e. bins 84, 85, 86, and 87 are collapsed and mapped to 84). In HRS, adjacent energies are collapsed to provide a resolution of 16 energies×128 TOF×4 s resolution. The same TOF collapsing as in burst is performed in HRS. In LRS and Cal, TOF data is collapsed into 16 energies×64 TOFs. In all modes, the TOF data is summed over all elevations and azimuths covered in the accumulation time.

4.2.2.1 JADE-E Sweeping by Mode

JADE-E sweeps through its 64 ESA steps in 1 s, regardless of mode. A low-resolution, 32-step sweep is taken every 0.5 s, hitting every other energy step in the first half second where voltage is increasing, then filling in the skipped voltages during the second half of the second where the voltages are decreasing. ESA steps are 0,2,4,…,30,31,29,…,1 (Fig. 73). The reason for sweeping up then down is to minimize the largest single step taken and therefore minimizing the HVPS settling time. Because the intrinsic resolution of JADE data is 1 s, no additional temporal resolution is required.

Open image in new window

There are 3 JADE-E sweep tables that can be updated; at launch, table 1 covered the full energy range from 0.1 to ∼100 keV, table 2 is a limited range solar wind sweep from 0.1–5 keV, and table 3 is a calibration table with 4 energies.

In HRS and Burst all 3 JADE-E sensors collect data, and deflectors are set to remain field-aligned according to the real-time magnetic field. The JADE software compensates for an ∼1 s latency in this vector by propagating from its last position to the time at the next sweep start, using the updated spin rate. In LRS only one sensor is operational. In Cal, all three sensors are active. Deflection is based on sweep tables, not magnetic field alignment.

4.2.2.2 JADE-I Sweeping by Mode

JADE-I makes use of low-resolution scans within the full ESA range to allow for greater temporal resolution over a large energy range. JADE-I has 32 ESA steps, but how long it remains in each step and how the voltages are ramped depend on the mode.

In HRS, one 32-step high-resolution ESA sweep is completed every 4 s, while each 8-step low-resolution ESA sweep is completed every 1 s. The low-resolution scans allow for greater temporal resolution than a sequential scan. ESA steps are 0,4,8,12,16,20,24,28,29,25,21,17,13,9,5,1,2,6,10,14,18,22,26,30,31,27,23,19,15,11,7,3 as shown in Fig. 74.

Open image in new window

In HRS, the deflector is scanned 8 DFL steps for each ESA step (covering ±45^∘ deflection), covering 64 ESA/DFL pairs per second. There are two sweep tables available for use in HRS that can be updated anytime by uploading new tables. The tables at launch are set to cover the full energy range. The first HRS sweep table sweeps the deflectors over 8 steps, spaced 11.25^∘ apart, while the second sweeps the deflectors over 8 steps, spaced 6^∘ apart.

In LRS and Cal, the ESA supply sweeps according to one of four sweep tables that command 32 voltage steps per second. The tables can be updated at anytime, with tables loaded at launch having 32 logarithmically spaced steps arranged in a pyramid sweep such that even numbered voltage steps ramp up in the first half second and the odd numbered voltage steps ramp down in the second half second (Fig. 75).

Open image in new window

There are 4 sweep tables available in LRS that can also be updated. At launch, the first table covered the energy rang of 10 eV to 45 keV, and is the default table to be used at Jupiter, while the second covers the energy range from 350 eV to 7.7 keV to be used during cruise to measure the solar wind; this energy range will cover the most probable solar wind velocities (Gosling 2007). The third table covers energies from 125 eV to 45 keV, and the fourth, energies from 5 eV to 25 keV. The deflector supplies do not sweep in this mode.

4.2.2.3. Histogram Binning

All histogram data is organized by spin phase. JADE calculates the spin phase at each observation cycle boundary based on the time stamp of the spin phase provided by the spacecraft. JADE-I bins translate to 56 near-constant solid angle sectors. In LRS the data is mapped per spin, such that the counts in each anode are mapped over approximately equal solid angles. Since anodes 4 and 11 look nearly along the spin axis, in half a spin they sweep out about the same amount of solid angle as anodes 7 and 8 sweep out in about 15^∘ of rotation, therefore in LRS, the count rates for each anode are mapped to different sized bins in spin phase (see Fig. 72).

