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Two solar eclipse events in 2023 appeared to produce considerable enhancements in the thermospheric column density ratio of monatomic oxygen to molecular nitrogen (ΣO/N2) as measured by TIMED GUVI. We quantify potential sources for eclipse‐induced ΣO/N2 changes and find that the observed enhancements arise from the ionospheric O+ radiative recombination contribution to the OI 135.6 nm emission from which ΣO/N2 is derived. Variations in the solar Extreme Ultra Violet (EUV) and X‐ray spectrum, due to the difference between the disk spectrum and the coronal spectrum, are also considered but shown to have negligible contributions to the ΣO/N2 enhancements. After accounting for the radiative recombination contribution, we constrain the real thermospheric compositional change to the uncertainty level of the measurements of 5%–10%. These results are valuable for the interpretation of eclipse‐induced ΣO/N2 changes that will further first‐principle model comparisons and lead to a better understanding of the response of the thermosphere to localized variations in solar EUV and X‐ray forcing. Plain Language Summary: The Thermosphere Ionosphere Mesosphere Energetics and Dynamics' Global Ultraviolet Imager provides an estimate of the column ratio of monatomic oxygen (O) to molecular nitrogen (N2) in the upper atmosphere (ΣO/N2) as a data product under normal, sunlit conditions. Eclipses are special conditions. In this paper, we model the measured ultraviolet emissions from O and N2 that go into the retrieval algorithm to understand why this increase in the radiance ratio occurs within the totality of the eclipse. There are two components to the O emission, the glow created by electrons (photoelectrons) that result from the ionization of the upper atmosphere and the glow created by recombination of ionospheric O+ ions. We find that the photoelectron source decreases significantly during the eclipse, but near the magnetic equator, the ionospheric recombination emission does not, which causes the retrieval algorithm to overestimate the ΣO/N2 column density ratio during eclipses. The observations and models are consistent with this explanation. Key Points: The apparent thermospheric ΣO/N2 enhancements (10%–30%) for two eclipse events are found to be predominantly of ionospheric originVariations in the solar spectrum due to wavelength‐dependent attenuation are found to have a negligible impact on ΣO/N2 during the eclipseThe true eclipsed‐induced thermosphere compositional changes are determined to be within 5%–10% of the pre‐eclipse values [ABSTRACT FROM AUTHOR]

STEVE, Strong Thermal Emission Velocity Enhancement, was often observed by ground‐based imagers in visible wavelengths and rarely detected by global FUV imagers. We present a new event, and revisit two reported STEVE events, that were observed by the DMSP/SSUSI FUV imager at O 135.6 nm, N2 LBHS, and LBHL emissions. Coincident particle and plasma drift observations showed that the events were associated with high ion drift speed, low ion density and no energetic particle precipitation. This is consistent with earlier findings (e.g., MacDonald et al., 2018, https://doi.org/10.1126/sciadv.aaq0030; Gallardo‐Lacourt et al., 2018a, https://doi.org/10.1029/2018ja025368, 2018b, https://doi.org/10.1029/2018gl078509; Archer et al., 2019). In this paper, several new features are identified: (a) Detection rates of FUV STEVE are much lower than that of visible STEVE; (b) The STEVE on 27 March 2008 covers 3‐hr local time with a length up to 2,700 km around 60° magnetic latitudes; (c) Different widths of STEVE observed in O 135.6 nm and N2 LBH images indicate a large altitude range in the STEVE FUV emissions; (d) The true ion (assuming O+) drift speeds during the 27 March 2008 STEVE event could be up to 20 km/s, well above the DMSP SSIES sensor limit. The kinetic energy of the high speed ions is larger than the excitation potential of the observed FUV emissions; (e) The requirement of such extreme high ion drift speed explains why FUV STEVE have been rarely observed, compared to the visible STEVE; (f) The three FUV STEVE events occurred during moderate geomagnetic activity; (g) Theses events were conjugate in both hemispheres. Plain Language Summary: STEVE, Strong Thermal Emission Velocity Enhancement, was discovered by ground‐based imagers in visible wavelengths. We present examples of STEVE detected by FUV