Global-scale Observations of the Limb and Disk (GOLD) is a PI-led NASA mission of opportunity that was launched on 25 January 2018 as a hosted payload on the SES-14 commercial communications satellite. GOLD science operations began in October 2018. The primary objective of GOLD is to answer fundamental scientific questions about how the Earth's thermosphere-ionosphere system responds to geomagnetic storms, solar radiation, and upward propagating tides. To help answer these questions, GOLD utilizes two identical but independent imaging spectrographs. From its vantage point in geostationary orbit at 47.5° W longitude GOLD images the Earth in the far-ultraviolet (FUV). GOLD observes the disk of the Earth for 18.5 hr per day while also performing routine limb scan and stellar occultation measurements. GOLD science algorithms use the observed spectra to produce Level 2 data products that include: daytime neutral temperatures near the peak of the N 2 LBH emitting layer (Level 2 data product TDISK); daytime and nighttime thermospheric molecular oxygen density profiles (O2DEN); daytime exospheric neutral temperature on the limb (TLIMB); daytime ratios of atomic oxygen and molecular nitrogen column densities (ON2); and integrated solar EUV energy flux between 1 and 45 nm (QEUV).The ON2 and QEUV data products are pertinent to several aspects of the GOLD science objectives. Changes in ΣO/N 2 are of particular interest.

We have measured the 30 and 100 eV far ultraviolet (FUV) emission cross sections of the optically allowed Fourth Positive Group (4PG) band system (A 1 Π → X 1 Σ + ) of CO and the optically forbidden O ( 5 S o → 3 P) 135.6 nm atomic transition by electron-impact-induced-fluorescence of CO and CO 2 . We present a model excitation cross section from threshold to high energy for the A 1 Π state, including cascade by electron impact on CO. The A 1 Π state is perturbed by triplet states leading to an extended FUV glow from electron excitation of CO. We derive a model FUV spectrum of the 4PG band system from dissociative excitation of CO 2 , an important process observed on Mars and Venus. Our unique experimental setup consists of a large vacuum chamber housing an electron gun system and the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission Imaging Ultraviolet Spectrograph optical engineering unit, operating in the FUV (110-170 nm). The determination of the total O I ( 5 S o ) at 135.6 nm emission cross section is accomplished by measuring the cylindrical glow pattern of the metastable emission from electron impact by imaging the glow intensity about the electron beam from nominally zero to~400 mm distance from the electron beam. The study of the glow pattern of O I (135.6 nm) from dissociative excitation of CO and CO 2 indicates that the O I ( 5 S o ) state has a kinetic energy of~1 eV by modeling the radial glow pattern with the published lifetime of 180 μs for the O I ( 5 S o ) state.Plain Language Summary Both Mars and Venus have upper atmospheres that are similar in composition: mostly CO 2 , CO, and N 2 are dominant molecular gases, with nearly identical UV spectra. The modeling studies of atmospheric UV emissions cannot presently be accurately conducted to the same accuracy as the planetary UV measurements, which avail themselves with state-of-the-art calibrated spectrographs. This dichotomy in accuracy between planetary observation and model occurs because the atomic and molecular emission cross sections with uncertainties of certain transitions are greater than 100%. Furthermore, the analysis is complicated by the spectral blending of the various emissions of the low-resolution spectral spaceborne instruments. We present in this paper a UV laboratory instrument unique in the world at the University of Colorado that can measure for the first time the excitation mechanisms with accurate emission cross sections of both allowed and optically forbidden transitions that are occurring in a planetary atmosphere. Key Points: • We measured 30 and 100 eV emission cross sections of Fourth Positive band system of CO and O I (135.6 nm) from electron impact on CO and CO 2 • We conducted a laboratory experiment measuring single-scattering electron-impactinduced-fluorescence FUV spectra from excitation of CO and CO 2 • Fragment kinetic energy measurement for O I ( 5 S o ) atoms found to be around~1 eV from both CO and CO 2 molecular dissociation Correspondence to: et al. (2019). UV study of the Fourth Positive band system ...

