The Global-Scale Observations of the Limb and Disk (GOLD) Mission

The GOLD mission is focused on four primary scientific questions regarding the response of the Earth’s thermosphere and ionosphere to forcing from above and below. Understanding the response to this forcing is one of the most prominent challenges for research in the Earth’s space environment. Each of the four primary questions are addressed through the examination of more focused questions that can be addressed using the high cadence, global-scale images observed by a ultraviolet (UV) imager in geostationary orbit. This orbit allows the GOLD imager to make repeated images, covering most of the hemisphere over the Americas, of the same geographic locations throughout a day, as portrayed in Fig. 1. Previous missions, e.g. TIMED, have demonstrated the value of imaging the thermospheric composition (O/N₂ density ratio) on the dayside, and peak ionospheric densities on the nightside. Such imaging has led to major advances in understanding the behavior of the T-I system. GOLD will make similar measurements of the O/N₂ density ratio, adding simultaneous measurements of the thermospheric temperatures at the same altitudes, and the observations will cover the American hemisphere for most of each day and at a high cadence.

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The first of the four primary science questions, “How do geomagnetic storms alter the temperature and composition structure of the thermosphere?”, is directed at understanding the most variable of the two major sources of forcing from above. The second primary science question, “What is the global-scale response of the thermosphere to solar extreme-ultraviolet variability?”, is directed at understanding the influence of what is typically the primary source of forcing from above, the extreme-ultraviolet (EUV) and soft X-ray (XUV) spectral range from \(\sim1~\mbox{nm}\) to \(\sim103~\mbox{nm}\). This is the most important solar spectral region for T-I variability and the entire spectral region will be referred to as the EUV in this paper. The third primary science question, “How significant are the effects of atmospheric waves and tides propagating from below on the thermospheric temperature structure?”, is directed at understanding the influence of forcing that may come from the lower atmosphere (e.g., gravity waves) or be generated within the T-I system as a result of the forcing. While this source of forcing to the T-I system is expected to be less than solar and geomagnetic forcing (the two primary sources) at most locations, it now appears to be a significant contributor to the day-to-day differences between observations and models. The fourth of the primary science question, “How does the structure of the equatorial ionosphere influence the formation and evolution of equatorial plasma density irregularities?”, is directed at understanding how the low latitude, nighttime ionosphere responds to the preconditioning that occurs on the dayside and to geomagnetic activity.

Although there are extensive measurements of the T-I system by instruments on low-Earth-orbiting (LEO) spacecraft and on the ground, we lack the data necessary to sufficiently answer the four questions summarized above. High-cadence, global-scale measurements of both the composition and temperatures in the lower thermosphere can provide the necessary perspective and insights into the variability and interactions. LEO spacecraft can only sample a narrow swath of the T-I system during each orbit. Since it takes 12 hours or more to obtain a global picture using LEO data, such observations do not allow definitive separation of spatial and temporal variations for dynamic events, which play an important role in the behavior of the T-I system. Imagers located in highly elliptical orbits have broader spatial coverage, but they observe varying geographic regions, limiting the insights that can be obtained. Due to these inherent limitations, previous missions have been unable to provide the continuous, high cadence, simultaneous observations of temperature and composition needed to relate complex plasma-neutral interactions to the state of the T-I system.

Thermospheric composition has been extensively studied by remote sensing, primarily through measurements of the ratio of atomic oxygen to molecular nitrogen or simply O/N₂ as well as by in-situ methods. Insights into the T-I response to geomagnetic storms have come from such measurements (e.g., Prölss 2011), and simultaneous temperature observations can leverage them to provide further insights. Temperature changes directly affect the density and composition of the thermosphere, and thereby the ionosphere, which is strongly coupled to the neutral atmosphere.