imager at O 135.6 nm, N2 LBHS (140–150 nm), and LBHL (165–180 nm) emissions. These FUV STEVE events were associated with high plasma drift speed and low plasma density in the ionosphere, and no energetic particle precipitation. These FUV STEVE events show several new features: (a) Detection rates of FUV STEVE are much lower than that of visible STEVE; (b) The STEVE length is up to 2,700 km; (c) There is a large altitude range of the FUV emission; (d) The true ion (assuming O+) drift speeds could be up to 20 km/s. Ions with such a high drift speed would carry sufficient kinetic energy to excite FUV emissions; (e) The FUV STEVE events occurred during moderate geomagnetic activity; and (f) The FUV STEVE events were conjugate in both hemispheres. Key Points: In addition to the visible emissions, STEVE is also observed in far ultraviolet emissionsThe STEVE on 27 March 2008 has a horizontal length up to 2,700 km and a large altitude extensionEstimated O+ ion drift speed for some STEVE events is ∼20 km/s and the ions' kinetic energy is greater than the FUV excitation potential [ABSTRACT FROM AUTHOR]

A comparison between storm‐time TIMED/GUVI FUV spectra in thermospheric composition disturbed (O/N2 <∼140 km> depleted and NO <∼110 km> enhanced) and undisturbed regions reveals the following features: (1) A clear decrease in N2 LBHS (140–150 nm) band intensities compared to that between 150 and 160 nm; (2) A significant spectral decrease around O 164.1 nm; (3) Two enhanced peaks between 170 and 185 nm. The feature (2) is clearly due to atomic oxygen density decrease in the low thermosphere. The feature (3) is caused by storm‐time production and transport of nitric oxide. An AURIC simulation reproduced feature (1) confirming that storm‐time enhancement in O2 density (∼120–170 km) and the wavelength dependent O2 absorption cross section are the source of observed feature (1). On the other hand, the feature (1) was not observed in the regions without O/N2 depletion. This further supports that O2 density enhancement is responsible for feature (1). Plain Language Summary: Observations by a FUV spectrograph imager on NASA TIMED satellite revealed two interesting features in the storm‐time disturbed thermosphere: (1) a relative decrease in N2 LBH intensities at 140–150 nm versus 150–160 nm, (2) significant decrease in O 164.1 nm radiance and enhanced emission between 170 and 185 nm. The feature (2) is well known and due to O density depletion and production of nitric oxide. The feature is not well understood. A model simulation indicates the feature (1) is due to storm‐time O2 density increase and wavelength dependent O2 absorption cross section which peaked around 140 nm. This result indicates that FUV spectrograph imager is capable to detect storm‐time O2 density changes. Key Points: (1) FUV spectral difference between storm‐quiet time shows a relative decrease in N2 LBH intensities at 140–150 nm versus 150–160 nmThe FUV spectrograph difference between undisturbed regions did not show the above feature(3) Simulations confirm indicate that enhanced O2 density and absorption causes the observed FUV spectral differences [ABSTRACT FROM AUTHOR]

The polar cap can become teardrop shaped through the poleward expansion of the dusk and dawn sectors of the auroral oval, to form what is called horse collar aurora (HCA). The formation of HCA has been linked to dual‐lobe reconnection (DLR) where magnetic flux is closed at the dayside magnetopause. A prolonged period of northward IMF is required for the formation of HCA. HCA have previously been identified in UV images captured by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument on‐board the Defense Meteorological Satellite Program (DMSP) spacecraft F16, F17 and F18. Events that have concurrent 630.0 nm all‐sky camera (ASC) data from the Redline Geospace Observatory (REGO) Resolute Bay site are now studied in more detail, making use of the higher cadence of the ASC images compared to DMSP/SSUSI. 