Measurements of SEPs

Abstract

Those who study solar energetic particles (SEPs) should be aware of the basic types of experiments that have contributed most of the observations studied in this book, and especially the tradeoff of their strengths and weaknesses, and how they fail. However, this is not a comprehensive review, only an introduction. We focus on dE/dx vs. E instruments that are the workhorses of SEP studies, and also study time-of-flight vs. E instruments that dominate precision measurements below 1 MeV amu⁻¹. Single-detector instruments and high-energy techniques are discussed briefly as are supplementary data and CME lists.

Nearly every experimenter who builds instruments thinks he has made the best tradeoff within the triple constraints of weight, power, and expense, to maximize the scientific return. Many instruments are designed to extend coverage to a previously unmeasured region: energy coverage, isotope resolution, heavy elements. Others hitchhike on spacecraft going to a new and interesting region of space. There are usually tradeoffs among resolution, geometry factor, background elimination, element or isotope coverage, and energy coverage. Higher-energy ions are much easier to resolve, but high intensities at lower energies sample rarer species and rarer events.

The rate of energy loss of an ion in a detector material is approximately

$$ \frac{dE}{dx}\approx \frac{4\pi {e}^2{n}_e}{mc^2{\beta}^2}\left(\frac{Q^2}{A}\right)\left[\ln \left(\frac{2{mc}^2{\beta}^2{\gamma}^2}{I}\right)-{\beta}^2\right] $$

(7.1)

where m and e are the mass and charge of an electron, n_e is the electron density, I is the “mean ionization potential” of the stopping material, and β and γ are the relativistic velocity and Lorentz factor of the ion as defined in Sect. 1.5.4. Here we have again used E = Ɛ/A = M_u (γ – 1) ≈ ½ M_u β², a function of velocity alone, to show that the only dependence on the stopping ion is Q²/A and its velocity β. M_u = m_uc² = 931.494 MeV.

Equation 7.1 is derived from the electron-ion scattering cross section (Rutherford scattering) where we view incoming electrons of the stopping material being scattered by the electric field of the ion. Energy transfers to the electrons are integrated from a minimum of I to a maximum of 2mc²β² γ², which is approximately the maximum energy that can be transferred to a scattered electron. Note that, when Q ≈ Z, the dominant energy dependence of dE/dx is ~β⁻², or nonrelativistically, ~E⁻¹, sometimes a useful approximation.

At relativistic energies, dE/dx reaches a broad minimum at ~2.5 GeV amu⁻¹ then rises slightly from density effects not included here. At low energies, dE/dx actually peaks, because Q decreases, but Q → Z at moderate energies. A simple approximation sometimes used for this is Q ≈ Z [1–exp (−β/β₀)]. For capture into the K orbital, β₀ ≈ Z/137; for the Fermi-Thomas model β₀ ≈ Z^2/3/137. Modern empirical tables use more complex expressions and tabulate both stopping power and range (Hubert et al. 1990). The particle range R = ∫dE (dE/dx)⁻¹. For energies down to 1 keV amu⁻¹, the tables of Paul and Schinner (2003) are available.

7.1 Single-Element Detectors

Conceptually, the simplest detector is that with a single sensitive element. Modern “solid-state” detectors are a Si wafer biased as a capacitor that collects the electron-hole pairs produced when an ionizing particle penetrates, loses energy, or stops within its volume. The charge collected, proportional to the energy loss, is measured as a pulse height by analog-to-digital converters. Single-element detectors are generally shielded to define the access geometry for low-energy particles.

Measuring the energy of each arriving particle works at low energies, but penetrating particles contribute as if they had a much lower energy (Fig. 7.1). When the SEPs have a steep energy spectrum, the contribution of high-energy particles may be small, but early in SEP events nearly all particles are penetrating and single-detector instruments falsely appear to show low-energy particles arriving from the Sun much earlier than they possibly could. This effect is sometimes called “punchthrough,” it occurs in every SEP event, and can cause serious misconceptions. These detectors also confuse heavier ions similarly, even though they deposit an increasing amount of energy. Single-element telescopes should never be used for SEP onset timing. They are more appropriate for the study of steep energetic particle spectra at interplanetary shock waves. Their obvious advantage is low weight, power, and cost.