GOLD will make global-scale thermospheric temperature measurement using observations of the rotational structure of \(\mathrm{N}_{2}\) LBH bands. Aksnes et al. (2006) have demonstrated the feasibility of this technique by comparing temperatures derived from data obtained with the High-resolution Ionospheric and Thermospheric Spectrograph (HITS) on the Advanced Research and Global Observation Satellite (ARGOS) to the Mass Spectrometer and Incoherent Scatter (MSIS) model. There is excellent agreement between their retrieved temperatures and the MSIS model, as shown in Fig. 2. Full end-to-end retrievals have also been demonstrated by running the GOLD retrieval algorithms with simulated data calculated using neutral densities from the NCAR TIE-GCM model.

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Modeling of the global thermosphere and ionosphere, using general circulation models such as the NCAR TIE-GCM, is mature, and includes the physical processes that control the coupled T-I system (e.g., Burns et al. 2007). Such models agree well with empirical models of the thermosphere and with the qualitative response of the T-I system. Yet, the ability of any models to match observed events is limited by uncertainty in the drivers from above and below (e.g., Fuller-Rowell et al. 2006). Comprehensive observations of T-I state parameters such as composition and temperature are therefore a key to reducing the uncertainties and advancing these models from climatological descriptions to realistic space weather simulations. The continuous, high-cadence, global-scale observations from the GOLD mission will resolve the temporal and spatial response to solar and geomagnetic inputs, which is not possible with current or historical data.

This paper will discuss the scientific background to these questions and the mission architecture. Additional details regarding the state of our knowledge about these questions and the measurements GOLD will make to address the questions as well as a full description the UV imager will be covered in forthcoming papers by S.C. Solomon and by W.E. McClintock respectively. An extensive review of previous UV observations is covered in a forthcoming review by A.G. Burns, the GOLD mission project scientist.

While the four primary science questions to be addressed by the GOLD mission are broad and UV remote sensing is only one of the measurement techniques that have been used in previous efforts to address them, the imaging capabilities of the GOLD mission allow it to address key, more specific issues about these questions from a new perspective. The primary science question as well as the more specific issues that the mission will address and the rationale for them are described below.

Geomagnetic storms drive large changes in the temperature and composition of the thermosphere. Forcing by geomagnetic storms occurs primarily in the high-latitude, auroral regions, but storm effects extend throughout the middle and low latitudes (e.g., Fig. 1). Previous studies of storms had limited ability to uniquely separate local time and longitude changes at global scales, which has made it difficult to understand the three-dimensional, time-varying response (e.g., Burns et al. 2007) of the T-I system. Observations from the ground and LEO satellites have not constrained the models sufficiently to substantially advance our current understanding. The ability of general circulation models and empirical models to describe storm-time changes is limited by an observational deficit, as they are currently based more on superposition of changing climatic states than on the full temporal and spatial analysis enabled by global-scale imaging.

Furthermore, since GOLD will image the same geographic locations continuously, the large-scale distribution of temperature and composition in the quiet-time thermosphere can be contrasted with the ∼10 or more moderate storms that typically occur every year. Maps of the response of temperature and composition, observed on the disk, will be compared to the equivalent maps for quiet times. Changes in \(\mathrm{O}_{2}\) density profiles, observed on the limb, will also be analyzed. The response of the nighttime equatorial ionosphere, which may also vary, will be quantified using nightside observations.

Quantifying the geomagnetic forcing requires the combining of high-latitude potential and conductance data from multiple sources. We will use the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure (Richmond and Kamide 1988) to combine these data. This method provides a comprehensive time-dependent description of magnetospheric forcing of the T-I system. AMIE can employ various data inputs, including space-based measurements; in its basic form it can be run using ground-based magnetometer data alone, which are certain to be available during the GOLD mission. AMIE runs will be used to provide high-latitude inputs to general circulation models.

To elucidate the physics underlying the T-I response to geomagnetic storms, three models will be employed to analyze GOLD observations: the Thermosphere-Ionosphere-Electrodynamics General Circulation model (TIE-GCM) (Richmond et al. 1992), Coupled Thermosphere Ionosphere Plasmasphere electrodynamics model (CTIPe) (Millward et al. 2001; Codrescu et al. 2008), and the semi-empirical NRLMSISE-00 model (Picone et al. 2002). Magnetospheric inputs to the T-I system will be combined with solar and tidal input parameters to drive TIE-GCM and CTIPe, in order to explore the compositional, thermal, and dynamical signatures present in GOLD images. Using different numerical models allows systemic deviations between the models to be studied and the important differences in parameterization of physical processes to be identified.

