DRAFT. The Global-scale Observations of the Limb and Disk (GOLD) Mission. Richard Eastes and the GOLD Team

The Global-scale Observations of the Limb and Disk (GOLD) Mission Richard Eastes and the GOLD Team Abstract The Earth s thermosphere and ionosphere constitute a dynamic system that varies daily in response to energy inputs from above and from below and can exhibit significant response at hour time scale to changes in those inputs, as plasma and fluid processes compete to control its temperature, composition, and structure. Within this system, short wavelength Solar radiation and charged particles from the magnetosphere deposit energy, and waves propagating from the lower atmosphere dissipate. Understanding the global-scale response of the thermosphereionosphere (T-I) system to these drivers is essential to advancing our physical understanding of coupling between the space environment and the Earth s atmosphere. Previous missions have successfully determined how the climate of the T-I system responds. The Global-scale Observations of the Limb and Disk (GOLD) mission will determine how the weather of the T- I responds, taking the next step in understanding the coupling between the space environment and the Earth s atmosphere. Operating in geostationary orbit, the GOLD imaging spectrograph will measure the Earth s emissions from 132 to 162 nm. These measurements will be used to image two critical variables thermospheric temperature and composition near 160 km on the dayside disk at half-hour time scales. At night they will be used to image the evolution of the low latitude ionosphere in the same regions that were observed throughout the preceding day. These measurements will allow the unambiguous separation of spatial and temporal variability. 1. Introduction The GOLD mission is focused on four primary scientific questions regarding the response of the Earth s thermosphere and ionosphere to the forcing from above and below. Understanding this forcing is one of the most prominent problems for research in the Earth s space environment. Consequently, one of the goals in the NRC 2012 Decadal Strategy for Solar and Space Physics is determine the dynamics and coupling of Earth s magnetosphere, ionosphere, and atmosphere and their response to solar and terrestrial inputs. GOLD responds to the strategic priorities of the Decadal Survey (and the scientific community) by addressing the top three science goals identified for the atmosphere-ionosphere-magnetosphere: Determine how the ionospherethermosphere system regulates the flow of solar energy through geospace ; understand how tropospheric weather influences space weather ; and understand the plasma-neutral coupling processes that give rise to local, regional, and global-scale structures and dynamics. Each of the four primary questions are addressed through the examination of more focused questions and with benefit of high cadence, global-scale images observed by a ultraviolet (UV) imager at geostationary orbit. This orbit allows repeated images, covering most of the hemisphere over the Americas, of the same geographic locations throughout a day, as portrayed in Figure 1.1. Previous missions, e.g. TIMED, have demonstrated the value of imaging the thermospheric composition (O/N 2 ) on the dayside, and peak ionospheric densities on the nightside, leading to major advances in understanding the behavior of the T-I system. GOLD will add 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. Gold Mission Description, working draft, August 2016 1

Figure 1.1: Modeled thermospheric temperature (K) progression at one-hour intervals, during a geomagnetic storm, near 160 km altitude, as sampled by GOLD from geostationary orbit. 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 ~1 nm to ~103 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 the best available 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 preconditioning that occurs on the dayside and the geomagnetic activity. Although there are extensive measurements of the T-I system by instruments on low-earthorbiting (LEO) spacecraft and by ground-based observatories, we have yet to understand T-I interactions and variability sufficient to answer the four questions summarized above. A lack of truly high-cadence, global measurements of both the composition and temperatures in the lower thermosphere will provide a new perspective and new insights into the variability and interactions. LEO spacecraft can only sample a narrow swath of the T-I system during each orbit. This coverage is inadequate, because T-I dynamic timescales are shorter than the twelve hours necessary to develop a global picture. In addition, LEO observations do not allow definitive separation of spatial and temporal variations. Imagers located in highly elliptical orbits have broader spatial coverage, but because they observe varying geographic regions, such measurements have not provided unambiguous global results. 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. Gold Mission Description, working draft, August 2016 2