The JADE-E lookup tables controlling the ESA and DFL voltages in LRS are fixed. These tables contain 64 steps of ESA and DFL voltage combinations. For example, one can chose a table with 64 energies with zero voltage on the DFL. Alternately, one can chose a different table with 16 ESA voltages (e.g. 16 energies) and 4 DFL voltages for each energy step. The voltage sweep is different, but the total of ESA-DFL voltage combinations is the same. The LRS data products for JADE-E are independent of the lookup table and are always binned in an array of counts with dimension 64×24. We reconstruct the count rate versus energy by look direction using the lookup tables with ground processing.

4.2.2.4. Determining Count Rate

The ion species and electron products in LRS are in units of rates (counts per views) instead of counts. If a sector is viewed 2 times and has 1 count, the rate is 0.5, which is saved as 0 (losing data with integer truncation). So, rates are multiplied by 512 before being sent to the ground. Then the ground software then divides the rates by 512 into floating point numbers, and preserves decimal rates.

4.4.2.1 Low Voltage Engineering (LVENG)

LVENG is entered immediately following successful Bootup and in this mode, the FSW first validates the Science Program Lookup Tables and then checks for the presence of the JSIB. If the JSIB is not present, or the Lookup Tables are corrupted, the Science Program stays in this mode and refuses to enter any other Science modes. In LVENG, all power supply voltages are set to 0, all JADE-E sensors are powered off (JADE-I low voltage remains powered), stims are disabled, and data buffers are cleared. Once in LVENG, the FSW produces housekeeping data and waits for commanding to one of the other modes.

4.4.2.2 High Voltage Engineering (HVENG)

Once commanded into HVENG, JADE-I is, and any number of the three JADE-E sensors can be, powered on. Default consists of all 3 JADE-E powered off, and macros enable the sensors for transition into the next mode. Upon initial entry into HVENG no voltages are changed. In each subsequent return to HVENG, the ESA and DFL are returned to the last values commanded while in HVENG. This ensures photoelectron rejection voltages are retained after the MCP voltages have been brought up after initial turn-on. Science and housekeeping data are produced in HVENG and JADE can only enter and leave this mode by command or autosafing.

4.4.2.3 MCP Calibration (Cal)

In Cal mode, JADE-I is on and any number of the three JADE-E sensors can be on, with a default is all three JADE-Es powered on. All prior MCP and TOF voltages from HVENG are retained and MCP voltages are stepped via command or macro to generate a gain curve (counts vs MCP voltage).

4.4.2.4 Low Rate Science (LRS)

Low rate science mode is used for the large portions (>90 %) of the Juno orbit away from Jupiter, where power needs to be conserved. In LRS, JADE-I and only one of the three JADE-E sensors is powered on. Deflector voltages are 0 for JADE-I and are swept via DFL sweep table for JADE-E. The inertial spin phase and inertial spin rate provided by spacecraft Attitude Control System (ACS) are used to map the JADE-I and JADE-E anodes into spin sectors (if no ACS message is received, the default spin phase is 0 and the default spin rate is set to the nominal spin rate of 2 rpm). These spin phase ordered observations are collapsed, saved and telemetered down at a low cadence, depending on the available telemetry (typically providing 5 or 10 minute temporal resolution for standard LRS but increasing temporal resolution to 30 or 60 s accumulation for intermediate rate LRS.

4.4.2.5 High Rate Science (HRS)

High rate science is the primary science mode for JADE, and is the mode that JADE is in for each of the critical perijove intervals, which includes the critical auroral crossings. In HRS, JADE-I and all three JADE-E sensors are nominally on with the DFL and ESA voltages sweeping at high rate to attain high temporal resolution observations. The broadcast magnetic field vector onboard Juno is received by JADE and used to determine which of the three JADE-E sensors has positive deflection, which has negative deflection, and which remains undeflected. Deflector voltages are swept via table.