The solar-driven thermospheric circulation, with upwelling in the dayside and summer hemispheres and downwelling in the nightside and winter hemispheres, produces variations in \(\mbox{O}/\mbox{N}_{2}\) (e.g., Burns et al. 2015) that will be observed by GOLD. The global effects of solar EUV variations are described by general circulation models of the coupled T-I system. These include solar forcing of the thermospheric temperature on timescales of the solar cycle, solar-rotation and solar flares. Comparisons of observations and general circulation models using measured solar EUV spectra as inputs (e.g., Solomon and Qian 2005) will elucidate whether model descriptions of thermospheric circulation and temperature variation due to solar forcing are sufficient.

Solar EUV radiation not only drives changes in O/N₂, it also causes variations in \(\mathrm{O}_{2}\), which GOLD also can measure. Solar occultation measurements from Solar Maximum Mission (SMM; Aikin et al. 1993) and Solar Ultraviolet Spectral Irradiance Monitor (SUSIM; Lumpe et al. 2007), incorporated into NRLMSISE-00, imply a 30% decrease in \(\mathrm{O}_{2}\) at 200 km from solar minimum to maximum, in approximate agreement with numerical models that include \(\mathrm{O}_{2}\) dissociative chemistry, but at variance with earlier empirical models that predicted a factor of two increase from minimum to maximum. GOLD stellar occultation measurements will provide \(\mathrm{O}_{2}\) densities over a wide range of latitudes and conditions, to determine which is correct.

Atmospheric tides are global-scale waves whose periodicities correspond to harmonics of 24 hours. A wide spectrum of tides exists, some of which propagate upward into the thermosphere from their source regions in the lower and middle atmosphere, while others are generated in situ in the thermosphere by absorption of solar UV and EUV. Tides are a major source of winds and temperature variations in the lower thermosphere, and they play a major role in the generation of ionospheric Electric fields through the E-region dynamo. Previous studies (e.g. Lastovicka 2006; England et al. 2006) have estimated that large-scale atmospheric waves such as tides are responsible for \(\sim40\%\) of the variability in the ionosphere at low latitudes. Upward-propagating tides vary in response not only to changes in their forcing, such as by tropospheric convective activity, but also to changes in conditions of the middle atmosphere through which they propagate.

Observations by the TIMED satellite (e.g., Zhang et al. 2006; Forbes et al. 2008; Oberheide and Forbes 2008) have identified the main tides that propagate from the lower atmosphere to the base of the thermosphere and have determined their variability with season, the quasi-biennial oscillation, and even El Niño. However, observations between the altitudes of ∼110 km (e.g., from TIMED) and 400 km (e.g., from CHAMP) are far less complete. Recently, Talaat and Lieberman (2010) analyzed WINDII data to show that tides penetrate into the middle thermosphere with amplitudes large enough to be observed by GOLD. Further, temperatures observed with the ENVISAT-MIPAS at altitudes up to 150 km have been used to identify a portion of the diurnal tidal spectrum that is seen just below the altitudes where GOLD will observe (García-Comas et al. 2016). Unlike observations made from low-Earth orbit, GOLD will be able to observe the temporal and spatial changes simultaneously. GOLD will stare at locations on the ground, observing the variability of tides for days. Such extended sampling at a location is similar to that provided by ground-based observations. However, GOLD will make simultaneous observations over a large geographical region, enabling a synthesis of measurements in a manner analogous to a ground-based network, but with the advantage of identical coverage over both land and sea. The GOLD mission will address this question by examining two more specific issues:

1. (a)

   which atmospheric waves propagate from the mesopause region to the middle thermosphere and

    
2. (b)

   what is the daily and seasonal variability of waves in the middle thermosphere.