Thermospheric composition has been extensively studied by remote sensing and in-situ methods, primarily through measurements of the ratio of atomic oxygen to molecular nitrogen, or simply O/N 2. Insights into the T-I response to geomagnetic storms have come from these measurements (e.g., Prölss, 2011), but simultaneous temperature observations are needed to provide crucial additional insight, especially concerning the interaction of storms with atmospheric tides. 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 thermospheric temperature measurement using observations of the rotational structure of N 2 LBH bands. Aksnes et al. (2006) have demonstrated the feasibility of this technique Figure 1.2: Comparison of temperatures measured by ARGOS HITS to the MSIS model (dashes). 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. The excellent agreement between their retrieved temperatures and the MSIS model, which is shown in Figure 1.2, validates the use of remote sensing for temperature measurement. Full end-to-end retrievals have also been demonstrated by running the GOLD retrieval algorithms through the NCAR TIE-GCM. Modeling of the global thermosphere and ionosphere, using general circulation models, is mature, and includes many 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 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 are therefore essential for providing the data necessary for 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 a detailed discussion of the measurements GOLD will make to address the questions as well as a full description the UV imager will be covered in detail by 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. Gold Mission Description, working draft, August 2016 3

2. Science Objectives DRAFT While the numerous types of measurements may be used to address the four primary science questions, there are related, more specific questions that data from the GOLD imager has been designed to address. These more specific questions and the rational for them are described below. Question 1: How do geomagnetic storms alter the temperature and composition structure of the thermosphere? 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., Figure 1.1). 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, timevarying response (e.g., Burns et al., 2007) of the T-I. Combined ground-based observations or LEO satellite observations alone have not provided the observations needed to understand this behavior. The ability of general circulation models and empirical models to describe storm-time changes is limited by this 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. GOLD will provide, for the first time, a large-scale depiction of storm driven changes in both temperature and neutral composition. This will enable the GOLD team to study geomagnetic storm effects by examining three more specific issues: a) whether the initial temperature and composition distribution affect the response to storms, b) how the vertical structure of O 2 changes during storms, and c) whether the T-I recovery from storms controlled by temperature or composition. Every geomagnetic storm is unique, since the interplanetary magnetic field, solar wind, and magnetospheric drivers are diverse. Not only does the magnetospheric convection and auroral precipitation vary from one storm to another, but the preconditioned state of the T-I system prior to storm onset may also contribute to its progression. The factors that control this may include solar activity, the time of year, the time of onset (which determines the position of the terrestrial magnetic field and auroral oval relative to the Sun), previous geomagnetic disturbances, flares or other changes in solar EUV, and lower atmosphere influences from waves and tides. Yet, in the T-I system, there are many common features (e.g., Wang et al., 2011), due to the general pattern of high-latitude heating, composition change driven by vertical motions, meridional winds that carry these changes to lower latitude, and alteration of the global ionosphere electric potential. Furthermore, since GOLD will image the same geographic locations continuously, the largescale 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 O 2 density profiles, observed on the limb, will also be analyzed. The response of the nighttime equatorial ionosphere is discussed in Section 2.4, below. Quantifying the geomagnetic forcing requires the combining of high-latitude potential and conductance data from multiple sources. We will use the 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, Gold Mission Description, working draft, August 2016 4

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: TIE-GCM, CTIPe, 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 inputs 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. Question 2: What is the global-scale response of the thermosphere to solar extremeultraviolet variability? Solar radiation in the ultraviolet and X-ray spectrum creates the ionosphere and is the principal driver of variation in thermospheric climate. The most important spectral region for T-I variability is the extreme-ultraviolet (EUV) and soft X-ray (XUV) spectral range from ~1 nm to ~103 nm, which we will collectively refer to as EUV. Comprehensive thermospheric measurements at global scale will enable the separation of spatial and temporal effects from this radiation, especially during solar flares, will be addressed by examining two more specific issues: a) the response of thermospheric temperature and composition to solar EUV forcing and b) whether the thermospheric response to flares is commensurate with measured solar emissions. There are still significant controversies concerning the consistency of solar measurements, thermospheric measurements, and the models that combine them. Measurements by the TIMED satellite Solar Extreme-ultraviolet Experiment (SEE) and Global Ultraviolet Imager (GUVI) have made important advances (e.g. Strickland et al., 2004a; DeMajistre et al., 2005; Meier et al., 2005; Qian et al., 2009). These issues are discussed by Woods et al. (2005) and motivate the next generation of spectrally resolved, high-cadence observations by the SDO Extreme-ultraviolet Variability Experiment (EVE) (Woods et al., 2009), as well as the NOAA GOES EUV and XRS monitors (Evans et al., 2010). EVE data, if it is available, will enhance the GOLD effort but is not essential because sufficient solar information could be derived from the NOAA GOES monitors, currently available solar proxy indices, and the solar models based on SEE and EVE data. A particularly valuable proxy index is the estimate of the ionizing radiation from 1 to 45 nm, Q EUV, that will be obtained from the intensity of GOLD dayglow observations themselves, using a procedure developed and proven using GUVI data (cf., Strickland et al., 2004b, 2007). The solar-driven thermospheric circulation, with upwelling in the dayside and summer hemispheres and downwelling in the nightside and winter hemispheres, produces variations in O/N 2 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. This includes solar forcing of the thermospheric temperature on timescales of the solar cycle, solar-rotation and solar flares. Using measured solar EUV spectra as inputs to general circulation models (e.g., Solomon and Qian, 2005) for comparison with observations will elucidate whether model descriptions of thermospheric circulation and temperature variation due to solar forcing are sufficient. Also, the variability of thermospheric O 2, and the relative importance of diffusion and chemistry, are not well known. Solar occultation measurements from SMM and SUSIM, incorporated into NRLMSISE-00, imply a 30% decrease in O 2 at 200 km from solar minimum to Gold Mission Description, working draft, August 2016 5

