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 (EUV) 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 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 neutral temperature algorithm. Effects on derived 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 neutral temperatures vary with local time, exhibit a strong seasonal dependence, 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.

The October 14, 2023 annular solar eclipse was visible from the US Pacific coast to Brazil’s east coast. NASA’s Global-scale Observations of Limb and Disk (GOLD) mission observed the first synoptic thermospheric temperature changes from a geo-stationary orbit above 47.5°W longitude between 17-20 UT during the eclipse. These daytime thermospheric changes were derived using GOLD’s disk FUV measurements. A significant decrease in the daytime disk temperatures (~ 100 K) was seen near the peak annularity compared to the day before (baseline). The temperature reduction’s spatial morphology is also like that of the eclipse shadow. Modelling simulations underestimate the eclipse-induced temperature decrease (expected ~ 30-40 K) by a factor of 2-3 compared to GOLD observations. These first of kind results provide new insight into the dynamic response of the coupled thermosphere and ionosphere system to transient solar events, including eclipses.

The Global-scale Observations of Limb and Disk (GOLD) imagers scan the Earth‘s Thermosphere-Ionosphere (TI) in the far ultraviolet wavelengths. Measurements from GOLD daylit spectrum are used to retrieve the column integrated atomic oxygen to molecular nitrogen density ratio (O/N2) over about one fourth of the globe. The present investigation assesses the impact of assimilating GOLD disk O/N2 on the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCMX) using the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter. Two Observing System Simulation Experiments (OSSEs) are performed, and improvements are quantified by calculating root mean square error (RMSE) and bias with respect to a true run. One of the OSSEs assimilates only the lower atmosphere (LA, <100 km) observations, referred to as LA experiment, and the second assimilates GOLD O/N2 and LA observations, referred to as Whole Atmosphere (WA) experiment. The WA-analysis O/N2 RMSE and bias are about 60% and 87% better compared to LA-analysis. Also, the O/N2 RMSE and bias for the WA-analysis are about 23% and 54% better compared to WA 1 hr forecasts. The improvement in WA electron column density (ECD), a model equivalent of Total Electron Content (TEC), is about 24% compared to the LA experiment. These results demonstrate that the assimilation of GOLD O/N2 improves both the thermosphere and ionosphere in a whole atmosphere data assimilation system.

A document by Deepak Kumar Karan. Click on the document to view its contents.

Using NASA’s Global-scale Observations of the Limb and Disk (GOLD) imager, we report nightside ionospheric changes during the G5 super geomagnetic storm of 10-11 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.

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.

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.

On 12 October 2020 and 26 December 2021, NASA’s Global-scale Observations of the Limb and Disk (GOLD) mission observed differently shaped EPBs simultaneously within ~10o longitude, near the subsatellite point and over the Atlantic, respectively which is unusual. On 12 October 2020, three EPBs with differing curvatures were observed in a ~12o longitude sector. The westside EPB was curved towards the east, in a C-shape. The middle was straight. The eastside EPB was curved westward, in a reversed C-shape. In the second case, 26 December 2021, in a smaller longitude range of ~6o adjacent C-shaped and reversed C-shaped EPBs were observed. EPBs’ zonal drift velocities at the magnetic equator and both EIA crests were compared. These occurrences of oppositely shaped EPBs simultaneously in a narrow longitude may indicate that small-scale longitudinal variations in the E-region density, electric field, neutral wind variations, or a combination of them were present.

A document by Richard W Eastes. Click on the document to view its contents.

On 12 October 2020, the NASA’s Global-scale Observations of the Limb and Disk (GOLD) mission observed three differently shaped EPBs within a 12o longitude range, near the subsatellite point. One is straight aligned to the magnetic field line, whereas the poleward extensions of the others are tilted eastward and westward from the magnetic field line resembling a C-shape and reversed C-shape structures. These EPBs were inside the GOLD imager’s field-of-view for a period of ~3 hours. This allowed us to compute their zonal motion and determine their drift velocities. EPBs’ drift velocities were derived from measuring their longitudinal shifts at the magnetic equator and at both EIA crests. This unique observation, showing three morphologies in a narrow longitude sector, indicates the occurrence of strong longitudinal gradients in the typical parameters associated with the dynamics of EPBs, like neutral winds, electric fields, or other parameters within such a narrow longitude range.

The nighttime ionospheric response to a geomagnetic storm occurred on 23-29 September 2020 is investigated over the South American, Atlantic, and West African longitude sectors using NASA’s Global-scale Observations of the Limb and Disk (GOLD) measurements. On 27 September the solar wind conditions were favorable for prompt penetration electric fields (PPEF) to influence the equatorial ionosphere over extended longitudes. The equatorial ionization anomaly (EIA) crests were shifted 8o-10o poleward compared to the quiet time monthly mean across ~65o- 35oW during the main phase. Ionosonde hmF2 (peak electron density height) measurements from Fortaleza (GG: -3.9oN and -38.4oW) indicated a stronger prereversal enhancement this evening than other nights. As a result, Equatorial Plasma Bubbles (EPB) occurred at these longitudes on this evening. This is the first simultaneous investigation of EIA morphology and EPB occurrence rate over an extended longitude range from geostationary orbit during a geomagnetic storm.

Observations of far-ultraviolet (FUV) dayglow by the Global-scale Observations of Limb and Disk (GOLD) mission provide a new opportunity to monitor relative composition changes in the upper atmosphere as well as solar extreme ultraviolet (EUV) variability. Relative composition changes are quantified by ΣO/N2, the column density ratio of atomic oxygen to molecular nitrogen, while QEUV provides a measure of the solar EUV energy flux from 1 to 45 nm into the upper atmosphere. This spectral range provides the ionizing radiation which ultimately results in FUV airglow emission produced by photodissociation and photoelectron impact. The quantities ΣO/N2 and QEUV are derived from GOLD FUV observations through lookup tables that are constructed using a first-principles photoelectron transport model. The two FUV emissions used are O I 135.6 nm and the N2 Lyman-Birge-Hopfield (LBH) bands. We present an overview of the theoretical basis for the algorithms and practical considerations for application to GOLD data. The effects of uncertainties in electron impact cross sections, off-nadir viewing, and instrument artifacts are reviewed. We also discuss GOLD Level 1C DAY, Level 2 data products ON2 and QEUV, and present representative samples of each.