    

From model simulations and observations at lower altitudes, it is expected that only tides with the longest vertical wavelengths will reach 150–180 km altitude, producing temperature perturbations in the disk observations. Originally the NCAR TIEGCM only included semi-diurnal tides which provided a representation of lower atmosphere forcing of the thermosphere (Richmond et al. 1992); later versions use GSWM (Hagan et al. 1999) to incorporate a wider range of lower atmosphere forcing. As shown in Fig. 3, the largest amplitude tides at these altitudes are expected to produce significant variations in thermospheric temperature, 30 K or more, peak-to-valley. Given GOLD’s uncertainty of \(\sim10~\mbox{K}\) for a two-hour imaging cadence, temperature changes from the larger amplitude tides in this altitude range should be identifiable.

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Plasma density irregularities at low latitudes and the ionospheric scintillations they cause, also known as “ionospheric bubbles,” are produced in the post-sunset ionosphere by the Rayleigh-Taylor (R-T) instability. The growth of the R-T instability into large-scale ionospheric bubbles has a significant optical signature (e.g., Weber et al. 1978; Tinsley et al. 1997; Kelley et al. 2003; Kil et al. 2004; Ledvina and Makela 2005) and is the greatest source of ionospheric irregularities at low latitudes (e.g., Martinis et al. 2005, 2009; Martinis and Mendillo 2007; Krall et al. 2009).

While bubble occurrence rates are known to depend on the state of the low latitude ionosphere—which is affected by geomagnetic storms, thermospheric tides, and other processes—the individual contributions are still unclear. During geomagnetic storms, changes in the high-latitude electric potential penetrate to the equatorial region, modifying its electrodynamics and drift velocities. The vertical drift velocities control the height, intensity, and separation of the equatorial ionization anomalies. Geomagnetic disturbances also alter the neutral winds which influence the electric potential and the latitude distribution of the anomalies. Tides and planetary waves also affect the vertical drifts, resulting in F-region longitudinal structures of considerable interest (e.g., England et al. 2010). GOLD will measure the peak electron density (\(N_{\max}\)) in the equatorial ionosphere at night, from the perspective of a fixed-longitude frame, providing a true time-evolving map on a global scale. The large-scale structure in \(N_{\max}\) contains information on the intensity and distribution of vertical \(\mathrm{E}\times\mathrm{B}\) drifts and the mechanisms that affect the formation of plasma density irregularities.

Space-based optical detection of ionospheric bubbles using FUV observations has been demonstrated from LEO, most prominently by the GUVI instrument on TIMED (Kil et al. 2004, 2009). Since the GUVI measurements are from a constant local time on any given night, they contain an inherent spatial-temporal ambiguity as the bubbles form and move. GOLD will observe ionospheric bubbles from a constant geographic frame, imaging the large-scale development of individual bubbles. This observing capability provides the means to tie bubble occurrence, drift, and evolution to the global-scale ionospheric morphology and hence to ionospheric electrodynamics.

Bubble drift rates are one phenomena that GOLD is well suited to examine. During geomagnetically quiet times, ionospheric bubbles drift eastward due to the eastward motion of the nighttime F-region ionosphere (Martinis et al. 2003). These post-sunset eastward drift velocities have been measured from the ground (by imagers and radars) and from space (by the IMAGE satellite; Park et al. 2007). However, during geomagnetic storms, the bubbles have been observed to drift westward in the middle of the night (Martinis et al. 2003; Martinis et al. 2015). GOLD observations of drifting bubbles will uniquely determine the sense of the bubble drift direction as a function of longitude under all geomagnetic conditions, both quiet and disturbed. The persistence of individual plasma depletions can also be uniquely determined by GOLD nighttime observations. It is also possible that new bubbles can be created away from the post-sunset production region, possibly triggered by existing bubbles, as in the model simulations of Huba and Joyce (2010). This process, if it occurs, cannot be discerned from ground-based or LEO observations, but GOLD can observe it unambiguously. GOLD will also be able to relate bubble drift and persistence to observations of the large-scale variability driven by storms and waves discussed under Questions 1 and 3.