maximum, in approximate agreement with numerical models that include 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 O 2 densities over a wide range of latitudes and conditions, to determine which is correct. Question 3: How significant are the effects of atmospheric waves and tides propagating from below on the thermospheric temperature structure? Atmospheric tides are global-scale waves whose periodicities correspond to harmonics of 24 hours. A wide spectrum of tides exist, 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 E-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 ~40% 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, associated with phenomena like planetary waves and stratospheric warmings. 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 determined their variability with season, the quasi-biennial oscillation, and even El Niño. However, observations at higher altitude are far less complete. There are few global-scale observations between the altitudes of ~110 km (e.g., from TIMED) and 400 km (e.g., from CHAMP). Recently, Talaat and Figure 2.1: Simulated temperature difference map calculated by the TIE-GCM, with and without tidal forcing at the lower boundary. Line-of-sight temperatures from geostationary orbit correspond to ~160 km altitude. Lieberman (2010) analyzed WINDII data to show that tides do penetrate into the middle thermosphere with amplitudes large enough to be observed by GOLD. Unlike observations made from low-earth orbit, GOLD will be able to observe the temporal and spatial changes simultaneously, and thus address this question by examining two more specific issues: a) which atmospheric waves propagate from the mesopause region to the middle thermosphere and 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 disk observations. The semi-diurnal migrating tide is probably the most prominent. As shown in Figure 2.1, the largest amplitude tides at these altitudes should produce significant variations in thermospheric temperature (30 K or more, peak-to-valley). Given GOLD s sensitivity of ~10 K for individual images and that the tides persist for days, allowing Gold Mission Description, working draft, August 2016 6

the uncertainties to be decreased further by averaging, temperature changes from the larger amplitude tides in this altitude range should be readily identifiable during periods of low geomagnetic activity. The geostationary observing platform offers a fundamentally new perspective on tides. Unlike observations from LEO, GOLD will stare at locations on the ground, observing the variability of tides on the order of several 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 complete coverage over both land and sea. Observations of temperature from a GEO platform present challenges for the analysis of tides that are different from those facing LEO missions, due to limitations in local time and longitude coverage. Similar limitations have been addressed by ground-based studies (e.g. Zhou et al., 1997). The observational parameters make GOLD preferentially sensitive to waves with vertical wavelengths larger than ~50 km, and horizontal wavelengths less than half of the globe. The dominant semi-diurnal (shown in Figure 2.1) and ter-diurnal tides have these characteristics, consequently GOLD will be sensitive to them and other important tides, such as the DE2 and DE3 non-migrating tides. Question 4: How does the nighttime equatorial ionosphere influence the formation and evolution of equatorial plasma density irregularities? 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, 2007, 2009; Krall et al., 2009). Bubble occurrence rates depend on the state of the low latitude ionosphere, which is affected by geomagnetic storms, tides, and other processes. During geomagnetic storms, changes in the high-latitude electric potential penetrate to the equatorial region, modifying its electrodynamics and drift velocities. The vertical drifts control the height, intensity, and separation of the equatorial ionization anomalies. Geomagnetic disturbances also alter the neutral wind system, influencing the electric potential and the latitude distribution of the anomalies. Tides and planetary waves additionally affect the vertical drifts, and the effects of this are transmitted to the F-region, resulting in the longitudinal structures of considerable topical interest (e.g., England et al., 2010). GOLD will measure the peak electron density (N max ) in the equatorial ionosphere at night, from the unique perspective of a fixed-longitude frame, providing a true time-evolving map on a global scale. The large-scale structure contains information on the intensity and distribution of vertical E B drifts, providing a connection to 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 recently by the GUVI instrument on TIMED (Kil et al., 2004, 2009). These measurements are from a constant local time on any given night, and hence contain an inherent spatial-temporal ambiguity as the bubbles form and move. GOLD will observe ionospheric bubbles from a geostationary frame, imaging the large-scale development of multiple 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. Gold Mission Description, working draft, August 2016 7

