Observations from the Global-scale Observations of Limb and Disk (GOLD) mission have provided a new remote-sensing data source of molecular oxygen profiles in Earth's lower-to-middle thermosphere (120-200 km). GOLD O2 observations indicate increasing densities of molecular oxygen at 170 km with rising solar activity between solar radio flux F10.7 values of 60 and similar to 120 solar flux units (sfu). This is also seen in comparisons with solar extreme ultraviolet irradiance QEUV between 1 and similar to 2.25 erg cm-2 s-1. However, the empirical Mass Spectrometer Incoherent Scatter radar 2.0 (MSIS 2.0) model overestimates O2 densities at 170 km at these low levels of solar activity and predicts a decreasing density with increasing solar flux below similar to 120 sfu. Additional data sets validate GOLD observations of O2 and their relationship with solar activity. Accurately determining and forecasting O2 is critical for accurately modeling plasma densities in the ionosphere and thermospheric density in the lower-to-middle thermosphere.

EUVS-C is one component of the Extreme Ultraviolet Irradiance Sensor (EUVS) instrument. EUVS together with the X-ray sensor (XRS) comprise the Extreme Ultraviolet and X-ray Irradiances Sensors (EXIS) investigation (Machol et al. 2020) aboard the GOES-R satellite series which includes GOES-16, -17, -18, and -19. From their vantage points in geostationary orbit, the EUVS-C instruments measure the solar Mg II Index, also referred to as the Mg II core-to-wing-ratio, which is a proxy for chromosphere activity and correlates with solar extreme ultraviolet (EUV) emission. Mg II produces two bright chromosphere emission lines appearing in the sun’s spectrum at 279.55 nm and 280.71 nm (Mg II k and h) that appear in the cores of their respective photospheric absorption lines. Measuring the ratio of emission from the core (chromsopheric) to that from the wings (photospheric) provides an index that is relatively insensitive to changes in instrument performance. In 2005, Snow & McClintock used 0.1nm resolution data to show that the intrinsic solar variability in the index (as opposed to instrument noise) is on the order of 0.2% on time scales 5-10 minutes. EUVS-C is designed to exceed these performance requirements. A companion paper describes the instrument design and its pre-flight calibration. This paper describes the operational implementation for the algorithm that produces the Index, flight calibrations, and the initial instrument flight performance. Each EUVS-C currently operating (GOES-16, -17, and -18) is providing high time-cadence (3 seconds), high precision (1 part in 104) Index determinations. Spectral shifts arising from spacecraft orbital motion introduce a systematic 0.1% diurnal variation in absolute index values. Additionally, wavelength dependent radiometric responsivity degradation leads to a systematic increase in the reported index on a timescale of years at an average rate of 0.2% per year. These systematic effects can be mitigated with additional post data processing.

This paper presents observations of upper atmospheric vortices following two geomagnetic storms: a Kp 9− storm on 10 October 2024 and a Kp 8+ storm occurring in April of 2023. These observations, using neutral composition and effective neutral temperatures from the NASA GOLD mission, reveal vortices similar to those initially reported by Evans, Correira, et al. (2024, https://doi.org/10.1029/2024GL110506 ) for the historic geomagnetic storm of May 2024. Development of vortices following more typical geomagnetic storms demonstrate that such vortices are not as rare as was originally speculated. For all three storms, vortices appeared in GOLD data following similar elapsed times after storm onset and developed in the Southern Hemisphere at similar locations. The size of the vortices varied for the three storms, as did whether or not a vortex appeared in the Northern Hemisphere.

EUVS-C is one component of the Extreme Ultraviolet Irradiance Sensor (EUVS) instrument. EUVS together with the X-ray sensor (XRS) comprise the Extreme Ultraviolet and X-ray Irradiances Sensors (EXIS) investigation aboard the GOES-R satellite series which includes GOES-16, -17, -18, and -19. From their vantage points in geostationary orbit, the EUVS-C instruments make high-precision (better than 1 part in 104), high-time-cadence (3 seconds) measurements of the solar Mg II Index with moderate (0.1 nm) spectral resolution. The Index, also referred to as the Mg II core-to-wing-ratio, is a proxy for chromosphere activity that correlates with solar extreme ultraviolet (EUV) irradiance. Mg II produces two bright chromosphere emission lines in the Sun’s spectrum at 279.55 nm and 280.71 nm (Mg II k and h) that appear in the cores of their respective photospheric absorption lines. Measuring the ratio of emission from the core (chromospheric) to that from the wings (photospheric) provides an index that is relatively insensitive to changes in radiometric performance that often occur when scientific instruments observe the Sun. EUVS-C design specifications were informed by earlier research reporting index variability of approximately 0.2% on time scales of 6 – 10 minutes, increasing to approximately 0.3% and approximately 0.55% for 30 and 80 minutes, respectively. This paper describes the EUVS-C instrument design and implementation, its ground calibration and characterization, and anticipated measurement performance. A companion paper describes initial EUVS-C flight measurement performance.