Fig. 7.1

A single-detector telescope measures the total kinetic energy of stopping ions (red) and the energy loss of penetrating ions (blue), which is much lower. The latter are (incorrectly) assumed to be rare. Access geometry is somewhat controlled by shielding (not shown) and a permanent magnet may be included to sweep away electrons, or to measure electrons by comparing detectors with and without magnets

Electrons are particularly difficult to measure since they do not travel in straight lines in detectors, and can suffer numerous large-angle scatters. The best remedy is extensive instrument calibration before launch.

Single-element and other limited telescopes are sometimes flown on deep-space or planetary missions where weight and power are severely limited and where SEPs are not the primary objective. Unfortunately, these low-priority hitchhikers may even be turned off during transit to the mission destination to save resources, further decreasing their limited value.

7.2 ΔE Versus E Telescopes

These telescopes consist of at least three active detector elements. Particles enter the first detector, penetrate into the second, and stop before entering the third anti-coincidence detector. The separation of the first two detectors, and their areas, determine the instrument geometry-factor. The detector thicknesses determine the minimum and maximum energy according to the range-energy relation in Si (e.g. Hubert et al. 1990). Front detector thicknesses of 10–20 μm set a lower bound of ~1 MeV amu⁻¹, depending upon species. Total thicknesses of D + E of up to 10 cm of Si are used for energies ~200 MeV amu⁻¹. The energy range can be extended to above ~400 MeV amu⁻¹ by observing the change in dE/dx between D and E, if penetrating ions are measured. The concept of a two-element telescope is shown in Fig. 7.2.

Fig. 7.2

A minimal ΔE vs. E or two-dimensional telescope requires coincidence of signals from the D and E elements and no signal from the A element to define stopping (red) ions. “Matrix” plots of pulse-heights of D vs. E are used to resolve elements and measure their energies (see Fig. 7.3). The anti-coincidence element or inert shielding may surround the telescope. Note that the energy deposit in D depends upon the angle of the particle trajectory with the telescope axis; this energy spread and thickness variations control charge and isotope resolution.

Most of the particle telescopes flown in space are of this general type, although multiple detectors may be used in place of the D and E elements. Early telescopes used plastic scintillator or even gas drift chambers, but most telescopes of the last 25 years are “solid state” Si detectors which have extremely high resolution and stability, i.e. their response does not change at all during decades of operation. Gas detectors soon suffer from out-gassing, contamination, or leakage.

Anti-coincidence detectors were sometimes wrapped around the whole telescope. However, at the high rates in a large SEP event these may be recording particles nearly all the time, and insure that the telescope is effectively turned off.

A geometry factor defined by surfaces S₁ and S₂ is

$$ A\Omega =\underset{S_1}{\iint }{dS}_1\underset{S_2}{\iint }{dS}_2\frac{\left({\mathbf{n}}_1\cdot \mathbf{r}\right)\left({\mathbf{n}}_2\cdot \mathbf{r}\right)}{r^4} $$

(7.2)

where n₁ and n₂ are unit vectors normal to the surface elements dS₁ and dS₂, respectively and r is the vector distance between them. Geometry factors are usually calculated numerically.

7.2.1 An Example: LEMT

Response of a telescope with a thin front detector, the Low-Energy Matrix Telescope (LEMT) on the Wind spacecraft (von Rosenvinge et al. 1995), is shown in Fig. 7.3. LEMT has three important virtues, large geometry (51 cm² sr), broad element coverage (H–Pb at ~2–20 MeV amu⁻¹), and, equally important, the author is familiar with it. Each LEMT consists of a domed array of 16 D-detectors 18 μm thick, followed by a large 1-mm-thick E-detector with coarse 5 × 5 position sensing and an anticoincidence detector (see von Rosenvinge et al. 1995).