11 HCA events are studied and classified based on the IMF conditions at the end of the event. Five of the events were found to end via a southward turning of the IMF, two end with positive By dominated IMF and four with negative By dominance. Under positive (negative) By the arcs move duskward (dawnward) in the northern hemisphere with the opposite true in the southern hemisphere. Under a southward turning the arcs move equatorward. One event is of particular interest as it occurred while there was a transpolar arc (TPA) also present. Understanding the evolution of HCA will allow DLR to be studied in more detail. Plain Language Summary: Horse collar auroras (HCA) form when the auroras move to high latitudes at dawn and dusk resulting in a teardrop‐shaped polar cap. When the interplanetary magnetic field (IMF) embedded in the solar wind is directed almost exactly northwards a process called dual‐lobe reconnection can occur which closes open magnetic flux on the dayside of the magnetosphere and has been proposed to be linked to the formation of HCA. We investigate 11 events in further detail that have concurrent data from Defense Meteorological Satellite Program and the 630.0 nm all‐sky camera from the Redline Geospace Observatory. The events are separated based on the IMF conditions at the end of the event which appears to determine their subsequent evolution. The IMF turns southward at the end of 5 events, for the other 6 the IMF becomes dominated by a dawnward (2 events) or a duskward (4 events) component. One event is of special interest as the HCA occurs alongside a transpolar arc, where a single band of aurora is seen poleward of the main auroral oval. Studying the evolution of HCA and the solar wind conditions under which they evolve allows us to gain new insights into the conditions necessary for DLR. Key Points: Horse collar aurora evolution is dependant on changes in the interplanetary magnetic field orientationHorse collar aurora evolution is consistent with existing modelsTranspolar arcs and horse collar aurora can occur simultaneously [ABSTRACT FROM AUTHOR]

We model an interval of remarkable interplanetary magnetic field (IMF), for which we have a comprehensive set of observational data. This interval is associated with the arrival of an interplanetary coronal mass ejection. The solar wind densities at the time are particularly high and the IMF is primarily northward over many hours. This results in strong auroral emissions within the polar cap in a cusp spot, which we associate with lobe reconnection at the high‐latitude magnetopause. We also observe areas of upwards field‐aligned current (FAC) within the summer Northern Hemisphere polar cap that exhibit large current magnitudes. The model can reproduce the spatial distribution of the FACs well, even under changing conditions in the incoming IMF. Discrepancies exist between the modeled and observed current magnitudes. Notably, the winter Southern Hemisphere exhibits much lower current magnitudes overall. We also model a sharp transition of the location of magnetopause reconnection at the beginning of the interval, before the IMF remained northward for many hours. The reconnection location changed rapidly from a subsolar location at the low‐latitude magnetopause under southward IMF conditions, to a high‐latitude lobe reconnection location when the field is northward. This occurs during a fast rotation of the IMF at the shock front of a magnetic cloud. Plain Language Summary: Under extreme incoming interplanetary magnetic field conditions following the impact of an Interplanetary Coronal Mass Ejection (CME) on the Earth's system, we observe a range of phenomena in the Northern Hemisphere ionosphere. This includes auroral emissions in the form of a cusp spot and associated precipitating particles, ionospheric flows, and strong field‐aligned currents (FACs) in the high‐latitude polar cap. These phenomena change in orientation and strength following variations in the incoming solar wind. We model the state of the magnetosphere during these observations. The modeled currents correspond well spatially with the observed currents, however, the current magnitudes are very different. The modeled FACs indicate that the site of magnetic reconnection can change rapidly from a lower‐latitude dayside position to a high‐latitude location in the magnetospheric lobes, which is reflected in field orientation within the magnetic cloud associated with the passing CME. Key Points: We model an interval of interplanetary BY‐dominated field and high solar wind densities during the impact of a Coronal Mass EjectionThe reconnection site moves rapidly from the subsolar to the high‐latitude magnetopause during a rotation of interplanetary magnetic fieldModeled and observed currents are spatially consistent in the polar cap although modeled to observed current magnitudes are often discrepant [ABSTRACT FROM AUTHOR]