During the intense geomagnetic storm of May 10–11, 2024, NASA's Global-scale Observations of the Limb and Disk (GOLD) imager revealed unprecedented changes in the nightside ionosphere. One of the most striking observations was the poleward expansion of the Equatorial Ionization Anomaly (EIA). The northern and southern crests of the EIA extended significantly, reaching around 38°N and 35°S magnetic latitude (Mlat), respectively. This poleward movement indicated the presence of a super plasma fountain effect. The southern EIA crest moved at speeds up to 450 m/s over a broad longitudinal range and merged with the aurora australis. Additionally, the southern crest drifted westward, forming a distinctive ‘V’ shape near 400W, which coincided with the peak of geomagnetic activity at that longitude as it neared sunset.

The scientific and societal importance of short-term changes in the thermosphere–ionosphere (T–I) system highlights a need to better understand short-term changes in the thermosphere. For collision avoidance, this need becomes increasingly important as the number of low-earth-orbiting satellites increases because geomagnetic activity can cause dramatic, unexpected increases in satellite drag. The thermospheric density changes responsible for changes in drag depend primarily on changes in the thermospheric temperature. However, for collision avoidance, the specification of drag could also be problematic during quiet periods when the number of satellites and the uncertainties in their orbits are large. While temperatures and densities at higher altitudes (≳250 km) have been extensively studied and modeled, there is a knowledge gap for densities at lower-middle-thermosphere altitudes (≲200 km). At these lower altitudes, the primary sources of thermospheric density data, in situ and drag data from satellites, are rarely available. Remote sensing of temperatures and composition by NASA's Global-scale Observations of the Limb and Disk (GOLD) mission can help fill this gap. The GOLD mission produces disk images of neutral temperature, a key parameter for understanding neutral density in the lower-middle thermosphere. However, while disk images of the temperature have been available since the launch of GOLD, recent improvements to its observational capability that are relevant to data interpretation may not be widely known. Also, other temperature retrieval techniques than GOLD's have been published. Comparisons indicate that GOLD's technique gives the most consistent results and yields the lowest uncertainties. This paper discusses both temperature retrieval techniques and issues in interpreting GOLD's images of temperatures.

Mars’ water history is fundamental to understanding Earth-like planet evolution. Water escapes to space as atoms, and hydrogen atoms escape faster than deuterium giving an increase in the residual D/H ratio. The present ratio reflects the total water Mars has lost. Observations with the Mars Atmosphere and Volatile Evolution (MAVEN) and Hubble Space Telescope (HST) spacecraft provide atomic densities and escape rates for H and D. Large increases near perihelion observed each martian year are consistent with a strong upwelling of water vapor. Short-term changes require processes in addition to thermal escape, likely from atmospheric dynamics and superthermal atoms. Including escape from hot atoms, both H and D escape rapidly, and the escape fluxes are limited by resupply from the lower atmosphere. In this paradigm for the escape of water, the D/H ratio of the escaping atoms and the enhancement in water are determined by upwelling water vapor and atmospheric dynamics rather than by the specific details of atomic escape.

Abstract Leveraging observations by two NASA missions—GOLD (Global‐scale Observations of the Limb and Disk) and ICON (Ionospheric Connection Explorer), we investigate concurrent responses of thermospheric composition, temperatures, and neutral winds to the geomagnetic storm on 3–4 November 2021, as well as their interplay at low and middle latitudes. The synergetic observations reveal remarkable depletions up to 60%–70% in GOLD O/N2, along with large enhancements in GOLD temperatures poleward of 30° in the middle thermosphere. Meridional winds from ICON observations are altered by ∼100 m/s equatorward of 25°N latitude and at 250 km, characterized by a reversal of prevailing northward winds to geomagnetic storm‐driven southward winds. This study fills a need, after a decade‐long gap, for observing concurrent and co‐located responses of composition, temperatures, and neutral winds in the thermosphere to geomagnetic storms.