Fig. 7.3

Response of the LEMT telescope (diagram shown in inset) to ions from a small ³He-rich event in 1995 is shown with “tracks” of species indicated. The telescope has only modest resolution of He isotopes. The track of O is heavily populated by anomalous cosmic rays during this period near solar minimum (see Reames et al. 1997 © AAS)

Particles entering LEMT are corrected for angle of entry, mapped in a log D vs. log E space, like that of Fig. 7.3, and binned onboard according to particle species and energy interval (see Reames et al. 2001). The right-hand ends of the particle tracks, especially noticeable for C, N, and O, occur just before the ions have enough energy to begin to penetrate into the anticoincidence detector.

In the region of the rarer elements with Z ≥ 34, “priority” measurements of individual ions are rare enough to be telemetered for later analysis. The performance of LEMT at high Z is shown in Fig. 7.4.

Fig. 7.4

High-Z response of LEMT is shown where resolution (i.e. track width) is comparable with that at Fe, but track locations are well calibrated using beams of Fe, Ag, and Au before launch. Energy varies along each calibration curve from left to right, from 2.5 to 10 MeV amu⁻¹

While the error at high Z is ±2 to 3 units, the resolution is adequate to show bands of enhanced abundances, such as that between Ge and Zr and the band near Sn, that reflect an abundance maximum at 50 ≤ Z ≤ 56. The absolute locations of the reference curves of the elements were well calibrated prior to launch using accelerator beams of He, C, O, Fe, Ag, and Au (see von Rosenvinge et al. 1995). By measuring at low energy with a fairly large geometry factor, LEMT can move up the steep energy spectra to get a rough measure of the abundances of the rare elements with 34 ≤ Z ≤ 82. For results of these measurements see Figs. 4.7 and 4.9.

7.2.2 Isotope Resolution: SIS

Accuracy can be affected by thickness variations and sec θ variations by particle trajectories inclined by an angle θ to the telescope axis. Both of these may be reduced by accurately measuring sec θ using two sets of x and y strip detectors (e.g. Stone et al. 1998). The additional detector thickness required for these measurements raises the energy threshold above ~10 MeV amu⁻¹, depending upon particle species, but also permits isotope resolution up to Fe—an important tradeoff. Fig. 7.5 shows the resolution of Ne isotopes by the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE) in two different SEP events. Isotopic abundances show the same A/Q variations we have seen in element abundance enhancements in both impulsive and gradual SEP events. Here, however, there is no question about the average value of Q, which we assume to be the same for all isotopes of an element.

Fig. 7.5

Panels show the resolution of Ne isotopes by the SIS telescope in two SEP events. Histograms are also shown enhanced by a factor of 5 to clarify ²²Ne measurement (Leske et al. 2007 © Springer). Isotope measurements show A/Q enhancements like those in element abundances

7.2.3 Angular Distributions

Most early missions outside the magnetosphere, such as IMP 8, ISEE 3, and even Wind, were spin stabilized and could choose the spin axis normal to the ecliptic, allowing easy measurement of each particle’s direction of incidence as a time in the spin phase relative to a Sun pulse or a directional pulse from the magnetic field instrument (see Figs. 2.3, 2.6, 3.11, 5.6, and 9.17). Since the magnetic field often lies in the ecliptic plane, this allows easy measurement of pitch-angle distributions. Instruments with apertures above and below the ecliptic could even have 3D coverage.

More-recent missions, like Voyager, SOHO, STEREO, PSP, and even ACE are controlled by stabilized or sun-pointing instruments such as cameras or coronagraphs, so that the particle instruments have great difficulty measuring meaningful distributions, even with multiple telescopes and look angles. Particle transport can be complex and the ability of older instruments to compare distributions of H, ³He, ⁴He, O, and Fe (e.g. Reames, Ng, and Berdichevsky 2001) can often help answer modern questions of ion transport when they arise (e.g. Reames 2019).

