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Previous studies have shown that Strong Thermal Emission Velocity Enhancement (STEVE) events occur at the end of a prolonged substorm expansion phase. However, the connection between STEVE occurrence and substorms and the global high‐latitude ionospheric electrodynamics associated with the development of STEVE and non‐STEVE substorms are not yet well understood. The focus of this paper is to identify electrodynamics features that are unique to STEVE events through a comprehensive analysis of ionospheric convection patterns estimated from SuperDARN plasma drift and ground‐based magnetometer data using the Assimilative Mapping of Geospace Observations (AMGeO) procedure. Results from AMGeO are further analyzed using principal component analysis and superposed epoch analysis for 32 STEVE and 32 non‐STEVE substorm events. The analysis shows that the magnitude of cross‐polar cap potential drop is generally greater for STEVE events. In contrast to non‐STEVE substorms, the majority of STEVE events investigated are accompanied by with a pronounced extension of the dawn‐cell into the pre‐midnight subauroral latitudes, reminiscent of the Harang reversal convection feature where the eastward electrojet overlaps with the westward electrojet, which tends to prolong over substorm expansion and recovery phases. This is consistent with the presence of an enhanced subauroral electric field confirmed by previous STEVE studies. The global and localized features of high‐latitude ionospheric convection associated with optical STEVE events characterized in this paper provide important insights into cross‐scale magnetosphere‐ionosphere coupling mechanisms that differentiate STEVE events from non‐STEVE substorm events.

 

This study presents multi‐instrument observations of persistent large‐scale traveling ionosphere/atmospheric disturbances (LSTIDs/LSTADs) observed during moderately increased auroral electrojet activity and a sudden stratospheric warming in the polar winter hemisphere. The Global Ultraviolet Imager (GUVI), Gravity field and steady‐state Ocean Circulation Explorer, Scanning Doppler Imaging Fabry–Perot Interferometers, and the Poker Flat Incoherent Scatter Radar are used to demonstrate the presence of LSTIDs/LSTADs between 19 UT and 5 UT on 18–19 January 2013 over the Alaska region down to lower midlatitudes. This study showcases the first use of GUVI for the study of LSTADs. These novel GUVI observations demonstrate the potential for the GUVI far ultraviolet emissions to be used for global‐scale studies of waves and atmospheric disturbances in the thermosphere, a region lacking in long‐term global measurements. These observations typify changes in the radiance from around 140 to 180 km, opening a new window into the behavior of the thermosphere.

 

Far ultraviolet (FUV) imaging of the aurora from space provides great insight into the dynamic coupling of the atmosphere, ionosphere, and magnetosphere on global scales. To gain a quantitative understanding of these coupling processes, the global distribution of auroral energy flux is required, but the inversion of FUV emission to derive precipitating auroral particles' energy flux is not straightforward. Furthermore, the spatial coverage of FUV imaging from Low Earth Orbit (LEO) altitudes is often insufficient to achieve global mapping of this important parameter. This study seeks to fill these gaps left by the current geospace observing system using a combination of data assimilation and machine learning techniques. Specifically, this paper presents a new data‐driven modeling approach to create instantaneous, global assimilative mappings of auroral electron total energy flux from Lyman‐Birge‐Hopfield (LBH) emission data from the Defense Meteorological System Program (DMSP) Special Sensor Ultraviolet Spectrographic Imager (SSUSI). We take a two‐step approach; the creation of assimilative maps of LBH emission using optimal interpolation, followed by the conversion to energy flux using a neural network model trained with conjunction observations of in‐situ auroral particles and LBH emission from the DMSP Special Sensor J and SSUSI instruments. The paper demonstrates the feasibility of this approach with a model prototype built with DMSP data from 17 February 2014 to 23 February 2014. This study serves as a blueprint for a future comprehensive data‐driven model of auroral energy flux that is complementary to traditional inversion techniques to take advantage of FUV imaging from LEO platforms for global assimilative mapping of auroral energy flux.

 

The dayside equatorial ionospheric electrodynamics exhibit strong variability driven simultaneously by highly changeable external forcings that originate from the solar extreme ultraviolet (EUV), magnetosphere, and lower atmosphere. We investigate this variability by carrying out comprehensive data‐driven ensemble modeling using a coupled model of the thermosphere and ionosphere, with the focus on the verticalE×Bdrift variability during a solar minimum and minor storm period. The variability of verticalE×Bdrift in response to the changes and uncertainty of primary forcings (i.e., solar EUV, high‐latitude plasma convection and auroral particle precipitation, and lower‐atmospheric tide and wave forcing) is investigated by ensemble forcing sensitivity experiments that incorporate data‐driven stochastic perturbations of these forcings into the model. Second, the impact of assimilating FORMOsa SATellite‐3/Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT‐3/COSMIC) electron density profiles (EDPs) on the reduction of uncertainty of the modeled verticalE×Bdrift variability resulting from inadequately specified external forcing is revealed. The Communication and Navigation Outage Forecasting System (C/NOFS) ion drift velocity observations are used for validation. The validation results support the importance of the use of a data‐driven forcing perturbation methods in ensemble modeling and data assimilation. In conclusion, the solar EUV dominates the global‐scale day‐to‐day variability, while the lower atmosphere tide and wave forcing is critical to determining the regional variability. The modeled verticalE×Bdrift is also sensitive to the magnetospheric forcing. The ensemble data assimilation of FORMOSAT‐3/COSMIC EDPs helps to reduce the uncertainty and improves agreement of the modeled verticalE×Bdrifts with C/NOFS observations.