This question will be addressed by using nighttime observations of the equatorial ionosphere to examine two more specific issues: a) What factors in the configuration of the equatorial ionosphere influence the formation of ionospheric bubbles and b) What is the persistence of ionospheric bubbles as they evolve and drift? The formation of ionospheric bubbles has significant dependence on solar activity, geomagnetic activity, season, and longitude sector (e.g., Comberiate and Paxton, 2010), and may depend on lower atmospheric conditions, including gravity wave activity, as well. The effect of geomagnetic activity is particularly complex, but the longitude where post-sunset bubbles are formed can be specifically related to the timing of a storm (Aarons, 1991; Martinis et al., 2005; Basu et al., 2007). However, the longitude extent of bubble formation and their progression in time cannot be determined uniquely from ground-based or LEO observations alone. GOLD images will create comprehensive nightly maps of bubble formation over ~100 of longitude during the period that bubbles form (~2000-2200 LT), enabling comprehensive comparisons of bubble formation rate to the state of the T-I and the equatorial ionosphere. During geomagnetically quiet times, ionospheric bubbles drift eastward after they are formed due to the eastward motion of the nighttime F-region ionosphere. These post-sunset eastward drift velocities have been measured by ground-based imagers and radars, and from high orbit 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). 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. 3. Mission Design A goal of the mission is to observe the T-I as a weather system, rather than as a series of climate states. Global-scale imaging at a high cadence and spatial resolution from geostationary orbit enables the GOLD mission to do this. Observing from geostationary orbit allows the changes in the Earth's lower thermosphere to be tracked on a global scale and to distinguish spatial from spatial changes. In contrast, previous in situ measurements and remote sensing observations from LEO covered more localized regions of space. Solar and geomagnetic forcing have global-scale effects, as do atmospheric tides and wave, on the T-I system and observing the effects at their natural scales - with sufficient precision, spatial resolution, and temporal cadence will provide a perspective needed for significantly advancing understanding of the system. The capabilities provided by the GOLD mission are essential for testing and improving models of the T-I region and the near-earth space environment. NASA missions of opportunity have typically been flown on satellites dedicated to science missions and the host mission is a critical part of such missions However, to obtain the perspective desired for the GOLD mission a host in geostationary orbit was needed. While there are limited opportunities for launch into geostationary orbit aboard missions dedicated to Gold Mission Description, working draft, August 2016 8

DRAFT science, there are numerous communications satellites in such orbits. The GOLD mission was proposed with a commercial partner, SES-GS, who is providing hosting on the SES-14 satellite (shown at the top of Figure 3.1), beginning in 2017. Two other essential components of the mission, the instrument (middle image in Figure 3.1) and science investigation are led by LASP/CU and FSI/UCF respectively. Figure 3.1. The GOLD Mission: (top) Artist s rendering of the SES-14 satellite. (middle) Drawing of the GOLD imager built by LASP/CU. (bottom) Simulated images of Earth s emission brightness at 135.6 nm, primarily from atomic oxygen, as observed from a geostationary orbit at 47.5 degrees west longitude where the GOLD imager will be stationed. The simulations shown are computed using the Global Airglow model with neutral and ion densities from the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model. These images of the Earth are calculated for 21 December of an arbitrary year at solar maximum, but with low geomagnetic activity. In this image, for 22:30 universal time, emissions from the dayside, nightside and the aurora (northern hemisphere) are visible. Brightness variations across the dayside range from ~10 kr (red) to ~100 R (green). The blue areas shown on the nightside of the Earth correspond to 10 to 100 R. Gold Mission Description, working draft, August 2016 9

