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The Earth's thermosphere and ionosphere constitute a dynamic system that varies daily in response to energy inputs from above and from below. This system can exhibit a significant response within an hour 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 thermosphere-ionosphere (T-I) system to these drivers is essential to advanc- ing 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 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 earlier during the day. Due to the geostationary orbit being used the mission observes the same hemisphere repeatedly, allowing the unambiguous separation of spatial and temporal variability over the Americas.

The NASA Global‐scale Observations of the Limb and Disk (GOLD) mission has flown an ultraviolet‐imaging spectrograph on SES‐14, a communications satellite in geostationary orbit at 47.5°W longitude. That instrument observes the Earth's far ultraviolet (FUV) airglow at ~134–162 nm using two identical channels. The observations performed include limb scans, stellar occultations, and images of the sunlit and nightside disk from 6:10 to 00:40 universal time each day. Initial analyses reveal interesting and unexpected results as well as the potential for further studies of the Earth's thermosphere‐ionosphere system and its responses to solar‐geomagnetic forcing and atmospheric dynamics. Thermospheric composition ratios for major constituents, O and N2, temperatures near 160 km, and exospheric temperatures are retrieved from the daytime observations. Molecular oxygen (O2) densities are measured using stellar occultations. At night, emission from radiative recombination in the ionospheric F region is used to quantify ionospheric density variations in the equatorial ionization anomaly (EIA). Regions of depleted F region electron density are frequently evident, even during the current solar minimum. These depletions are caused by the “plasma fountain effect” and are associated with the instabilities, scintillations, or “spread F” seen in other types of observations, and GOLD makes unique observations for their study.

[1] New models of solar extreme ultraviolet (EUV) irradiance variability are constructed in 1 nm bins from 0 to 120 nm using multiple regression of the Mg II and F 10.7 solar activity indices with irradiance observations made during the descending phase of cycle 23. The models have been used to reconstruct EUV spectra daily since 1950, annually since 1610, to forecast daily EUV irradiance and to estimate future levels in cycle 24. A two-component model developed by scaling the observed rotational modulation of the two solar indices underestimates the solar cycle changes that the Solar EUV Experiment (SEE) reports at wavelengths shorter than 40 nm and longer than 80 nm. A three-component model implemented by including an additional term derived from the smoothed Mg II index better reproduces the measurements at all wavelengths. The three-component model is consistent with variations in the EUV energy from 0 to 45 nm that produces the far ultraviolet (FUV) terrestrial dayglow observed by the Global Ultraviolet Imager (GUVI). However, the spectral structure of this third component is complex, and its origin is uncertain. Analogous two-and three-component models are also developed with absolute scales determined by the NRLEUV2 spectrum of the quiet Sun rather than by the SEE average spectrum. Assessment of the EUV absolute spectrum and variability of the four different models indicate that during solar cycle 23, the EUV irradiance (0 to 120 nm) increased 100 ± 30%, from 2.9 ± 0.2 to 5.8 ± 0.9 mWm −2 , and may have been as low as 1.9 ± 0.5 mWm −2 during the 17th-century Maunder Minimum. Near the peak of upcoming solar cycle 24, EUV irradiance is expected to increase 40% to 80% above the 2008 minimum values.

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.