4.4.2.6 Burst Science (Burst)

Burst science mode is simply an extension of HRS, but at higher resolution (no collapsing of dimensions and no accumulation of multiple samples). Data products are sent directly to the spacecraft over dedicated serial lines and include all ion species. Owing to the very large data content, only short intervals of Burst can be stored and fit into the JADE telemetry allocation.

JADE Lookup Table (LUT)

The JADE Lookup Table (LUT) is a partition in JADE memory that contains the definitions of tables, macros and parameters. The LUT contains default parameters, hidden parameters, ion species TOF definitions, voltage sweep tables, compression definition, MCP rate limiting definitions, macros, autosafing definitions, and the magnetometer parameters.

Upon transition from Bootup to LVENG, the Science Program performs a checksum and EDAC check of the table images stored in EEPROM. If table image 1 has no errors, it is used. If image 1 of a table has errors and image 2 does not, image 2 is used. If both table images have errors, JADE remains in LVENG mode. The error-free table is stored in SRAM, where it is used to define and execute Science Program tasks. The contents of the LUT can be updated in LVENG or HVENG modes. These updates can be temporary (written to SRAM; reverts to default upon power cycle) or permanent (written to EEPROM; establishes new default).

Parameter Tables

All default parameters are captured within the LUT. Examples of these parameters are default sweep table for JADE-E and JADE-I, default JADE-E for LRS, and default additional ion species. These parameters can be updated individually using the Set Parameter command or as part of a LUT upload. Updates can be either temporary or permanent.

ESA & DFL Sweep Tables

When in specified science modes, the ESA and DFL voltages sweep according to predefined sweep tables stored in the LUT. In LRS, JADE-I has 4 ESA sweep tables available and JADE-E has 3. In LRS JADE-E also sweeps their DFL using one of their 3 available DFLs tables. In HRS and Burst, JADE-I has 2 sweep tables that define ESA+DFL voltages. JADE-E has 3 ESA sweep tables available, and the DFL voltages are set according to the Mag parameters table in order to track the magnetic field direction. Large tables like the ESA and DFL sweep tables are updated by either overwriting the whole sweep table or the entire LUT. Updates can be either temporary or permanent.

Ion TOF Tables

The TOF of an ion is measured between the START and STOP signals generated by JADE-I electronics. For each of the 32 JADE-I energy steps, the JSIB converts the time between the START and STOP into digital numbers, and uses it for the subsequent ion data processing. The delta time between the START and STOP signals is proportional to the M/q of the ion and is used to distinguish ion species. See Sect. 3.3.2 for more detail. Large tables like the Ion TOF table can be updated by either replacing the whole sweep table or the entire LUT. Updates can be either temporary or permanent.

4.4.3.2 JADE Macros

JADE macros are a series of relative-timed commands stored in the LUT. These macros are called using the Execute Macro commands. When JADE receives an Execute Macro command, it sends the series of time-tagged commands stored in SRAM. Note that these macros are completely internal to JADE and are different from spacecraft command blocks, although the concept behind spacecraft blocks and JADE macros is the same.

There are 40 macro “slots” available for use in JADE; half contain at least one interactive command and half contain only non-interactive commands. A non-interactive command does not change power draw by any more than 1 W. The first 20 macros are dedicated to interactive commands and contain commanding to change Science modes, decrease voltage prior to an orbital maneuver, increase voltages after an OTM, and prepare JADE to be turned off by the spacecraft. The second 20 macros contain only non-interactive commands that contain commanding to configure settings within a mode, i.e. change the LRS histogram accumulate rate, change the optional ion species transmitted. Large tables like the macros can be updated by either replacing the whole macro table or the entire LUT. Updates can be either temporary or permanent.

4.4.4.1 Lossy Compression

Most JADE products are lossy compressed by JADE except for Ion Direct Events, HVENG products, and housekeeping telemetry. Burst products transmitted over the high speed interface are also not compressed by JADE although they are subsequently compressed by the spacecraft.