3.1 Host mission SES Government Solutions (SES-GS) will fly the GOLD imager as a hosted payload on the SES-14 satellite, which is being launched by their parent company, SES, an industry leader with decades of experience launching satellites into geostationary orbit and operating them. Launch of SES-14 is scheduled for late 2017. SES-GS has successfully hosted optical instruments - e.g., the Commercially Hosted Infrared Payload (CHIRP) for the Department of Defense - on their geostationary, communications satellites. SES and SES-GS will control the satellite, uplink instrument commands generated by the GOLD team, and transfer downlinked data to GOLD facilities for processing and distribution to the science community and other users. For decades, SES has produced reliable satellites that typically operate on-orbit for 15 years or more. The SES-14 spacecraft, a Eurostar 3000e, is being built by Airbus DS. The spacecraft is scheduled for launch on a SpaceX Falcon9 in late 2017. After launch, the spacecraft will use electric propulsion for transfer to geostationary orbit at 47.5 W longitude, and for station-keeping. Once the satellite is on station, commands will be uplinked through the Mission Operations Center (MOC) through an SES ground station in Brazil which will also monitor the satellite and the GOLD instrument. More extensive monitoring of the data from the instrument will be conducted by the SOC at LASP/CU which will also generate the commands. GOLD data are not stored aboard the satellite but are routed to a transponder to be immediately downlinked to an SES ground station in VA. Consequently, the typical latencies for the lowest level GOLD data products is expected to be less than one hour. Processed data will be available to the science community through the GOLD Science Data Center at UCF. The data will also be available to drive and improve space weather nowcasting and forecasting capabilities. Figure 3.2. Drawing of the GOLD imager Gold Mission Description, working draft, August 2016 10

3.2 GOLD imager The Far Ultraviolet imager that the GOLD mission will fly is built at LASP/CU. It has two identical and independent channels, each capable of performing every required measurement and is shown in Figure 3.2. Each channel has a plane scan mirror - two sided, slightly tilted such that one side scans northern latitudes and the other southern - enabling each to scan the entire disk of the Earth by rotating the mirror through ~190 degrees, and each has three selectable entrance slits providing 0.166, 0.332 or 2.160 nm resolution - chosen to match the desired observational capabilities. Each channel contains an imaging detector, and the communications bandwidth available enables the position and pulse height of individual photon detection events to be downlinked as they are collected, through a dedicated communications link from the satellite to the ground station. The data are stored at the ground station until transmission to LASP is confirmed. Binning and averaging are performed only during the processing at UCF, which maximizes the analysis flexibility. Additional details for the instrument are provided later in this paper. 3.3 Science investigation and data processing The University of Central Florida (UCF) is the lead institution for the GOLD mission and is responsible for the science investigation. The GOLD Science Data Center (SDC) will be located at University of Central Florida (UCF) the lead institution for the GOLD mission. While the Science Operations Center (SOC) at LASP will perform checks on the Level 0 data and generate Level 1A data, the SDC produces the remaining data products, L1B and higher, and analysis. The SDC is also capable of generating Level 1A data if needed. The standard science data products that will be produced by GOLD are all Level 2. Products range from empirical (emission brightnesses) to ones requiring substantial modeling or fitting of data, and they include both disk and limb products. The disk products can be produced pixel by pixel, while the limb products can be produced profile by profile. The Level 2 products are: (1) Daytime O/N 2 ratios (Disk); (2) Nighttime peak electron density (Disk); (3) Daytime neutral temperatures (Disk); (4) Q EUV (disk); (5) Neutral (exospheric) temperature (Limb); (6) O 2 density profiles (Limb); and (7) O and N 2 emission brightnesses. These data products are summarized in Table 3.1. Distribution and local archiving of the GOLD data is also provided by the SDC which will supply data to the Space Physics Data Facility (SPDF), the NASA designated archive. Table 3.1. GOLD Science Data Levels Level Brief Description 1A time tagged data 1B binned and mapped in GOLD coordinates 1C calibrated and geolocated data 1D quick-look plots 2 O/N 2, N max, Q EUV, T(disk), T(limb), O 2 density profiles, emission brightnesses 3 multiple orbits and images of all parameters 4 higher level analysis and images 4. Observations/Measurements by the GOLD Mission The mission will use existing remote sensing techniques for the Far Ultraviolet (FUV) in order to determine the temperature, density and composition of the Earth s space environment. The information obtained from the observed emissions, in addition to their brightness, are: Gold Mission Description, working draft, August 2016 11