After days of intense solar activity, active region AR3664 launched seven CMEs towards Earth producing an extreme G5 geomagnetic storm commencing at 16:45 UT on Saturday May 10, 2024. The storm impacted power grids, disrupted precision navigational systems used by farming equipment, and generated aurora seen around the globe. The storm produced remarkable effects on composition, temperature, and dynamics in the Earth’s thermosphere that were observed by NASA’s Global-scale Observations of the Limb and Disk (GOLD) mission and are reported here for the first time. We use synoptic disk images of ΣO/N2 and neutral temperature (at ~160 km) measured by GOLD to directly link dynamics resulting from the storm with dramatic changes in thermospheric composition and temperature. We observe an apparent rotation simultaneously in ΣO/N2, neutral temperature, and total electron content. Equator-to-pole temperature differences reach 400 K with peak neutral temperatures near 160 km exceeding 1400 K at high latitudes.

During a minor geomagnetic storm occurring from Aug 2 (day-of-year (DOY) 214) to Aug 4 (DOY 216), 2021, the National Aeronautics and Space Administration Global-scale Observations of the Limb and Disk (GOLD) mission observed different column density ratio of O to N-2 (Sigma O/N-2) variations in storm main and recovery phases. The percentage difference of Sigma O/N-2 between DOY 215 (disturbed day, main phase) and DOY 213 (quiet reference day) exhibits a depletion on the east side of the GOLD field-of-view (FOV). However, that of Sigma O/N-2 between DOY 216 (recovery phase) and 213 shows depletions on the west side of GOLD FOV. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model qualitatively reproduced the observations. Analysis of the model output illustrates that the Sigma O/N-2 patterns in the two days are both formed due to the classical thermospheric composition theory and formed on DOY 214 and 215, respectively. Further investigation found that the Sigma O/N-2 depletion on DOY 214 and 215 both initially formed near 120-180 degrees E, but the one on DOY 215 then quickly moved westward into the GOLD FOV, from local post-midnight to pre-midnight, near 19 UT. Then it moves equatorward and slowly westward. This results in the observed depletion structure on the west side of GOLD FOV. Model simulations show that the quick westward movement near 19 UT is due to the dominant positive Interplanetary Magnetic Field east-west component (B-y) conditions.

The recovery of the thermosphere after a strong geomagnetic storm on 12 May 2021 is investigated using lower and middle thermospheric temperature (Tdisk), ratio of atomic oxygen and molecular nitrogen column densities (O/N2), cooling due to Nitric Oxide (NO), and model simulations. The peak influence of the geomagnetic storm lasted about five hours and generated latitudinal gradients in Tdisk and O/N2. Following the storm, the latitudinal gradient in temperature recovered faster compared to that of O/N2. Measured NO-cooling rate and simulated NO-densities are enhanced on the storm day compared to a quiet-day, consistent with a rapid recovery in temperatures. Based on this, we conclude that the faster recovery of the Tdisk latitudinal gradient is driven by the combined effect of NO cooling and strong thermal conductivity. Furthermore, the slower recovery of the O/N2 latitudinal gradients can be attributed to the lack of poleward advection after the storm.

This Letter presents first observations of distinct solar atmospheric oscillations from signals collected by the Extreme Ultraviolet and X-ray Irradiance Sensors EUVS-C instrument, which is part of each Geostationary Observational Environmental Satellite R-Series instrument payload. The EUVS-C instrument is a full-disk, normal-incidence spectrograph that covers a narrow band in the mid-ultraviolet between 276 and 284 nm, where it can measure the magnesium ii emission doublet at similar to 280 nm and the photospheric continuum. The primary goal of EUVS-C data is to construct the well-known Mg ii index, which is often used as a proxy for chromospheric activity. Because of the high temporal and spectral resolution of EUVS-C measurements, the data provide a unique opportunity to observe discernible solar atmospheric waves that have definite signatures of 3 and 5 minute oscillation periods, where the frequency response of these signals is dependent on what part of the spectrum is analyzed (e.g., Mg emission lines). Furthermore, both photospheric and chromospheric waves can simultaneously be examined. With the recent increase in solar activity for Solar Cycle 25, these waves exhibit enhanced amplitudes and phase shifts during the impulsive shock of strong solar flares. This Letter will discuss the analysis for deriving these waves, and results from both a quiescent Sun and an X-class solar flare event will be presented.