7.2.4 Onboard Processing

Early instruments telemetered pulse-height measurements of each particle to the ground for processing. Often a new particle could not be sampled until the current one was telemetered. Onboard sampling criteria for H, He, and heavies were soon invented to insure that rarer species were sampled, but telemetry was limited to about one particle s⁻¹. Onboard microprocessors can now handle up to ~10,000 particles s⁻¹, correcting for geometry and identifying particle species and energy by lookup in 64 × 64 element log-log tables (one for ³He-⁴He, one for 6 ≤ Z ≤ 26) that would overlay regions of Fig. 7.3, for example. This is a major statistical improvement; the telemetry is now filled with bin counts by particle species and energy with some space reserved for pulse heights of rare species and samples for quality control.

Buffering is also possible, so that rates can be sampled for longer or shorter periods, some sampled vs. spin phase, and all are queued in a buffer for readout. However, for SEPs near 1 AU, we rarely need less than 5-min averages. Typically, for an energy interval from E₁ to E₂, it is not worthwhile to study time intervals less than t₁ – t₂, where t_i is the typical travel time from the Sun for particles of energy E_i. Higher time resolution can be useful for more local sources, e.g. for spacecraft nearer the Sun or for study of local shocks. For the broad energy intervals on some SEP monitors, the mean energy can decrease markedly with time.

7.3 Time-of-Flight Versus E

Measurement of a particle’s time of flight over a fixed distance determines its velocity. If the particle subsequently stops in a Si detector its total kinetic energy can be measured, and the pair of measurements determines the particle mass. The design of the SupraThermal Energetic Particle (STEP) system flown on the Wind and STEREO spacecraft is shown in Fig. 7.6.

Fig. 7.6

The STEP telescope measures time of flight vs. energy (see text; von Rosenvinge et al. 1995 © Springer)

A particle penetrating the entrance Ni foil in STEP may knock off ~4–30 electrons that are accelerated and deflected by the 1 kV electric field into the “start” microchannel plates that multiply the signal by ~100. If the particle then enters the Si detector, backscattered electrons are accelerated into the “stop” microchannel plates, and energy is measured in the Si detector. The time between the start and stop signals, 2–100 ns, is processed by a time-to-amplitude converter (TAC). The TAC and energy signals are combined into a weighted analog sum that assigns a priority that controls further processing. Heavies, with A > 4, are assigned the highest priority, He next, and then H. The response of STEP to a small ³He-rich SEP event is shown in Fig. 7.7.

Fig. 7.7

The response of the STEP telescope shows the time-of-flight (ns) vs. the total kinetic energy (MeV) for a sample of ions during a small ³He-rich SEP event (see von Rosenvinge et al. 1995; Reames et al. 1997 © AAS)

The resolution using this technique can be greatly improved by adding an additional timing plane, using electrostatic mirrors to reflect the electrons, and using microchannel plates with position-sensing anodes. This was done for the ULEIS instrument on the ACE spacecraft (Mason et al. 1998). This instrument produced the resolution seen in Fig. 4.19.

7.4 NOAA/GOES

The Geostationary Operational Environmental Satellites (GOES), operated by the National Oceanic and Atmospheric Administration (NOAA), are a series of satellites intended to give continuous time coverage of the space environment. A new GOES spacecraft with equivalent capabilities is launched every few years.

Energies of interest for SEP observations are proton energies in five channels from 4 to 500 MeV measured by two-element telescopes behind different thicknesses of shielding in the Energetic Particle Sensor (EPS). In addition, the High Energy Proton and Alpha Detector (HEPAD) adds a Cherenkov detector to measure protons in the intervals 350–420, 420–510, 510–700, and >700 MeV. These are extremely useful high-energy measurements. GOES data since 1986 are available at https://www.ngdc.noaa.gov/stp/satellite/goes/index.html (although the web site has been known to change). Note that the low-energy channels of the EPS should not be used for onset timing since they are contaminated by higher-energy particles. Geometry factors for high-energy particles are too uncertain to allow channel differences to exclude all contamination in EPS. However, GOES provides an excellent synoptic summary of SEP events (see Fig. 5.1) and the >700 MeV channel may be a better indicator of high-energy protons than neutron monitors (Thakur et al. 2016).