Two 32–8 bit compression tables are provided for LRS and Cal Science products. This compression is implemented as a binary search through a step table of 256 values, where at most 8 comparisons are made before determining the compressed value. In contrast, HRS utilizes two 16–8 bit LUT compression tables where the value to be compressed is treated as an index into the table. Before each data product gets lossy compressed to 8 bits, the Science Program subtracts the minimum data value from each product element and reports the subtracted value in telemetry.

The JADE compression cuts the data volume in half, while maintaining losses below 14 % across the entire range of fluxes measured. Steps are spaced such that there is ∼4 % error in 16–8 compression and up to 14 % error in 32–8 compression. The data processing software assigns a value to the data that is the average value of the compression bin; for example, if data with original values of 1000–1040 were compressed together, the data processing software could output 1020±20 as the value of the original data.

4.4.4.2 Lossless Rice Compression

In addition to the lossy compression, most products are also subsequently lossless compressed using the Rice algorithm (exceptions are Ion Direct Events, HVENG products, and housekeeping). Burst products are not lossless compressed by JADE although they may subsequently be lossless compressed by the spacecraft. Lossless compression can be enabled/disabled via the Set Parameter command.

4.4.4.3 Internal Data Policing

The JADE instrument can produce far more telemetry than the spacecraft can store for transmission to the ground. JADE uses lossless compression techniques to reduce data size, but the compression ratio varies. Once JADE reaches a predefined orbit data limit in the spacecraft, all remaining data in the orbit is lost. To avoid losing high-priority data, JADE monitors data volume and omits low priority data as needed.

The orbit is broken into time intervals with a data limit set for each interval. A priority mask, configurable as a table parameter, designates each data product as high or low priority. While data volume for an interval is below the limit, all products are produced. Once the limit has been reached, only the high-priority products are produced. This ensures high-priority products are transmitted regardless of how well they compress. If an interval ends up below the data limit, the excess data allocation is saved for future use.

4.4.5.1 Bin Recording Burst

JADE records data as a series of bins, which are regions of memory designed to hold N words of recorded burst data. The number of bins, size of bins, and amount of data recorded in the bin leading up to the quality factor data is configurable by command. During any one record session, all bins are the same size, with the number and size configurable via command, subject to a minimum of 10 s of data. Once the bin is no longer being written to, it is a “saved bin” and is typically associated with a Waves quality factor. If no Waves quality factors are received with empty bins remaining, JADE data fills them with high-rate data, and those bins are saved without an associated quality factor. When there are no remaining empty bins the spacecraft first records over any bins without an associated Waves quality factor in a circular manner. If there are no empty bins and no bins without an associated Waves quality factor, data with an associated higher Waves quality factor overwrites the lowest quality factor data.

Another key aspect of Burst is the “quality point,” which is where the recorded data associated with the quality factor resides. As part of the configuration of bins (number of bins, size of bins, etc.), the number of 32-bit words desired before the quality point is specified. If, at the time the quality factor is received from Waves, the scratch bin contains the specified number of words or more, the recording is allowed to continue until exactly the number of specified words precedes the quality point. If there are less than the specified points, the scratch bin is allowed to fill, and the resulting quality point will not have the specified number of 32-bit words preceding the event. It is important to note that the timestamp associated with the quality factor is not where the quality point nominally resides.

4.4.5.2 Continuous Burst

The second mechanism, continuous recording, is independent of the Waves instrument. This recording is used to turn on recording and record up to a commanded number of data words.

4.5.1.1 Science Data

Reformatted Engineering Data Records (REDRs) take the raw data and divide it into separate files depending on the JADE Data Product IDentifier (DPID) number. Table 18 separates these out by data type (rows) and telemetry mode (columns).