After days of intense solar activity, active region AR3664 launched seven CMEs toward Earth producing an extreme G5 geomagnetic storm commencing at 17:05 UT on 10 May 2024. The storm impacted power grids, disrupted precision navigational systems used by farming equipment, and generated aurora seen around the globe. The storm produced remarkable effects on composition, temperature, and dynamics in the Earth's thermosphere that were observed by NASA's Global-scale Observations of the Limb and Disk (GOLD) mission and are reported here for the first time. We use synoptic disk images of Sigma O/N2 and neutral temperature (at similar to 160 km) measured by GOLD to directly link dynamics resulting from the storm with dramatic changes in thermospheric composition and temperature. We observe a heretofore unseen spatial morphology simultaneously in Sigma O/N2, neutral temperature, and total electron content. Equator-to-pole temperature differences reach 400 K with high latitude peak neutral temperatures near 160 km exceeding 1400 K. On Saturday 10 May 2024, the sun launched a wave of energized plasma toward the Earth. A large disturbance in the Earth's magnetic field associated with the solar wind resulted in an extreme geomagnetic storm. The storm impacted power grids, disrupted navigational systems used by farming equipment, and produced aurora seen around the globe. The storm produced remarkable effects in the Earth's upper atmosphere that were observed by NASA's Global-scale Observations of the Limb and Disk (GOLD) mission. In this letter, we use images measured by GOLD to directly link atmospheric dynamics resulting from the May 10-12 superstorm with dramatic changes in composition, temperature, and global circulation in the Earth's upper atmosphere. We observe previously unseen structure in the upper atmosphere associated with equator-to-pole temperature differences exceeding 400 K. Peak neutral temperatures near 160 km exceed 1400 K at high latitudes. GOLD disk images of Sigma O/N2 and neutral temperature link storm time dynamics with changes in thermospheric composition and temperature We observe a previously unseen spatial morphology in Sigma O/N2, neutral temperature, and total electron content Peak equator-to-pole temperature differences exceed 400 K but relax to pre-storm conditions well before Sigma O/N2

Abstract A coronal mass ejection erupted from the Sun on 21 April 2023 and created a G4 geomagnetic storm on 23 April. NASA's global‐scale observations of the limb and disk (GOLD) imager observed bright equatorial ionization anomaly (EIA) crests at ∼25° Mlat, ∼11° poleward from their average locations, computed by averaging the EIA crests during the previous geomagnetic quiet days (18–22 April) between ∼15°W and 5°W Glon. Reversed C‐shape equatorial plasma bubbles (EPBs) were observed reaching ∼±36° Mlat (∼40°N and ∼30°S Glat) with apex altitudes ∼4,000 km and large westward tilts of ∼52°. Using GOLD's observations EPBs zonal motions are derived. It is observed that the EPBs zonal velocities are eastward near the equator and westward at mid‐latitudes. Model‐predicted prompt penetration electric fields indicate that they may have affected the postsunset pre‐reversal enhancement at equatorial latitudes. Zonal ion drifts from a defense meteorological satellite program satellite suggest that westward neutral winds and perturbed westward ion drifts over mid‐latitudes contributed to the observed latitudinal shear in zonal drifts.

A unique phenomenon {\textendash} merging of Equatorial Ionization Anomaly (EIA) crests, leading to an X-pattern (EIA-X) around the magnetic equator {\textendash} has been observed in the night-time ionospheric measurements by the Global-scale Observations of the Limb and Disk (GOLD) mission. A whole atmospheric general circulation model simulation reproduces this pattern. The pattern is also produced in an assimilative ionosphere model that assimilates slant Total Electron Content (slant-TEC) from Global Navigation Satellite System (GNSS) and Constellation Observing System for Meteorology, Ionosphere, and Climate 2 (COSMIC-2). Due to the observed similarity between measurements and simulations, the latter is used to diagnose this heretofore unexplained phenomenon. The simulation shows that the EIA-X occurs in the afternoon to evening sector at a longitude where the vertical drift is negative, which is a necessary but not sufficient condition. The simulation was performed under constant low-solar and quiescent-geomagnetic forcing conditions, therefore we suggest that one of the drivers of this phenomenon is from lower-atmospheric processes.