The mission will use proven remote sensing techniques to derive the temperature, density and composition of the Earth’s space environment. More specifically, the information obtained from the observations, in addition to the brightness of Earth’s FUV emissions, are:

The intrinsic capability of the instrument exceeds that necessary to meet the requirements summarized in Table 2. The spatial resolution capability in the longitudinal direction, is either ∼40 km or 100 km for observations of atmospheric emissions, depending on which slit (of the two included for disk observations) is used (resolution during occultations depends on the timing accuracy). In the latitudinal direction the spatial resolution is ∼50 km. During processing the photon events are binned spatially. This improves the signal to noise ratios and decreases the uncertainties to less than those listed in Table 2.

The instrument is designed to meet the requirements for observations of the sunlit disk during solar minimum (\(\mbox{F}10.7=70\)) at high, 70 degrees SZA and nadir viewing, approximately the worst-case solar illumination and viewing conditions. At the beginning of operations the Sun is expected to be near solar minimum. The instrument is also fully capable of observations at solar maximum.

GOLD measurements will be used with some of the most advanced space environment models available (e.g. TIE-GCM and CTIPe). The GOLD mission provides a continuous data set, over a large part of the T-I system, that is ideal for improving such large scale models and developing new ones.

The GOLD mission uses a pair of independent, identical channels that interface to the SES-14 spacecraft through a single processor assembly. Each channel is an independently commandable, imaging spectrograph that can measure the Earth’s airglow emissions from 132 to 162 nm. This wavelength coverage, at the spectral resolution planned for the observations, enables the use of existing remote sensing techniques for retrieving densities, composition and temperature. The baseline mission is two-years of operations with both channels performing the measurements summarized in Table 2 and with sufficient data completeness to determine the seasonal variability.

Figure 6 shows the optical layout for one of the two identical channels and Table 3 summarizes the resource requirements for the total instrument and describes some of the important channel characteristics. In each channel a plane scan mirror is located in front of a spherical telescope mirror with a 150-mm focal length, which images the Earth onto a spectrograph entrance slit. A concave toroidal mirror redirects the light passing through the slit before it impinges on a concave toroidal grating. The grating disperses the input beam and forms a spectral × spatial image of the slit on an ‘open faced’ microchannel plate (MCP) detector equipped with an opaque CsI photocathode and a cross-delay line (XDL) anode (Siegmund et al. 2004). A 3-position mechanism inserts a 0.2 mm, 0.4 mm, or 2.6 mm wide entrance slit into the telescope focal plane.

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Table 3

GOLD instrument summary

Resources (total)
Mass	37 kg
Power	72 W (average)
Data Rate	6 Mbits/second
Telescope (each)
Focal length (mm)	150
Entrance pupil size (mm)	30 × 30
Scan Mirror (each)
Angular range	±100°
Spectrograph (each)
Wavelength Coverage (nm)	132–162
3-Position Slits (each)
Slit widths (mm)	0.2, 0.4, 2.6
Spectral resolution (nm)	0.20, 0.35, 2.2
Field of View (degrees)	(0.05, 0.15, 0.95) × 10.0
Detectors (each)
Detector size (mm)	27 × 32
Pixel format	1600 × 1800

GOLD’s field-of-regard (FOR) is 31.0° E-W and 19.75° N-S. This FOR enables each channel to image the disk over ∼160°× ∼160° (latitude × longitude) as well as perform limb scans and occultations to altitudes of 600 km (at the equator). This FOR also accommodates a requirement that the spacecraft’s earth-pointing axis may be adjusted by up to ±5° E-W and ±1° N-S in order to optimize its communications payload.

Spectroscopic observations by each channel of the GOLD instrument will be in one of three modes—high spectral resolution, HR (0.2 mm slit); low spectral resolution, LR (0.4 mm slit); or occultation, OCC (2.6 mm slit). HR mode provides the spatial resolution necessary for limb altitude profiles; disk images can be made in either HR or LR mode, depending on the desired tradeoff between spectral resolution and signal-to-noise; and stellar occultations will be performed in OCC mode. There is also a “Solar Safe” mode, which will be used when the Sun is within 30° of the instrument field of regard (FOR), (approximately \(\pm3\) hours around local midnight). An diagram of the nominal observations performed during a day is shown in Fig. 7.