Table 18

All JADE science products

Open image in new window

In addition to the above science data there are five operations-related data products concerning the whole JADE instrument suite: (1) BOOT_HK—Engineering/Housekeeping packet of the Boot Program (i.e. General monitors and power status’), (2) BOOT_MEM_STAT—Memory Status packet of the Boot Program (i.e. checksums and counts of single/double-bit errors), MEM_DMP—Memory Dump packet (a complete dump of the memory), SCI_ENG—Engineering/Housekeeping packet of the Science Program (i.e. General monitors and power status), SCI_EVENT—Event Message packet of the Science Program (describes an unusual or unexpected occurrence, i.e. a command rejection).

4.5.1.2 Ion Species Identification

The JADE-I sensor uses a loadable LUT to separate the measured events on the anodes to different species mappings. Events that register on a single or two adjacent STOP anodes get mapped to a species. Events that result in only a START event, Background events, or any events that get mapped to non-adjacent STOP anodes are not mapped to a species.

There are 8 potential species, numbered 0 through 7. Species 0 is defined as any event that is seen on either a single or two adjacent STOP anodes. No START is required, so species 0 includes events that do not have a valid TOF measurement (a valid TOF measurement includes a valid START and STOP within a defined time range). For species 1–7 there is additional event masking possible so that events that have a ‘bad’ TOF can be masked out. Bad TOF events are durations shorter than 1 ns or longer than 330 ns. Also, events that have multiple starts prior to the stop signal, or events that result in STOPS in adjacent anodes can be masked out of species 1–7. In the case of measuring STOPS in adjacent anodes, the event is mapped to the lower anode number in addition to incrementing a counter of the number of adjacent events. The mask setting is adjustable based on table values. At launch, adjacent events are included in species 1–7, but bad TOF events and events with multiple starts are not.

Species 1 are all events that do have a valid TOF measurement. Species 1 is a subset of the species 0 events. Species 2–7 are reserved for specific mass ranges and each of species 2–7 are a subset of the species 1 data. Events are mapped to species 2–7 based on a LUT that defines a TOF range for each species and each ESA step. The TOF table in the LUT can be updated by uploading a new table or a new LUT. The species TOF table is currently defined to have species 2 for M/q of 1 amu/q, species 3 for M/q between 2 and 5, and species 4 to be for M/q greater than 5 amu/q.

4.5.1.3 Ion Direct Events

There are two types of primary Science data products: histograms and direct events. Histograms are binned data and retain a complete history of the number of events that occurred within a given bin during an accumulation period. The duration of the accumulation period can be commanded, as noted earlier; low accumulation LRS has histograms that cover 300 or 600 s periods while intermediate accumulation LRS has histograms that cover 30 or 60 s periods. Because these events are simply counted, you can derive the number of particles from a given region from histograms but have no data on the order in which the particles were collected (i.e. if one section of the sky filled in preferentially earlier in the accumulation period and another section filled in later).

Direct events are individual events stored with time of collection, TOF, ESA and DFL information associated with them. A direct event carries with it all relevant information to describe the stored event, and thus is a much more detailed data product. Because the level of detailed information associated with just one direct event is large, telemetering many direct events quickly becomes a bandwidth issue. Juno is a telemetry-limited mission, so histograms are given priority storage. Direct events are stored on a low-priority basis and are transmitted when downlink allows.

4.5.1.4 Electron Data

The electron data are returned as histograms of energy vs. pitch angle, as described in Sect. 4.3.4. For electrons, there is no complexity of TOF data, nor species separation, as there is for JADE-I.

4.5.2.1 Conversion of Counts per Accumulation to Counts per Second

4.5.2.2 Convert to Differential Energy Flux

4.5.2.3 Convert to Differential Number Flux

4.5.2.4 Convert to Phase Space Density

Note that this equation assumes there is zero spacecraft potential accelerating/decelerating the incident electrons/ions. This equation also shows the necessity of partitioning the ion counts into separate ion species. Note the strong dependence of this conversion of DEF to phase space density on the mass and charge of the particles: H⁺ has an (mq)²=1 amu²e², O⁺ has (mq)²=256 (=16²) amu²e², while S⁺⁺ has (mq)²=4096 amu²e². The net effect is that the resulting PSD, the desired scientific measurement, is very sensitive to assumption of what species is being detected.

