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

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.

The Global-scale Observations of the Limb and Disk (GOLD) Mission images middle thermosphere temperature and the vertical column density ratio of oxygen to molecular nitrogen (ΣO/N2) using its far ultraviolet imaging spectrographs in geostationary orbit. Since GOLD only measures these quantities during daylight, and only over the ~140º of longitude visible from geostationary orbit, previously developed tidal analysis techniques cannot be applied to the GOLD dataset. This paper presents a novel approach that deduces two specified non-migrating diurnal tides using simultaneous measurements of temperature and ΣO/N2. DE3 (diurnal eastward propagating wave 3) and DE2 (diurnal eastward propagating wave 2) during October 2018 and January 2020 are the focus of this paper. Sensitivity analyses using TIE-GCM simulations reveal that our approach reliably retrieves the true phases, whereas residual contributions from tides assumed to be absent, the restriction in longitude, and random uncertainty can lead to ~ 50% error in the retrieved amplitudes. Application of our approach to GOLD data during these time periods provides the first observations of non-migrating diurnal tides in measurements taken from geostationary orbit. We identify discrepancies between GOLD observations and TIE-GCM modeling. Retrieved tidal amplitudes from GOLD observations exceed their respective TIE-GCM amplitudes by a factor of two in some cases.

Global-scale Observations of Limb and Disk (GOLD) disk measurements of far ultraviolet molecular nitrogen band emissions are used to retrieve column integrated disk temperatures (Tdisk), which are representative of the lower-and-middle thermosphere. The present work develops a new approach to assimilate the Tdisk in the Whole Atmosphere Community Climate Model with thermosphereâ\euro?ionosphere eXtension (WACCMX) using the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter. Nine days of data,1 to 9 November 2018, are assimilated. Analysis state variables such as thermospheric effective temperature (Teff, airglow layer integrated temperature), ratio of atomic oxygen to molecular nitrogen column densities (O/N2), and column electron content are compared with a control simulation that is only constrained up to ~50 km. It is observed that assimilation of the GOLD Tdisk improves the analysis states when compared with the control simulation. The analysis and model states, particularly, Teff, O/N2, and Electron Column Density (ECD) are also compared with their measurement counterparts for a validation of the assimilation. Teff and O/N2 are compared with GOLD Tdisk and O/N2. While, the ECD is compared with ground based Total Electron Content (TEC) measurements from Global Navigational Satellite System (GNSS) receivers. Root Mean Square Error (RMSE) improvements in Teff and O/N2 are about 10.8% and 22.6%, respectively. The RMSE improvement in analyses ECD is about 10% compared to control simulation.

The middle thermospherefrom ~150 to 250 km is characterized by rapid increase in temperature with altitude and rapid ionization. The entire thermosphere is believed to be home to atmospheric waves that propagate through it, originating both in the atmospheric layers below and in the thermosphere itself. Within the middle thermosphere, direct observations of such waves are extremely sparse. The GOLD far-UV imaging spectrometer is able to observe the middle thermosphere from geostationary orbit. During October 2018 a special observational campaign was performed, designed to identify atmospheric waves. Signatures in the 135.6 nm O airglow were seen that move northwards with time, away from the Southern polar region. These are consistent with a large-scale atmospheric gravity wave. These results are the first time 135.6 nm airglow has been used to track such a wave and highlight the ability of GOLD to observe such waves, even when at a modest amplitude, and track their motion.

Travelling ionospheric disturbances (TIDs) and their neutral counterparts known as travelling atmospheric disturbances (TADs) are believed to play a central role in redistributing energy and momentum in the upper atmosphere and communicating inputs to other locations in the fluid. While these two phenomena are believed to be connected, they may not have a one-to-one correspondence as the geomagnetic field influences the TID but has no direct impact on the TAD. The relative amplitudes of the perturbations seen in the ionosphere and atmosphere have been observed but rarely together. This study reports results from a three-day campaign to observe TIDs and TADs simultaneously over a broad latitudinal region over the eastern United States using a combination of GOLD and a distributed network of ground based Global Navigation Satellite System (GNSS) receivers. These results demonstrate that GOLD and the ground-based total electron content (TEC) observations can see the atmospheric and ionospheric portions of a large-scale travelling disturbance. The phase difference in the perturbations to the GOLD airglow brightness, O/N2 and thermospheric disk temperature are consistent with an atmospheric gravity wave moving through this region. The ionospheric signatures move at the same rate as those in the atmosphere, but their amplitudes do not have a simple correspondence to the amplitude of the signal seen in the atmosphere. This campaign demonstrates a proof-of-concept that this combination of observations is able to provide information on TIDs and TADs, including quantifying their impact on the temperature and chemical composition of the upper atmosphere.

The present investigation evaluates the assimilation of synthetic data which has properties similar to actual Global-scale Observations of the Limb and Disk (GOLD) level-2 (L2) and other conventional lower atmospheric observations. The lower atmospheric and GOLD L2 temperature (Tdisk) are assimilated in the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCMX) using Data Assimilation Research Testbed (DART). It is found that inclusion of the GOLD Tdisk improves the forecast root mean square error (RMSE) and bias by 5% and 71%. When compared to lower atmosphere only assimilation the improvements in RMSE and bias are 20% and 94%. An investigation of the global DW1 and local diurnal tidal characteristics shows that inclusion of the GOLD temperatures improves the DW1 by about 8% and diurnal tide by more than 17%. The percentage improvement in tides is higher at lower thermospheric altitudes. Considerable improvements in the model state are also seen at times and locations where there are no GOLD observations available. These results and the background data assimilation procedure are presented here, which demonstrates that GOLD thermospheric temperature is an excellent dataset which can be used for thermospheric assimilation studies and operational purposes.