 

We determine the primary modes of field‐aligned current (FAC) variability and their hemispheric asymmetry by nonlinear regression analysis of a multiyear global data set of Iridium constellation engineering‐grade magnetometer data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment program. The spatial and temporal FAC variability associated with three major categories of solar wind drivers, (1) slow flow, (2) high‐speed streams (HSS), (3) transient flow related to coronal mass ejections (CMEs), and (4) a combination of these, is characterized as empirical orthogonal functions (EOFs) and their time‐varying amplitude. For the combined solar wind category, the order of the modes of variability are strengthening/weakening of (1) EOF1—all FACs; (2) EOF2—Region 2 (R2) FACs; and (3) EOF3—dayside/nightside FACs. The first two EOFs are associated with solar wind coupling; EOF3 is associated with the ecliptic components of the interplanetary magnetic field (IMF). We also find hemispheric asymmetry in FACs. Northern Hemisphere EOFs show clearer spatial features and higher correlation coefficients with solar wind drivers. The Northern Hemisphere also shows higher correlation coefficients in all seasons except winter. We find transient flow EOFs to be better correlated with solar wind drivers such as IMFB_zand coupling functions, while HSS EOFs are better correlated with solar wind plasma parameters. CME‐related transient flow EOFs also show R2 FAC variabilities that are not found in other separate wind drivers. Application of the EOF analysis to the Iridium magnetometer data shows significant promise for greater understanding of geoeffectiveness of solar wind interactions with geospace.

 

We present examples of high‐latitude field‐aligned current (FAC) and toroidal magnetic potential patterns in both hemispheres reconstructed at a 2‐min cadence using an updated optimal interpolation (OI) method that ingests magnetic perturbation data provided by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) program. A solstice and an equinoctial event are studied to demonstrate the reconstructed patterns and to provide scientific insights into FAC response to different solar wind drivers. For the 14 June 2011 high‐speed stream event with mostly northwardB_zdriving, we found persistently stronger FACs in the Northern Hemisphere. Extreme interhemispheric asymmetry is associated with the interplanetary magnetic field (IMF) direction and large dipole tilt, consistent with earlier studies. FAC asymmetries seen during an isolated substorm can be attributed to dipole tilt. During relatively low geomagnetic activity, the FAC response to IMFB_xchanges is identified. For the 17–18 March 2013 period, we provide global snapshots of rapid FAC changes related to an interplanetary shock passage. We further present comparisons between instantaneous and mean behaviors of FAC for the solar wind sheath passage and interplanetary coronal mass ejection southwardB_zinterval and northwardB_zintervals. We show that (1) sheath passage results in strong FAC and high variation in the dayside polar cap region and pre‐midnight region, different from the typical R1/R2 currents during prolonged southwardB_z; (2) four‐cell reverse patterns appear during northwardB_zbut are not stable; and (3) persistent dawn‐dusk asymmetry is seen throughout the storm, especially during an extreme substorm, likely associated with a dawnside current wedge.

 

Abstract. This paper presents a new technique to derive thermospheric temperature from space-based disk observations of far ultraviolet airglow. The technique, guided by findings from principal component analysis of synthetic daytime Lyman–Birge–Hopfield (LBH) disk emissions, uses a ratio of the emissions in two spectral channels that together span the LBH (2,0) band to determine the change in band shape with respect to a change in the rotational temperature of N2. The two-channel-ratio approach limits representativeness and measurement error by only requiring measurement of the relative magnitudes between two spectral channels and not radiometrically calibrated intensities, simplifying the forward model from a full radiative transfer model to a vibrational–rotational band model. It is shown that the derived temperature should be interpreted as a column-integrated property as opposed to a temperature at a specified altitude without utilization of a priori information of the thermospheric temperature profile. The two-channel-ratio approach is demonstrated using NASA GOLD Level 1C disk emission data for the period of 2–8 November 2018 during which a moderate geomagnetic storm has occurred. Due to the lack of independent thermospheric temperature observations, the efficacy of the approach is validated through comparisons of the column-integrated temperature derived from GOLD Level 1C data with the GOLD Level 2 temperature product as well as temperatures from first principle and empirical models. The storm-time thermospheric response manifested in the column-integrated temperature is also shown to corroborate well with hemispherically integrated Joule heating rates, ESA SWARM mass density at 460 km, and GOLD Level 2 column O/N2 ratio. 

Polar ionospheric electrodynamics plays an important role in the Sun–Earth connection chain, acting as one of the major driving forces of the upper atmosphere and providing us with a means to probe physical processes in the distant magnetosphere. Accurate specification of the constantly changing conditions of high-latitude ionospheric electrodynamics has long been of paramount interest to the geospace science community. The Assimilative Mapping of Ionospheric Electrodynamics procedure, developed with an emphasis on inverting ground-based magnetometer observations for historical reasons, has long been used in the geospace science community as a way to obtain complete maps of high-latitude ionospheric electrodynamics by overcoming the limitations of a given geospace monitoring system. This Chapter presents recent technical progress on inverse and data assimilation procedures motivated primarily by availability of regular monitoring of high-latitude electrodynamics by space-borne instruments. The method overview describes how electrodynamic state variables are represented with polar-cap spherical harmonics and how coefficients are estimated from the point of view of the Bayesian inferential framework. Some examples of the recent applications to analysis of SuperDARN plasma drift, Iridium, and DMSP magnetic fields, as well as DMSP auroral particle precipitation data are included to demonstrate the method.