On the disk: Neutral temperature, during the day, from N 2 LBH band rotational structure. O/N 2 column density ratios, during the day, from atomic oxygen 135.6 nm and N 2 LBH emission intensities. Peak electron density, during the night, from radiative recombination of O + resulting in the atomic oxygen emission at 135.6 nm. Solar proxy (Qeuv), during the day, from the brightness of the FUV emissions. On the limb: Exospheric neutral temperature, during the day, from N 2 LBH emission profiles. O 2 density profiles, during day and night, from stellar occultation. The uncertainties, spatial resolution, temporal coverage and time coverage which these measurements are required to satisfy in order for the mission to meet all the objectives are summarized in the Level 1 science requirements for the GOLD mission, which are listed Table 4.2. Table 4.2. Baseline (Level 1) Science Requirements 1. GOLD shall make disk images of atomic oxygen (O) 135.6 nm emissions and molecular nitrogen (N2) Lyman-Birge-Hopfield (LBH) emissions over a latitude range of ±60 and a longitude range of ±70 relative to spacecraft nadir. 2. GOLD shall construct, on the sunlit portion of the disk, images of: a. lower thermosphere temperature with a precision of 55 K with 60 minute cadence and spatial resolution of 250 km 250 km (at nadir); and b. thermosphere column composition (O/N2 radiance ratio) with a precision of 10% with 30 minute cadence and spatial resolution of 250 km 250 km (at nadir). 3. GOLD shall construct, on the nighttime portion of the disk, images of Nmax F2, at the peak of the equatorial arcs, with a precision of 10% and a latitude resolution of 2. 4. GOLD shall track ionospheric bubbles (depletions) within a single equatorial arc with a precision of 20% in brightness and a spatial resolution (at nadir) of 100 km in the longitudinal direction. 5. GOLD shall measure near-equatorial limb profiles of the N2 LBH emissions up to an altitude of approximately 350 km. 6. GOLD shall measure exospheric temperature (near-equatorial) with a precision of 40 K in the daytime. 7. GOLD shall measure O2 line-of-sight column densities at an altitude of 160 km with a precision of 10% and a vertical resolution of 10 km in the nighttime and daytime by stellar occultation. 8. GOLD shall perform all of the above from geostationary orbit for two years. The intrinsic capability of the instrument exceeds that necessary to meet the requirements summarized in Table 4.2. The spatial resolution capability in the longitudinal direction, is either ~40 km or 100 km for observations of atmospheric emissions, depending on which slit width (of the two included for disk observations) is used (resolution during occultations depends on 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 in the derived parameters to less than is listed in Table 4.2. Gold Mission Description, working draft, August 2016 12

The instrument is designed to meet the requirements during worst-case solar illumination and viewing conditions: solar minimum (F10.7=70) at high, 70 degrees SZA and nadir viewing. Since it is not known what the range of solar activity will be during the mission, the scientific analysis strategy is designed to exploit any phase of the solar cycle and level of geomagnetic activity. Highly-disturbed conditions will favor storm studies; quiet conditions will facilitate analysis of waves; and solar variation and flares will be used to address the response to solar inputs. GOLD measurements will be used with some of the most advanced space environment models available (e.g. The National Center for Atmospheric Research Thermosphere- Ionosphere-Electrodynamics General Circulation model, TIE-GCM, and the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics model, CTIPe). High-latitude geomagnetic inputs will be determined using the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) method. The GOLD mission provides a continuous data set, over a large part of the T-I system, that can be used to evaluate and improve such large scale models and to develop new ones. 5. The GOLD Imager 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 which can measure the Earth s airglow emissions from 132 to 162 nm. This wavelength coverage enables the use of existing techniques (which rely on O 135.6 nm and N 2 LBH <160 nm) to determine O/N 2 and to isolate other emissions (e.g., atomic nitrogen 149.3 nm). The baseline mission is two-years of operations with both channels performing measurements needed to produce the Level 2 data products described in Table 3.1 at the cadence and measurement performance summarized in Table 4.2 with sufficient data completeness to determine the seasonal variability. Figure 5.1 shows an instrument Grating Telescope layout drawing and Table 5.1 summarizes its characteristics. A Detector plane scan mirror is located in front of a spherical telescope mirror with a 150-mm focal length, which images Light Shade the Earth onto a spectrograph entrance slit. A concave toroidal Scan Mirror mirror redirects the light passing Slit Mechanism through the slit before it impinges on a concave toroidal grating. The Imaging Mirror grating disperses the input beam and forms a spectral spatial image of Figure 5.1: Instrument Layout (one of two channels) the slit on the input face of 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, which varies with observing mode, into the telescope focal plane. Gold Mission Description, working draft, August 2016 13