GOES also provides 1–8 Å soft X-ray peak intensities that is a classic measure of heating in solar flares. The X-ray “CMX class” specifies the decade of X-ray peak intensity with Cn for n × 10⁻⁶ W m⁻²_, Mn for n × 10⁻⁵ W m⁻², and Xn for n × 10⁻⁴ W m⁻² (e.g. lower panel of Fig. 7.8; also see Fig. 4.15).

Fig. 7.8

CME height-time plots (center) are shown together with GOES X-ray (below) and particle (above) data in the SOHO/LASCO CME catalog (see Gopalswamy et al. 2009)

A very useful data base is provided by the SOHO/LASCO CME catalog which lists CME data from January 1996 on (Gopalswamy et al. 2009; https://cdaw.gsfc.nasa.gov/CME_list/). Plots of GOES particle and X-ray data are also shown correlated with CME height-time plots as shown in the example in Fig. 7.8. Note the steep height-time plots (i.e. fast CMEs) in the middle panel near the onsets of the two large gradual SEP events in the upper panel. The SEP event on 7 January has a prominent associated X-ray increase (lower panel), while that on 6 January does not, even though both SEP events have similar intensities of >100 MeV protons. Other data in the catalog include movies of CME evolution and EUV flares along with radio data from Wind/WAVES.

There are also data bases of CMEs produced automatically such as the Solar Eruptive Event Detection System (SEEDS; http://spaceweather.gmu.edu/seeds/; Olmedo et al. 2008) which generates listings of CMEs for both SOHO/LASCO and STEREO/SECCHI and CME movies. The SEEDS web site contains links to other data bases.

7.5 High-Energy Measurements

Ground-level neutron monitors have provided the historic information on SEPs above ~0.5 GeV by observing the products that rain down from nuclear interactions of energetic protons with atomic nuclei of the upper atmosphere. When the signal from the SEPs can be seen above the background produced by galactic cosmic rays we have a ground-level event (GLE). However, most GLEs rise less than 10% above GCR background, providing rather poor information on timing.

As noted previously, high-energy protons are often strongly beamed along the interplanetary magnetic-field line, so a particular neutron monitor on Earth sees an intensity maximum when its asymptotic look direction is aligned with that field. Since the field direction can vary, neutron monitors often see sudden increases or decreases, or even multiple peaks and valleys of intensity as their look direction scans across the pitch-angle distribution as the interplanetary magnetic-field direction swings around. Nevertheless, integrating over an event at multiple stations can produce creditable spectra, that compare well with those from GOES and IMP, as obtained by Tylka and Dietrich (2009) and shown in Figs. 6.2 and 6.3. This was a significant advance in high-energy spectra.

Two newer instruments, the Payload for Antimatter Exploration and Light-nuclei Astrophysics (PAMELA) mission and the Alpha Magnetic Spectrometer (AMS) are large complex instruments that were justified and funded for particle physics and cosmology, which may also prove useful for high-energy SEP measurements. These instruments use transition-radiation detectors, time-of-flight detectors, a permanent magnet and tracking system, Cherenkov systems, and calorimeters to measure each incident particle. They were designed to search for antimatter, such as anti-helium, strange quark matter, and dark matter.

The PAMELA satellite (2006–2015) is in a near-polar, 70°-inclination, orbit. It can measure protons and He above about 80 MeV amu⁻¹ and reported spectra for the 13 December 2006 SEP event (Adriani et al. 2011), for events in 2010–2012 (Bazilevskaya et al. 2013), and for 26 events from 2006 to 2014 (Bruno et al. 2018). AMS is on the International Space Station. It can measure protons and isotopes of light ions above about 200 MeV amu⁻¹.

While these instruments must deal with geomagnetic-field limitations, as neutron monitors do, they can directly measure spectra and abundances, and represent a great improvement in the accuracy of measurements at high energies.

7.6 Problems and Errors

The single most difficult problem in measuring SEPs is exploring rare species and small events while still dealing with the high intensities in large events. Most high-resolution instruments fail or degrade during periods of high SEP intensity.