4.5.3.1 Pitch-Angle Distributions

In order to calculate pitch-angle distributions with respect to the magnetic field, one must know the magnetic field direction at the time of the particle measurement as accurately as possible, ideally with level 3 data from Juno’s magnetometer. Calculating pitch-angle distributions requires reordering the data to refer to the particle’s (ion or electron) direction of motion relative to the magnetic field direction, such that 0^∘ is parallel to the magnetic field, 90^∘ is perpendicular, and 180^∘ is anti-parallel. A dot product between the particle’s flow direction and the magnetic field vector (in the same co-ordinate system) is then used to find the pitch angle. Note that the instrument FOV measures incoming particles, so that the flow FOV is the opposite direction (i.e. the instrument aperture must include the direction opposite to the particle motion).

4.5.3.2 Methods of Calculating Plasma Moments

Methods of calculating single species moments of the velocity distribution function, f(v), are well known (Baumjohann and Treumann 1999; Boyd and Sanderson 1969; Paschmann and Daly 1998) and particularly well suited to spinning spacecraft and electron data.

This technique is excellent for electron data where the plasma species has a known mass and charge (at Jupiter we do not expect to find the negative ions that are found near Titan or Enceladus at Saturn), and can give a cadence of one set of moments values per spin per sensor. Another benefit is that no plasma distribution (Maxwellian, Kappa, or other) is assumed.

However it is more complicated to use this technique for ion data because of the multiple species with differing masses and charges. Partitioning the ion data into single species to calculate f(v) is complicated because the different species often overlap in velocity space. Therefore, a forward modeling method is generally preferred for magnetospheric ions.

4.5.3.3 Methods of Calculating Plasma Parameters via Forward Modeling

If multiple plasma species exist and overlap in velocity space, then plasma parameters can also be obtained by a forward modeling. This requires assuming which plasma species are present and the nature of their velocity distribution (i.e. Maxwellian, kappa, etc.). These assumptions are then used to create a virtual model of the instrument response for a specified density, temperature and velocity of each plasma species. The simulated histogram (for a given density, temperature and velocity per plasma species) is then compared with the measured histogram from JADE, and the model plasma properties adjusted until the model matches the measured data (e.g. as quantified by a reduced Chi-square measure of goodness of fit).

This technique is computationally expensive, but does not require partitioning of the data in to separate species, allows exploration of the parameter space (usually not unique) that reasonably fit the data, and estimates statistical uncertainties of the output plasma properties. The disadvantage is it is necessary to assume the plasma species and distributions to use for the calculation; a poor assumption will give poor values.

4.5.3.4 Temperature Anisotropies and Plasma-β

4.5.4.1 Time

Time is expressed as a UTC string (yyyy-dddTHH:MM:SS.sss) and the corresponding Spacecraft clock time. Additionally, time to/since perijove is provided. These times and conversions are calculated using NAIF SPICE.

4.5.4.2 Spacecraft Position/Orientation

Spacecraft position and velocity is provided in a variety of co-ordinate systems, including (but not limited to) Jupiter-centered radial distance, latitude and local time, SYS3 (with west longitude), RTN and J2000. The spacecraft’s orientation is provided along with transformation matrices to convert this in to several other co-ordinate systems. Finally, the spacecraft spin rate and a spin phase are given, where zero spin phase is when Juno scans past ecliptic north. All this information originates from NAIF SPICE routines, so the particular SPICE kernels used are also noted.

4.5.4.3 JADE Instrument Information

JADE can take a variety of data products, but often runs a subset in order to meet telemetry bandwidth allowances, therefore the data products “on” at any given time are listed. In addition, quality and warning flags are provided to help give context to the data. Magnetic field information is provided, along with a flag describing its source (i.e. onboard uncalibrated or ground-calibrated CODMAC level 3 data from the Juno MAG team).

Different energy tables can be used for each of the JADE instruments. This information includes which table is used, and efficiency parameters required to convert raw data in to calibrated units for CODMAC level 3 products.