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As depicted in Fig. 8, the scan mirror in each channel sweeps its field-of-view (which depends on the spectrograph entrance slit used) across the limb and disk, building up spatial-spectral image cubes beginning at 0300 hours and ending at 2100 hours Satellite Local Time (SLT). At each mirror position, the detectors record the x-y locations of individual photons as they arrive at the spectrograph focal plane. On the ground these locations are binned into \(800 \times 600\) spectral-spatial arrays associated with each mirror position, providing 0.02° along-the-slit spatial sampling and ∼0.04 nm spectral sampling. Further summing and binning occurs during ground processing to produce spectral-spatial image cubes summarized in Table 2.

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Science and engineering data from the instrument are routed to a dedicated transmitter for immediate downlink to an SES ground station, from which they are transmitted to the LASP Science Operations Center (SOC). There the engineering data are displayed in real time to monitor instrument health and safety. Level 1A science data are produced and transmitted to the GOLD Science Data Center (SDC) at UCF. In addition, a subset of GOLD critical engineering data is routed to the spacecraft over a 1553 interface and downlinked in real time as part of the spacecraft housekeeping data stream to the spacecraft Mission Operations Center (MOC), also referred to as the Satellite Control Center (SCC), for monitoring of the instrument by SES. Commands to the instrument are transmitted over the same 1553 interface after being uplinked from the MOC

Dayside disk imaging and dayside limb scans are performed by sweeping the instantaneous field-of-view of each spectrometer across the sunlit disk in two swaths. One swath covers the northern latitudes and the other covers the southern (Panel 3a and 3b of Fig. 8). Scan rate and range of travel vary with SLT to ensure that the desired areas are observed and that each swath is completed in the ∼12 minutes available (24 minutes to image the sun-lit part of the disk) aside from the limb observations. The maximum rate of 0.025° per second (ground speed of 15.6 km per second at the sub spacecraft point) occurs at noon SLT.

In addition to the disk observation performed on the dayside, GOLD also makes either dayside limb scans or a stellar occultation each half hour. As shown in Fig. 8, dayside limb scans are made in both the northern and southern hemispheres (Panels 4a and 4b). The scans begin on the disk at a tangent altitude of \(-50~\mbox{km}\) at the equator and scan to ∼430 km with a step size of 12 km. Approximately 6 minutes is required to complete the scans. The order of scanning (e.g., limb-disk) and the scan direction (either east-west or west-east) for the disk and limb are programmable, as is the scan rate. Background contributions from dark counts and radiation are also measured each half hour by turning the scan mirrors to block photons for 30 seconds. The entire sequence—limb, disk and dark measurements—is timed to complete in 30 minutes.

Exceptions to the limb scan sequence occur when stars selected for occultation measurements pass through either limb (Panel 1) during a 30-minute observing period. In those cases the limb scan is omitted and the disk scan is interrupted to slew the FOVs to the star and the slit mechanism inserts the 1° wide slit to perform the occultation (OCC mode). Occultations (Panel 2 of Fig. 8) require \(\sim7\mbox{--}8\) minutes to execute. The occultation slit is wide enough to allow the scan mirror (and slit image) to be held stationary for the entire occultation. Once the occultation is complete, the scan mirror returns to its point of departure from the imaging observation, and the disk scan is resumed (Panel 3a of Fig. 8). Since limb scans will be skipped when an occultation is performed during a 30-minute observing period, the impact on the time devoted to disk imaging is insignificant. Approximately 10 occultations per day are anticipated.

At 1700 SLT, one channel continues dayside observations while the other begins nightside observations (Panels 5a and 5b of Fig. 8). Beginning at 2000 SLT the second channel is also dedicated to scanning the night side, increasing cadence by a factor of 2. The nightside scans begin ∼15° in longitude behind the terminator and extend up to 75° eastward. During the last hour of observations (2000–2100 SLT) all limb observations will be omitted in order to improve the SNR of the disk images.

