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Electron detachment from O− is important for understanding of lightning‐induced upper atmospheric discharges. Contrary to previous studies, Rayment and Moruzzi (1978) (RM78) argue that the associative detachment reaction of O− with N2 proceeds with N2 in its ground state. Here, we analyze the experimental setup in RM78 and demonstrate that vibrationally excited N2 may have in fact contaminated the results, the theoretical approach in RM78 requires corrections, and the rate calculations provided in RM78 are inconsistent. As the vibrational temperature of N2 remains relatively low in the initial stages of gas discharges in air, i.e., streamer formation, we conclude that if in fact vibrationally excited N2 is required for the O− + N2 → N2O + e reaction to proceed, the process will happen only in later stages of the discharge, e.g., during streamer to leader transition. Controlled experiments are required to reconcile the literature on the reaction of O− with ground state N2. Plain Language Summary: The development of lightning induced transient luminous events in the middle atmosphere (i.e., sprite discharges) usually proceeds on very short time scales measured in milliseconds. The modeling of these discharges requires accounting for a set of relatively fast reactions, that includes the electron impact ionization of ambient oxygen and nitrogen molecules, the dissociative attachment of electrons and the electron impact excitation of electronic states of N2 that have relatively short lifetimes and are responsible for observed visible, infrared, and ultraviolet signatures of these events detected from various (orbital and ground based) remote sensing platforms. Here we discuss the existing knowledge on the associative detachment of electrons from O− ions obtained in previous current growth laboratory experiments under steady state conditions. This detachment process has been included in several recently published models of transient luminous events. This process may be applicable to later stages of discharge development when significant quantities of vibrationally excited nitrogen molecules accumulate. This work provides evidence that it may not be applicable during initiation of these discharges. Key Points: Applicability of the associative detachment of electrons from O− ions to modeling of lightning induced transient luminous events is discussedThe detachment rate obtained in previous steady state laboratory experiments requires correctionControlled experiments are required to reconcile the literature on the detachment reaction of O− with ground state N2 and vibrationally excited N2 [ABSTRACT FROM AUTHOR]

The balance of processes affecting electron density drives the dynamics of upper‐atmospheric electrical events, such as sprites. We examine the detachment of electrons from negatively charged atomic oxygen (O−) via collisions with neutral molecular nitrogen (N2) leading to the formation of nitrous oxide (N2O). Past research posited that this process, even without significant vibrational excitation of N2, strongly impacts the dynamics of sprites. We introduce updated rate coefficients derived from recent experimental measurements which suggest a negligible influence of this reaction on sprite dynamics. Given that previous rates were incompatible with the observed decay of the light emissions from sprite glows, our findings support that glows actually result from electron depletion in sprite columns. Plain Language Summary: Sprites are transient, filamentary luminous structures appearing between approximately 50 and 85 km above Earth's surface. While the primary sprite activity is ephemeral, lasting mere thousandths of a second, certain luminous features persist up to a hundred times longer. The key to understanding these enduring structures lies in the evolution of free electron populations that facilitate electrical conductivity. Here we show that a process that influences this population is slower than previously thought. This may explain why luminous structures can persist for so long. Key Points: Associative detachment of electrons in sprites proceeds almost exclusively from vibrationally excited N2We provide updated rate coefficients for electron associative detachmentIn models with the updated rates, sprite glows persist tens of milliseconds, in agreement with observations [ABSTRACT FROM AUTHOR]

The high‐resolution Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension (WACCM‐X) is used to study the impacts of gravity waves (GWs) on the thermospheric circulation and composition. The resolved GWs are found to propagate anisotropically with stronger eastward components at most altitudes. The dissipation of these waves in the thermosphere produces a net eastward forcing that reaches peak values between 200 and 250 km at mid‐high latitudes in both hemispheres. Consequently, the mean circulation is weakened in the winter hemisphere and enhanced in the summer, which in turn impacts the thermospheric composition. Most notably, the column integrated O/N2 in both hemispheres is reduced and agrees better with observations. The mean thermospheric GW forcing in the meridional direction has comparable amplitude and acts to modify the gradient‐wind relationship. Plain Language Summary: Small‐scale waves originate from the lower atmosphere have been shown to propagate into the thermosphere. To study their effects a high‐resolution whole atmosphere model has been employed. Using this high‐resolution model, which can partially resolve the small‐scale waves, we can directly quantify the force exerted by these waves on the general circulation in the thermosphere. We found that such force is strong, and affects the thermospheric circulation in both winter and summer hemisphere. This consequently changes the distribution of important thermospheric species. One measure of the thermospheric composition is the ratio of atomic oxygen and molecular nitrogen, which is an indicator of the relative abundance of atomic and molecular species. This ratio has been grossly over‐estimated in previous modeling studies. It is reduced as a result of the circulation change, and is much better agreement with observations. Key Points: Gravity waves (GWs) resolved by high‐resolution WACCM‐X displays anisotropic propagationGW forcing alters thermospheric circulationThe circulation change leads to a much improved thermospheric O/N2 [ABSTRACT FROM AUTHOR]