Observations of far‐ultraviolet (FUV) dayglow by the Global‐scale Observations of Limb and Disk (GOLD) mission provide an opportunity for quantifying the global‐scale response of the thermosphere to solar extreme‐ultraviolet variability and geomagnetic activity. Relative temperature changes can be measured by monitoring changes in the rotational structure observed in molecular nitrogen Lyman‐Birge‐Hopfield (LBH) band emissions. We present a new technique for deriving effective neutral temperatures from GOLD FUV observations using optimal estimation fits to spectra containing LBH band emissions. We provide an overview of the theoretical basis for the GOLD Level 2 TDISK algorithm. Effects on derived effective neutral temperatures from instrument artifacts and particle background are reviewed. We also discuss GOLD Level 1C DAY and Level 2 TDISK data products and present representative examples of each. We show that effective neutral temperatures vary with local time, exhibit a strong dependence on season and solar zenith angle, and correlate strongly with geomagnetic and solar activity. Finally, we present results from a preliminary data product validation that show good agreement with coincident GOLD exospheric temperatures and predictions from a global reference atmospheric model.

AbstractUsing NASA's Global‐scale Observations of the Limb and Disk (GOLD) imager, we report nightside ionospheric changes during the G5 super geomagnetic storm of 10 and 11 May 2024. Specifically, the nightside southern crest of the Equatorial Ionization Anomaly (EIA) was observed to merge with the aurora near the southern tip of South America. During the storm, the EIA southern crest was seen moving poleward as fast as 450 m/s. Furthermore, the aurora extended to mid‐latitudes reaching the southern tips of Africa and South America. The poleward shift of the equatorial ionospheric structure and equatorward motion of the aurora means there was no mid‐latitude ionosphere in this region. These observations offer unique insights into the ionospheric response to extreme geomagnetic disturbances, highlighting the complex interplay between solar activity and Earth's upper atmosphere.

Each day the Global‐scale Observations of the Limb and Disk imager observes the equatorial ionization anomaly (EIA) near sunset from ∼10°E to ∼80°W geographic longitude. Most images cover ∼45° of longitude (∼3 hr), and most longitudes are observed multiple times. Monthly averages of EIA crests' latitude (EIA lats) versus longitude during March, September, and December 2020 have been analyzed. The EIA lats reflect the combined influence of winds, solar radiation, and fields (electric and magnetic) in the equatorial region. Winter solstice differs significantly from the equinoxes, which are similar, but there are notable similarities between all three. The similarities in the EIA lats during the seasons examined indicates that the magnetic equator to subsolar point separation influences them in all three seasons and that it has a more distinct, possibly more significant, influence than winds on the average latitudes.

Using observations from the Global‐scale Observations of the Limb and Disk (GOLD) mission, we investigate post‐sunset ionospheric responses to the September 2019 Antarctic sudden stratospheric warming –first ever from a synoptic perspective. Observations reveal a prevalent quasi‐6‐day periodicity in the equatorial ionization anomaly region over South America and the Atlantic, coincident with enhanced quasi‐6‐day wave (Q6DW) activity in the mesosphere (Liu et al., 2021, https://doi.org/10.1029/2020JA028909 ). The atmosphere‐ionosphere coupling via large‐scale waves is rarely studied over the ocean due to the lack of observations. More importantly, further analyses suggest that multiple pathways are involved in transmitting the quasi‐6‐day periodicity from the middle atmosphere into the post‐sunset F‐region ionosphere, including modulation of F‐region field aligned winds and pre‐reversal enhancements by the tides and or Q6DW. A remarkable depletion in electron density, attributable to the overall change in thermosphere composition driven by the dissipative tides and or Q6DWs, is also seen during the period of enhanced Q6DW activity.

capabilities for understanding the T-I system, especially its weather.The instrument is hosted on a communication satellite in geostationary orbit (GEO) and takes full disk images of the thermospheric composition and temperature during the day and O + density during the night.One striking example of the results from GOLD is shown in Figure 1, where structuring in the nighttime equatorial ionosphere appears across a broad longitudinal region.The two main bands are the EIA crests.Relatively small, latitudinally extended plasma depletions, sometimes called bubbles, in the EIA crests are associated with plasma instabilities and radio wave scintillation.While bubbles have previously been seen at regional scales from the ground and space, GOLD's imaging reveals that they appear frequently and are often spaced somewhat evenly in longitude across 1000s of km.This cannot be seen using ground-based observations due to their sparseness in the equatorial region.Expanding the current ~ 140˚ longitude coverage of FUV remote sensing from GEO will enable investigations that cannot be accomplished using a single imager.