All the observations are spectrally resolved data at wavelengths of 132–162 nm. A model calculation of the daytime airglow spectrum at these wavelengths is shown in Fig. 9. The spectrum contains the emissions, from atomic oxygen at 135.6 nm and the \(\mathrm{N}_{2}\) Lyman-Birge-Hopfield bands, used by the GOLD mission for remote sensing of the T-I system. A spectrum of this wavelength region will be assembled for each pixel on the dayside.

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At night the 135.6 nm atomic oxygen emission, which persists due to production by recombination of the ionosphere, will be used to derive information about the nighttime ionosphere.

As discussed in the previous section, GOLD assembles spatial-spectral image cubes by sweeping the field-of-view of each channel across the Earth. A depiction of one instantaneous slit position is show on the left side of Fig. 10 as a white rectangle superimposed on the simulated image of oxygen 135.6 nm emission. A simulation of the Earth’s spatially resolved spectrum along the slit, which contains both 135.6 nm emission and LBH 132–162 nm emission, is shown on the right side of Fig. 10. Since the slit is projected onto the Earth near sunset (the terminator is diagonal across the earth) the spectrum shows significant spatial variation. At the lower latitudes, the dayglow emissions from atomic oxygen and molecular nitrogen are both seen. At slightly higher latitudes, where the slit images the night side, only the atomic oxygen 135.6 nm emission is seen. Near the top, the slit crosses the auroral region, which produces the brightest emissions observed in the spectrum.

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GOLD will also measure the limb brightness profile at the equator on the dayside every 30 minutes, unless an occultation is scheduled during the period. The shape of the \(\mathrm{N}_{2}\) limb emission profile varies with exospheric temperature and will be used to derive the temperature. Since the tangent points observed on the limb are fixed in longitude, the exospheric temperature changes at the equator can be tracked throughout the day and from day to day. From the planned orbit of the SES-14 satellite, geostationary at 47.5°W longitude, the tangent points at the equator will be located at approximately 129° West and 34° East longitude. A model simulation of the \(\mathrm{N}_{2}\) limb emission profile data from GOLD is shown in Fig. 11, as well as a fit to the simulated data.

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While \(\mathrm{O}_{2}\), one of the three most abundant constituents of the Earth’s thermosphere, does not have emissions at the wavelengths observed by the GOLD imager, it does absorb photons at those wavelengths. Consequently, occultations of ultraviolet stars rising or setting through the Earth’s limb can be used to determine the \(\mathrm{O}_{2}\) densities. Shown is Fig. 12 are some simulated retrievals of \(\mathrm{O}_{2}\) densities from occultations by the GOLD imager. The signal to noise and uncertainty in the retrieved densities depends on the brightness of the star being observed, as demonstrated by the three signal to noise levels (low, medium and high) shown.

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The data products will be produced at the GOLD Science Data Center (SDC) at the University of Central Florida, with the exception of L1A data which LASP will produce. The GOLD SDC will also provide public access to the science data products as will NASA’s SPDF, the NASA designated archive for data from the mission.

The Global-scale Observations of the Limb and Disk (GOLD) mission will provide a new perspective on the global-scale response of the thermosphere-ionosphere (T-I) system to the forcing from above and below. As articulated in the most recent NRC Decadal Strategy for Solar and Space Physics, understanding the global-scale response of the thermosphere-ionosphere (T-I) system to these drivers is essential to advancing our physical understanding of coupling between the space environment and the Earth’s atmosphere. Data from the GOLD imager, at geostationary orbit, enable the mission to follow the response of the T-I system as plasma and fluid processes compete to control its temperature, composition, and structure. The data can provide simultaneous images of two critical variables—thermospheric temperature and composition, near 160 km—on the dayside disk. At night, the mission will follow changes in the same geographic regions that were observed earlier in the day, enhancing our understanding of the low latitude ionosphere’s evolution. Previous missions have successfully determined how the “climate” of the T-I system responds. The GOLD mission will take a long awaited step to understanding the coupling between the space environment and the Earth’s atmosphere.