Early instruments sampled particles randomly and sent the measurements to the ground for analysis. However, since telemetry was slow and the H/O ratio can exceed 10⁴ at fairly high energies, H and He consumed all the telemetry and heavy ions were almost never seen. Later instruments incorporated priority schemes to distinguish H, He, and “heavies” and selectively telemetered them at different priorities, keeping track of the number received onboard for re-normalization. Most modern instruments determine particle species and energy and bin them onboard. The higher onboard processing rates have allowed geometry factors to profitably expand, improving statistics and observing rare species.

As rates increase, the first problem to solve involves “dead-time corrections.” An instrument cannot process a new particle while it is still busy processing the previous one. Knowing the processing times, these corrections are usually already included while calculating intensities. However, it does make a difference whether the telescope has become busy because too many high-energy particles traverse the anticoincidence detector, or because too many low-energy particles are striking the front detector. Some instruments can determine coincidence and priority at high rates before they decide to perform the slower pulse-height analysis; they can handle much higher throughput. Instruments that must pulse-height analyze every above-threshold signal in every detector are more limited in speed, by factors of 10 or more, since many of the pulse heights are not of interest; perhaps because they do not even meet the coincidence conditions.

Eventually, problems come from multiple particles in the telescope within the resolution time. A proton stops in the back detector and triggers the coincidence while a low energy Fe stops in the front detector, or while an energetic He or heavier ion crosses the front detector at some large angle. Background in LEMT during the first day of the Bastille-Day SEP event on 14 July 2000 is shown in Fig. 7.9. Background stretches all the way up the ordinate in the 2 < E < 20 MeV band. Calibration curves that are shown have omitted the 2.5–3.3 MeV amu⁻¹ interval which would extend into this band (also compare Fig. 7.3). The added background not only contaminates measurements but also reduces the time available for real particles. Fortunately this is a rare problem for LEMT and it fails quite gracefully in this case, i.e. abundances and spectra above 3.3 MeV amu⁻¹ are still quite valid.

Fig. 7.9

Sampled response of LEMT is shown during the first day of the Bastille-Day event, 14 July 2000. Calibration curves are only shown from 3.3–10 MeV amu⁻¹, to emphasize the band of background covering the region where the 2.5–3.3 MeV amu⁻¹ interval would be. The low-energy SEP ions are throttled by the streaming limit (Sect. 5.1.5) and only background survives. Compare the region 2 < E < 20 MeV with that in Fig. 7.3

The upper limit of E of the background band in LEMT occurs because it is difficult for a proton, the most abundant species, to deposit more than 10 or 15 MeV into the E detector before penetrating into the anticoincidence detector. Instruments that require three energy measurements on each ion can use consistency to eliminate much background, but they have higher energy thresholds than LEMT.

One easy way to detect background is to check for unrealistic abundances, such as measurable ratios of F/O or B/O. If you discover something really unusual, it is wise to check the pulse-height matrix before publishing your amazing new finding.

Different instruments have different problems and some have interesting solutions. Some early instruments suffered gain changes in large events so the particle tracks moved around with time. Many instruments saturate at high particle rates, the smaller, faster instruments on GOES and Helios do not. ULEIS has a restricting aperture that can be rotated into place to reduce intensities. Other telescopes turn off detector elements to reduce their effective geometry factor.

The data base for many measurements from many spacecraft, including SEP intensities, is http://cdaweb.gsfc.nasa.gov/sp_phys/. Generally, however, pulse-height data are not widely available, since the more-extensive data and the specialized processing and software required are only developed by the instrument teams. This software is generally not modified to keep up with evolution of computer hardware and operating systems. Software longevity can become a general problem for instruments that last over two solar cycles.

Last, but not least, good calibrations of instruments before launch are important. A wide variety of beam species from particle accelerators are now commonly available over a vast energy range. Not only does this validate the nominal particle response derived from the range-energy relation, but multifaceted telescopes like LEMT were mounted in a rotatable fixture that allowed each detector to be exposed to an ⁵⁶Fe beam at normal incidence to measure thickness distributions that could be used in corrections. This, and confirmation of viewing angles, could only be done with terrestrial monoenergetic beams and not with “inflight” calibrations sometimes used.