Observations of far-ultraviolet (FUV) dayglow by the Global-scale Observations of Limb and Disk (GOLD) mission provide a new opportunity for determining how geomagnetic storms alter the temperature of the thermosphere and enable us to quantify the global-scale response of the thermosphere to solar extreme-ultraviolet (EUV) variability. Relative temperature changes can be measured by monitoring changes in the scale height of molecular nitrogen as observed in Lyman-Birge-Hopfield (LBH) band emissions. Mean scale heights are derived from GOLD FUV observations using Chapman function fits to altitude profiles of LBH band emissions. We provide an overview of the theoretical basis for the GOLD Level 2 exospheric temperature algorithm, including a generalization of a standard Chapman function fitting technique to allow for gravity and temperature gradients. Effects on derived exospheric temperatures from instrument artifacts and stars in the GOLD field of view are reviewed. We also discuss GOLD Level 1C LIM and Level 2 TLIMB data products and present representative examples of each. We show that exospheric temperatures vary with local time and correlate weakly with solar activity but more strongly with geomagnetic activity. Finally, we present results from a preliminary data product validation that show good qualitative agreement with predictions from a global reference atmospheric model. Plain Language Summary Observations of the Earth's daytime ultraviolet emission by the Global-scale Observations of Limb and Disk (GOLD) mission provide a new opportunity to determine how geomagnetic storms alter the temperature of Earth's upper atmosphere and enable us to quantify the global-scale response of the upper atmosphere to solar variability. We provide an overview of the GOLD upper atmosphere temperature data product and present representative measurement examples. We show that derived temperatures vary with local time, correlate with both geomagnetic and solar activity, and show good qualitative agreement with reference atmospheric model predictions.

We have measured in the laboratory the far ultraviolet (FUV: 125.0-170.0 nm) cascade-induced spectrum of the Lyman-Birge-Hopfield (LBH) band system (a 1 Π g →X 1 Σ þ g ) of N 2 excited by 30-200 eV electrons. The cascading transition begins with two processes: radiative and collision-induced electronic transitions (CIETs) involving two states (a′ 1 Σ − u and w 1 Δ u → a 1 Π g ), which are followed by a cascade induced transition a 1 Π g →X 1 Σ þ g at the single-scattering pressures employed here. Direct excitation to the a-state produces a confined LBH spectral glow pattern around an electron beam. We have spatially resolved the electron-induced glow pattern from an electron beam colliding with N 2 at radial distances of 0-400 mm at three gas pressures. This imaging measurement is the first to isolate spectral measurements in the laboratory of single-scattering electron-impact-induced fluorescence from two LBH emission processes: direct excitation, which is strongest in emission near the electron beam axis; and cascading-induced, which is dominant far from the electron beam axis. The vibrational populations for vibrational levels from v′ = 0-2 of the a 1 Π g state are enhanced by radiative cascade and CIETs, and the emission cross sections of the LBH band system for direct and cascading-induced excitation are provided at 40, 50, 100, and 200 eV.

The Global-scale Observations of the Limb and Disk (GOLD) instrument was launched on 25 January 2018 onboard the SES-14 commercial communications satellite and began nominal science operations in October 2018. Operating from geostationary orbit at 47.5°W longitude, GOLD images the Earth's thermosphere and ionosphere in the far-ultraviolet (132-162 nm), measuring critical geophysical parameters by continuously scanning the Earth's disk and limb 18 hours per day. GOLD also performs stellar occultation measurements using bright type O and B stars. Up to 10 occultations per day are obtained at latitudes ranging from 60°S to 45°N and two fixed longitudes,~33°E and~128°W. The occultation data provide a direct measurement of atmospheric absorption in the O 2 Schumann Runge continuum, which is used to retrieve the O 2 density profile from 130-to 200-km altitude. This paper describes the GOLD occultation measurement technique and the operational retrieval algorithm used to derive the Level 2 O2DEN data product. The spatial, temporal, and local time sampling of the GOLD O 2 data product is summarized, and the data quality, retrieval errors, and validation plans are discussed. Occultations are observed under both daytime and nighttime conditions, providing the O 2 density over a complete range of local times. GOLD retrievals have an estimated precision of 10% and a vertical resolution of 5-10 km, depending on altitude. The data have sufficient accuracy and resolution to provide scientifically useful constraints on the O 2 abundance and variability. Initial comparisons show qualitative agreement, but also notable differences, between the GOLD O 2 data and the Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar (NRLMSISE-00) empirical model predictions. Key Points:• Thermospheric O 2 density profiles derived from stellar occultation measurements by the GOLD instrument are described • The GOLD O 2 data set covers altitudes from 130 to 200 km, all local times and latitudes from 60°S to 45°N, with 10% precision and 5-to 10-km vertical resolution • Preliminary validation shows overall agreement with empirical models but also significant differences in magnitude and local time dependenceCorrespondence to: