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.

The Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X) is used to investigate the influence of stratosphere polar vortex variability on the mesosphere, thermosphere, and ionosphere during Northern Hemisphere winter. Based on 40 simulated Northern Hemisphere winters, the mesosphere and lower thermosphere (MLT) residual circulation is found to depend on whether the stratosphere polar vortex is strong or weak. In particular, during weak stratosphere polar vortex time periods, the MLT circulation anomalies are characterized by clockwise and anti-clockwise flow in the Northern and Southern Hemispheres, respectively. Opposite, though weaker, anomalies are found to occur during time periods when the stratosphere polar vortex is strong. The MLT circulation anomalies influence the composition of the lower thermosphere, leading to ±5% changes in the thermosphere column integrated atomic oxygen to molecular nitrogen ratio (O/N2). Large differences between strong and weak stratosphere polar vortex events are also found to occur in the semidiurnal migrating tide (SW2) in the MLT, which leads to ±15-20% differences in the SW2 component of the ionosphere total electron content (TEC) at low latitudes. The WACCM-X simulation results indicate that variability in the stratosphere polar vortex can explain ~30% and ~18% of the quiet time variability in thermosphere O/N2 and the SW2 component of TEC during Northern Hemisphere winter, respectively.

This work examines the weak stratospheric polar vortex (SPV) event of February 2008 and the strong SPV event of February 2021 to understand the potential influence of SPV strength on midlatitude thermospheric composition and ionospheric plasma density. The weak SPV event shows a reduction in thermospheric O/N2 at all longitudes, changes in the ionospheric total electron content (TEC) that vary with longitude, and an intensification of the semidiurnal tides. The strong SPV event shows O/N2 elevated by up to 25% in the Asian sector and a 25% decrease in semidiurnal tidal amplitude. The TEC response during the strong SPV event is complex, but shows enhanced TEC in the Asian sector where O/N2 is elevated. These are the first observations of anomalies in the thermospheric composition and ionospheric plasma associated with a strong polar vortex event.

The influence of atmospheric planetary waves on the occurrence of irregularities in the low latitude ionosphere is investigated using Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X) simulations and Global Observations of the Limb and Disk (GOLD) observations. GOLD observations of equatorial plasma bubbles (EPBs) exhibit a ~6-8 day periodicity during January-February 2021. Analysis of WACCM-X simulations, which are constrained to reproduce realistic weather variability in the lower atmosphere, reveals that this coincides with an amplification of the westward propagating wavenumber-1 quasi-six day wave (Q6DW) in the mesosphere and lower thermosphere (MLT). The WACCM-X simulated Rayleigh-Taylor (R-T) instability growth rate, considered as a proxy of EPB occurrence, is found to exhibit a ~6-day periodicity that is coincident with the enhanced Q6DW in the MLT. Additional WACCM-X simulations performed with fixed solar and geomagnetic activity demonstrate that the ~6-day periodicity in the R-T instability growth rate is related to the forcing from the lower atmosphere. The simulations suggest that the Q6DW influences the day-to-day formation of EPBs through interaction with the migrating semidiurnal tide. This leads to periodic oscillations in the zonal winds, resulting in periodic variability in the strength of the prereversal enhancement, which influences the R-T instability growth rate and EPBs. The results demonstrate that atmospheric planetary waves, and their interaction with atmospheric tides, can have a significant impact on the day-to-day variability of EPBs.        

It is well-known that equatorial plasma bubbles (EPBs) are highly correlated to the post-sunset rise of the ionosphere on a climatological basis. However, when proceeding to the daily EPB development, what controls the day-to-day/longitudinal variability of EPBs remains a puzzle. In this study, we investigate the underlying physics responsible for the day-to-day/longitudinal variability of EPBs using the Sami3 is A Model of the Ionosphere (SAMI3) and the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X). Simulation results on October 20, 22, and 24, 2020 were presented. SAMI3/WACCM-X self-consistently generated midnight EPBs on October 20 and 24, displaying irregular and regular spatial distributions, respectively. However, EPBs are absent on October 22. We investigate the role of gravity waves on upwelling growth and EPB development and discuss how gravity waves contribute to the distributions of EPBs. Of particular significance is that we found the westward wind associated with solar terminator waves and gravity waves causes midnight vertical drift enhancement and collisional shear instability, which provides conditions favorable for upwelling growth and EPB development. The converging and diverging winds associated with solar terminator waves and midnight temperature maximum also affect the longitudinal distribution of EPBs. The absence of EPBs on October 22 is related to the weak upward drift induced by weak westward wind associated with solar terminator waves.

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.

OI 630.0 nm post-sunset emission enhancement as an effect of tidal activity over low-latitudesSovan Saha1, Duggirala Pallamraju1, Sunil Kumar1, Fazlul I. Laskar2, and Nicholas M. Pedatella31 Space and Atmospheric Sciences Division, Physical Research Laboratory, Ahmedabad, India2Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA3High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO, USACorresponding author: Sovan Saha ([email protected]). Key Points:·         OI 630.0 nm post-sunset emission enhancement over low-latitudes is consistent with the presence of poleward meridional winds.·         WACCM-X simulated meridional winds show a poleward wind reversal during post-sunset hours, on occasion.·         Quarter-diurnal tides seem to play significant role in reversing the meridional winds after sunset. Abstract:   The OI 630.0 nm airglow emission variability provides salient information on the dynamical changes taking place in the upper atmosphere at around 250 km. The emission rates vary with the changes in the ambient electron densities and the neutral constituents that are associated with these emissions. On several occasions, enhancements in these emissions are observed during post-sunset hours as measured from Mt. Abu (24.6oN, 72.7oE, 19oN Mag), a low-latitude location at Indian longitudes. These enhancements occur following the typical monotonic decrease in emission intensity after sunset. The presence of poleward meridional wind was shown to be the cause for such observed emission enhancements. However, climatologically, meridional winds are equatorward during these times. In this study, the cause of such reversal in winds in the post-sunset hours has been investigated using the variation in electron densities and winds simulated by the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X), which also shows enhancements in electron densities similar to those observed in the post-sunset OI 630.0 nm nightglow emissions, and simultaneous reversal in winds as well. The amplitudes and phases of different components of tides obtained from WACCM-X meridional winds reveal a significant contribution of quarter-diurnal tides to the observed reversal in the meridional winds during post-sunset hours. The change in the tidal amplitudes is proportional to the changes in wind magnitude during that wind reversal.1. Introduction:The OI 630.0 nm (redline) nightglow emissions typically decrease monotonically after sunset along with the decrease in the incidence of solar radiation. The nightglow emission intensities continue to be small throughout the night and again start increasing towards morning twilight time. In one of our recent studies, an enhancement in OI 630.0 nm nightglow emissions after sunset was reported (Saha et al., 2021) in measurements over Mt. Abu (24.6oN, 72.7oE, 19oN Mag), a low-latitude location in the Indian longitude sector. The enhancements in nightglow emissions were observed between 20-22 hrs local time. Firstly, the strength of pre-reversal enhancement in the zonal electric field was investigated as a possible cause as it has the potential to bring in equatorial and off-equatorial plasma to the tropical latitudes, such as Mt. Abu. However, it could not satisfactorily explain the observations of enhancement in redline emissions during post-sunset time. The variation in meridional winds, however, showed that they were either poleward or that there was a cessation in the equatorward direction during those times. This observation was explained in terms of the altitudinal movement of the ionosphere to be responsible for the observed enhancements in the redline airglow. As the poleward wind contributes to a decrease in the ionospheric height (Saha et al., 2021), thereby, it provides greater reactants for the dissociative recombination mechanism to cause an enhancement in the emissions. Such a decrease in the height of the ionosphere in the nighttime has been observed during midnight hours, and it is called as the midnight temperature maximum (MTM) (e.g., Mayr et al., 1979; Herrero and Spencer, 1982; Herrero et al., 1983; Sastri and Rao, 1993; Colerico et al., 1996; Fesen, 1996; Mesquita et al., 2018). Consequently, brightness in redline emission was also enhanced during those events (Colerico and Mendillo, 2002).The upper atmosphere shows different kinds of variability associated with neutral winds (e.g., Meriwether et al., 1985; Gurubaran et al., 1995; Jyoti et al., 2004; Abdu et al., 2006; Kumar et al., 2022) and electrodynamics (e.g., Pallamraju et al., 1996; Karan et al., 2016; Karan and Pallamraju, 2017; Saha and Pallamraju, 2022). The atmospheric waves have a significant impact on the upper atmosphere at different altitudes (Vadas, 2007; Yiğit and Medevdev, 2009; Miyoshi et al., 2014; Singh and Pallamraju, 2017; Mandal et al., 2020; 2022). Optical and radio techniques have been used to characterize atmospheric waves (e.g., Oliver et al., 1994; Pallamraju et al., 2014, 2016; Mandal et al., 2019; Kumar et al., 2023a). Several model studies have also been carried out, which include the lower atmospheric forcing and describe the thermospheric variation quite satisfactorily (Roble and Ridley, 1994; Fesen, 1996; Laskar et al., 2013; Liu et al., 2018). It is known that atmospheric tides play an essential role in the variation of temperature and winds in the mesosphere and the thermosphere. Several dynamical processes that occur in the upper atmosphere have been explained using the variation in atmospheric tides (e.g., Thayaparan, 1997; Akmaev, 2001; Chau et al., 2009; Guharay and Franke, 2011; Laskar et al., 2014; Guharay et al., 2018; Pedatella and Harvey, 2022; Kumar et al., 2023b). The atmospheric tidal waves are generally generated in the lower atmosphere and they can propagate to the thermosphere where they eventually dissipate due to the molecular viscosity and thermal diffusivity. In the thermosphere, the tides can be generated in-situ as well. The influence of semidiurnal and other higher-order tides was seen during MTM (e.g., Mayr et al., 1979; Herrero et al., 1983; Fesen, 1996; Colerico and Mendillo, 2002), which caused an increase in the nightglow emission intensity during midnight and post-midnight hours. A significant magnitude of MTM was seen in simulations by the Whole Atmosphere Model, and lower atmospheric forcing was found to contribute to the MTM (Akmaev et al., 2009; Fang et al., 2016).In this work, we have used a free-running version of the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X) to understand the dynamics prevalent during the post-sunset hours over low-latitudes. Electron densities and meridional winds have been obtained from the WACCM-X at 250 km altitude (which is the altitude of the peak emission of OI 630.0 nm). Interestingly, WACCM-X simulated electron density also occasionally shows an increase after sunset, and the increase is consistent with the presence of poleward winds in the simulation. Therefore, these simulations provide an independent confirmation of the interpretation made in our earlier work that the poleward turning of the usually equatorward meridional wind, or cessation of equatorward wind during post-sunset hours is the cause for the observed enhancements in the OI 630.0 nm emissions from Mt. Abu (Saha et al., 2021). The question we ask in this study is what is the cause for the reversal of meridional wind from its usual equatorward direction at post-sunset hours over low-latitudes?2. Data used:2.1. Optical data (OI 630.0 nm nightglow emissions):An optical spectrograph, High Throughput Imaging Echelle Spectrograph (HiTIES) (Chakrabarti et al., 2001), is used to measure the nocturnal OI 630.0 nm airglow emission variability over a low-latitude location, Mt. Abu. The OI 630.0 nm nightglow emissions originate from an altitude region peaking at around 250 km, with a half-value width of around 70 km. HiTIES is a slit spectrograph, and the spectra around OI 630.0 nm nightglow emissions are imaged onto a 1K×1K CCD chip. Image processing has been carried out to remove the dark counts, scattered lights that arise due to starlight, zodiacal light, and the vignetting and Van-Rhijn effects.2.2. WACCM-X simulation:The WACCM-X is a community developed whole atmosphere model that provides simulations of the variabilities in the Earth’s atmosphere-ionosphere-thermosphere regions (Liu et al., 2018). The model captures chemical, thermodynamical, electrodynamical, ionization, and physical processes from the surface of the Earth to 500 to 700 km (depending on solar activity) altitudes. The simulation used WACCM4/CAM4 physics, as described in Marsh et al. (2013), Neale et al. (2013), and Liu et al. (2018). The chemistry used in the middle atmosphere chemistry as described in Davis et al. (2023) as well as the ionosphere-thermosphere modifications described in Liu et al. (2018). The model is capable of providing upper atmospheric neutral and ionospheric parameters and dynamics, which are coupled to the lower atmosphere. In this study, the analysis has been carried out using hourly data obtained from a free-run simulation of WACCM-X, where the lower atmospheric dynamics and day-to-day variability are internally generated by the model, for a constant solar flux value of 70 solar flux unit and Ap value of 1 to represent moderate solar activity and geomagnetic quiet conditions, to which the measured data pertains. The global variations in meridional winds, temperatures, and electron densities obtained from WACCM-X are used in this study. The variations in these parameters are analysed to understand the post-sunset emission enhancement observed in the nightglow data. The analyses and results are discussed below.3. Data analysis and results:Typically, OI 630.0 nm nightglow emissions show a monotonic decrease after sunset as the ionization stops, but the recombination process of ions and electrons continues uninterruptedly. On several occasions, an enhancement in emissions is observed around 20 to 22 LT following a decrease after sunset (Saha et al., 2021). Figure 1 shows the variations in OI 630.0 nm nightglow enhancements during post-sunset hours for the period of Jan-Mar for the years 2013 to 2016 over Mt. Abu. The thick blue line of Figure 1 shows the typical OI 630.0 nm nightglow variation for a given night in 2016. A total of 72 nights (out of 185 nights) during that period showed an enhancement in post-sunset emissions, as depicted in this figure. Note that all these days were geomagnetically quiet with Ap<20 nT. The left-side y-axis represents the emission rates (in Rayleighs) for the years 2013, 2014, and 2015, whereas, the values shown on the right-side y-axis are for the year 2016. Due to the decrease in the values of solar flux in 2016, the nightglow emission rates are relatively lower as expected, owing to reduced electron number densities. The amount of increase in emissions during post-sunset hours also varies depending on the solar flux (Saha et al., 2021) ranging from a few tens of Rayleigh to around 200 Rayleigh. The occurrence time of nightglow enhancement does not show any dependence on solar flux and seasons.