Table 5.1. GOLD Instrument Summary Resources Mass 37 kg Power 72 W (average) Data Rate 6 Mbits/second Telescope Focal length (mm) 150 Entrance pupil size (mm) 25 x 25 Scan Mirror Angular range ±100 Spectrograph Wavelength Coverage (nm) 132-162 3-Position Slits Slit widths (mm) 0.2, 0.4, 2.6 Dl atmosphere (nm) 0.17,0.33, 2.2 Field of View (degrees) (0.xx, 0.yy,0.95) x 10.0 Detectors Detector size (mm) 25 x 25 Pixel format 1024 x 1024 GOLD s field-of-regard is 31.0 E-W and 19.75 N-S. This FOR enables disk mapping over 160 160 (latitude longitude) as well as limb scans and occultations to altitudes of 600 km (at the equator). It also includes a margin for errors in spacecraft pointing (up to ±0.2 in pitch and roll and up to ±0.5 in yaw) and instrument-spacecraft alignment (up to ±0.05 ). Additionally, the FOR accommodates a requirement that the spacecraft s earth-pointing axis may be adjusted by up to ±5 E-W and ±1 N-S (one time, during commissioning) in order to optimize its communications payload. GOLD s pointing knowledge accuracy (0.04, 0.05, and 0.2 in pitch, roll, and yaw) and stability (±0.007 ) requirements were driven by the requirement for 50 km disk image registration and less than 10 km jitter and smear combined during the limb scans. Since the design details on which knowledge of GOLD s pointing depends had to be fixed significantly before the host spacecraft was known, the design had to accommodate the pointing capabilities of multiple candidate spacecraft. 6. Science Observing Profile & Instrument Operations When making observations, each channel can use any of the three slits (identical in each channel). We associate a named mode with each slit width: low-spectral-resolution (GOLD-LR, 0.15 wide slit), high-spectral-resolution (GOLD-HR, 0.05 wide slit), and stellar occultation (GOLD-OCC, 1 wide slit). HR mode will be used for limb altitude profiles and dayside disk imaging, LR will be used for nightside disk imaging, and OCC will be used for stellar occultations. There is also a Solar Safe mode for when the Sun is within 30 of the instrument field of regard (FOR), typically ±3 hours around local midnight. An example of how these modes may be used during a day is shown in Figure 6.1. Gold Mission Description, working draft, August 2016 14

Figure 6.1 Example of observations by channel through 24 hours As shown in Figure 6.2, the scan mirror in each channel sweeps the entrance slit across the limb and disk, building up spatial-spectral image cubes, performing daily observations from 0300 hours to 2100 hours Satellite Local Time (SLT). At each mirror position, the detectors record the x-y locations of individual photons with 10-bit precision, as they arrive at the focal plane. On the ground these locations are binned into 1024 1024 spectral-spatial arrays associated with each mirror position, providing 0.01 along-the-slit spatial sampling and ~0.03 nm spectral sampling. Further summing and binning occurs during ground processing to produce the spectral-spatial image cubes required by the science measurement objectives (Table 4.2). High-rate science and engineering data are routed to a dedicated spacecraft communications transmitter for immediate downlink to an SES-GS 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) located at UCF. In addition to the high-rate data, a subset of GOLD critical engineering data is routed to the spacecraft over a standard 1553 interface and downlinked to the spacecraft Mission Operations Center (MOC), also referred to as the Satellite Control Center (SCC), in real time as part of the spacecraft housekeeping data stream for monitoring of the instrument. Commands to the instrument are transmitted over the same 1553 spacecraft interface after being uplinked from the MOC. Gold Mission Description, working draft, August 2016 15

Figure 6.2: Observing scenario. Sunlit disk imaging in both hemispheres (3a and 3b) for temperature and O/N2 is followed by dayside limb scans of N 2 emission in both hemispheres (4a and 4b). Nightside O emission is imaged in both hemispheres (5a and 5b). Occultations (2) interrupt disk imaging (1). Imaging resumes after occultation (3a and 3b). Figures 6.1 and 6.2 illustrate the standard GOLD observing scenario. Dayside disk imaging and dayside limb scans are performed using both channels operating in HR mode. Measurements begin at 0300 SLT and continue until 1700 SLT. During this time, the field-of-view of each spectrometer, an image of the entrance slit, is swept across the sunlit disk in two swaths by rotating the scan mirror. One swath covers the northern latitudes and the other covers the southern (Panel 3a and 3b of Figure 6.2). Scan rate and range of travel vary with SLT to ensure that the desired regions are observed and that each swath is completed in the ~12 minutes available (24 minutes for a complete sun-lit disk map) 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. Following the disk observation, each channel scans both the north and south hemispheres of the dayside limb (Panels 4a and 4b of Figure 6.2). The scans begin on the disk at a limb-height of -120 km at the equator and scan to ~600 km with a step size of 16 km. Approximately 6 minutes is required to complete them. During the approximately 1 hour, centered on spacecraft noon, when both limbs are illuminated, one channel scans the east limb while the other scans the west limb. 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. Detector dark counts are measured with the scan mirrors turned inward for 30 seconds, once per image, in order to monitor particle backgrounds. The entire sequence limb, disk and dark measurements is timed to complete in 30 minutes. Exceptions to the sequence described above occur when stars selected for occultation measurements approach either limb (Panel 1) during a 30 minute observing period. In those Gold Mission Description, working draft, August 2016 16