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.

We report on a new method to calibrate the FORMOSAT-7/COSMIC-2 (F7/C2) ion velocity meter (IVM) in-situ ion density data using the Tri GNSS (Global Navigation Satellite System) Radio-occultation System (TGRS) differential total electron content (TEC) data. This calibration is made using collocated measurements from the IVM and TGRS instruments on the same satellite. We found that the IVM ion density is about 8-15% lower than the TGRS derived density at the insertion orbit (~ 710 km) and 5% higher at the mission operation orbit (~ 540 km). Using a linear correction specific to orbit altitude and satellite, an adjustment is implemented for the IVM density data. These corrections remove the offsets between the IVM and TGRS density measurements. We believe the corrected densities eliminate any inter-spacecraft discrepancy in the IVM density data, making it suitable for use in multi-satellite scientific investigations of longitudinal and local time variations and space weather operational applications.

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.

Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (~0–50km) are associated with a complete reversal of the climatological wintertime westerly winds. SSWs are fundamentally caused by the breaking of planetary-scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid warming of the polar air column. This rapid warming and descent of the polar air column affects tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode (NAM), including increased frequency of cold air outbreaks over North America and Eurasia. SSWs affect the whole atmosphere above the stratosphere producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (non-ionized) particle and electron densities, and electric fields. These effects span the surface to the thermosphere and across both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal predictions. This work reviews the current knowledge on the most important aspects related to SSWs from the historical background to involved dynamical processes, modelling, chemistry and impact on other atmospheric layers.

Themospheric conditions during a minor geomagnetic event of 3 and 4 February 2022 has been investigated using disk temperature (T$_{disk}$) observations from Global-scale Observations of the Limb and Disk (GOLD) mission and model simulations. GOLD observed that the T$_{disk}$ increases by more than 60 K during the storm event when compared with pre-storm quiet days. A comparison of the T$_{disk}$ with effective temperatures (i.e., a weighted average based on airglow emission layer) from Mass Spectrometer Incoherent Scatter radar version 2 (MSIS2) and Multiscale Atmosphere-Geospace Environment (MAGE) models shows that MAGE outperforms MSIS2 during this particular event. MAGE underestimates the T$_{eff}$ by about 2\%, whereas MSIS2 underestimates it by 7\%. As temperature enhancements lead to an expansion of the thermosphere and resulting density changes, the value of the temperature enhancement observed by GOLD can be utilized to find a GOLD equivalent MSIS2 (GOLD-MSIS) simulation $\textendash$ from a set of MSIS2 runs obtained by varying geomagnetic ap index values. From the MSIS2 runs we find that an ap value of 116 nT produces a T$_{eff}$ perturbation that matches with the GOLD T$_{disk}$ enhancement. Note that during this storm the highest value of the 3 hr cadence ap was 56 nT. From the MSIS-GOLD run we found that the thermospheric density enhancement varies with altitude from 15\% (at 150 km) to 80\% (at 500 km). Independent simulations from the MAGE model also show a comparable enhancement in neutral density. These results suggest that even a modest storm could impact the thermospheric densities significantly.