cases the scans are 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 Figure 6.2) require ~7 8 minutes to execute (1 minute to configure the instrument, 1 minute for above-theatmosphere observation, 3 4 minutes for the actual occultation, 1 minute measuring emission (background) when observing on the sunlit limb, and 1 minute to return to HR. 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 0.05 -wide slit is reinserted, the scan mirror returns to its departure point in the original mapping observation, and HR mode is resumed (Panel 3a of Figure 5.2). 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, reducing the disk imaging duration by approximately 2%. At 1700 SLT, one channel continues dayside observations while the other begins nightside observations using LR mode (Panels 5a and 5b of Figure 6.2). The nightside sequence consists of a pair of 10-minute swaths, one of the north and one of the south, followed by a pair of 20- minute swaths of the southern hemisphere only. 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. Since the 135.6 nm brightness typically decreases at the later local times, as the scan extends further into the night, the dwell time per step at the later local times is increased improve the SNR. Imaging of the nighttime ionosphere is interrupted for occultations just as on the dayside which was described above. 7. Observations As discussed earlier, all observations by the mission yield spectrally resolved data at wavelengths of 132-162 nm. An example of the daytime airglow spectrum in this wavelength range is shown in Figure 7.1. Emissions from atomic oxygen at 135.6 nm and the N 2 Lyman- Birge-Hopfield bands, resulting from absorption of the Sun s short wavelength radiation in the Earth s atmosphere, play an important role in the remote sensing techniques that the mission will use with the dayside data. A spectrum of this wavelength region will be assembled for each pixel analyzed on the dayside. Figure 7.1. Daytime far-ultraviolet model airglow spectrum Gold Mission Description, working draft, August 2016 17

At night the 135.6 nm atomic oxygen emission from the Earth s atmosphere, which persists due to production by recombination of the ionosphere while the other emissions decrease below detectable levels, will be used to derive information about the nighttime ionosphere. As discussed in the previous section, the entrance slit of each channel, one of which is show on the left side of Figure 7.2 as a white rectangle superimposed on the simulated Earth image at 135.6 nm, will be swept across the Earth during. Although the image only shows the 135.6 nm emission, primarily atomic oxygen, the complete spectrum is measured. As shown in the image/spectrum on the right, a full spectrum is seen on the dayside and in the aurora, while at night the 135.6 nm emission from atomic oxygen is the only emission. The structure in the nighttime ionosphere is seen in the image on the left. A simulation of the Earth s spectrum across the entire length of the slit, is shown on the right side of the figure. The slit projected onto the Earth near sunset at the local time of the satellite (the terminator is diagonal across the earth observations.) At the lower latitudes, the dayglow emissions from atomic oxygen and molecular nitrogen are both seen. At slightly higher latitudes, where the nightside is filling the slit, only the atomic oxygen 135.6 nm emission appears until the auroral region is reached, at the top of the image/slit/spectrum. Figure 7.2. Left: Representation of Earth s emissions at 135.6 nm calculated using atmospheric densities from the TIE-GCM model and radiances calculated by the GLOW code, with an image of the entrance superimposed in white (vertical rectangle). Right: Model spectrum that would be seen on the detector when viewing the emissions within the entrance slit of a channel. As discussed earlier, GOLD will also perform limb scans on the dayside. Since the altitude dependence of the N2 emission brightness depends on the exospheric temperature, the data will be used for deriving that temperature. A model simulation of the N2 limb emission profile data from GOLD is shown in Figure 7.3, as well as a fit to the simulated data. The limb tangent points observed by the GOLD mission are essentially fixed in longitude, allowing temperature changes to be tracked at those locations throughout the day and from day to day. Gold Mission Description, working draft, August 2016 18