Leveraging observations by two NASA missions—GOLD (Global‐scale Observations of the Limb and Disk) and ICON (Ionospheric Connection Explorer), we investigate concurrent responses of thermospheric composition, temperatures, and neutral winds to the geomagnetic storm on 3–4 November 2021, as well as their interplay at low and middle latitudes. The synergetic observations reveal remarkable depletions up to 60%–70% in GOLD O/N2, along with large enhancements in GOLD temperatures poleward of 30° in the middle thermosphere. Meridional winds from ICON observations are altered by ∼100 m/s equatorward of 25°N latitude and at 250 km, characterized by a reversal of prevailing northward winds to geomagnetic storm‐driven southward winds. This study fills a need, after a decade‐long gap, for observing concurrent and co‐located responses of composition, temperatures, and neutral winds in the thermosphere to geomagnetic storms. Plain Language Summary: Geomagnetic storms, arising from solar wind shocks emitted from the Sun, deposit a large portion of the absorbed solar energy into the Earth's high‐latitude atmosphere through Joule heating and high energy particle heating. Accurate prediction of the responses to geomagnetic storms in the thermosphere‐ionosphere is one of the core scientific objectives of space weather. The recently launched Global‐scale Observations of the Limb and Disk (GOLD) and Ionospheric Connection Explorer (ICON) missions provides an unparalleled opportunity to assess storm‐induced concurrent changes in multiple key parameters. During a G3 geomagnetic storm on 3 November 2021, GOLD observed the substantial changes of 60%–70% in thermosphere column density ratios of atomic oxygen to molecular nitrogen and temperatures. Exceptional meridional winds of 100–200 m/s were also seen by ICON over a broad altitude range. Such observations fill a decades‐long need for simultaneous observations of the key variables in the ionosphere‐thermosphere system. Key Points: Remarkable depletions in O/N2 and enhancements in temperature, up to 60%–70%, are seen by Global‐scale Observations of the Limb and Disk during the geomagnetic stormAveraged southward wind deviations of ∼100 m/s are observed by Ionospheric Connection Explorer, coincident with the largest gradient in O/N2 depletionsBoth O/N2 and temperatures recover rapidly from the disturbed states to pre‐storm states [ABSTRACT FROM AUTHOR]

This study investigates thermospheric seasonal variations composed of annual and semiannual oscillations (AO and SAO) in the column density ratio of atomic oxygen to molecular nitrogen observed by Global‐scale Observations of Limb and Disk. On the basis of multiple analysis approaches, it is revealed that the AO is the predominant component at middle latitudes, while the SAO tends to be increasingly important toward the lower latitudes. The SAO is also evident in the southern mid‐latitudes. Quantitative analyses demonstrate that AO (SAO) peaks around winter solstices (equinoxes). Additionally, the seasonal variation at low‐latitudes exhibits greater amplitudes in the early morning and around noon, whereas it gradually weakens from early morning toward late afternoon at mid‐latitudes. A hypothesis is that seasonal variations are likely modulated by atmospheric tides, including those generated in the thermosphere and those propagating upward from the lower atmosphere. Plain Language Summary: Annual and semiannual variations are the most prominent phenomenon standing out on the seasonal scale in the thermosphere‐ionosphere system. Despite being recognized for decades, the characteristics of the AO and SAO are not yet well delineated, and relevant physics is not fully understood, due largely to limited observations and sampling constraints of low‐Earth‐orbit (LEO) satellites. For instance, distinguishing the local time versus longitude variations in the thermosphere is not feasible with LEO satellites alone. Leveraging the synoptic observations enabled by the Global‐scale Observations of Limb and Disk (GOLD) mission, we identified the AO and SAO in the column density ratios of atomic oxygen to molecular nitrogen at low and middle latitudes. More importantly, since GOLD provides full coverage of daytime local times on a daily basis, we quantified the local time dependence of seasonal variations in composition. Prevalent local time modulations could be driven by atmospheric tides. Key Points: Pronounced annual and semiannual oscillations (AOs and SAOs) are seen in Global‐scale Observations of Limb and Disk ∑O/N2AO is predominant at mid‐latitudes, and SAO is evident in both low‐latitudes and southern mid‐latitudesLocal time modulations of seasonal variations are prominent at low‐ and mid‐latitudes [ABSTRACT FROM AUTHOR]

The electronic kinetics of N2 triplet states A3Σu+, B3Πg, W3Δu, B'3Σu−, and C3Πu in the upper and middle atmosphere of the Earth during relativistic electron precipitation is considered. Intramolecular and intermolecular electron energy transfers in inelastic collisions of electronically excited molecular nitrogen with N2 and O2 molecules are taken into account in the calculations. The calculations indicate an influence of atmospheric density increase on the calculated vibrational populations of the B3Πg state. For the first time it is shown that there is a dependence of the calculated column intensity ratios of first and second positive bands on the energy of relativistic electrons. The calculated column intensity ratios I(749 nm)/I(337 nm) and I(669 nm)/I(337 nm) decrease with increasing relativistic electron energy. Key Points: The electronic kinetics of N2 triplet states in the upper and middle atmosphere during relativistic electron precipitation is consideredIntramolecular and intermolecular electron energy transfers are taken into account in calculations of vibrational populations of the statesIt is shown that there is a dependence of calculated column intensity ratios I749/I337 and I669/I337 on the energy of relativistic electrons [ABSTRACT FROM AUTHOR]

Global‐scale Observations of Limb and Disk (GOLD) disk measurements of far ultraviolet molecular nitrogen band emissions are used to retrieve temperatures (Tdisk), which are representative of lower thermospheric altitudes. The present investigation studies the response of lower thermospheric temperatures to geomagnetic activities of varying magnitudes. In this study, it has been observed that Tdisk increases over all latitudes in response to enhanced geomagnetic activity. The increase in temperature is proportional to the strength of the geomagnetic activity and is greater at higher latitudes. Temperature enhancements vary from 10s to 100s of Kelvins from low‐ to mid‐latitudes. Local time behavior shows that pre‐noon enhancements in temperatures, during relatively stronger geomagnetic activities, are greater compared to afternoon, which may be caused by the combined action of daytime dynamics and geomagnetic forcing. This study, thus, demonstrates the utility of GOLD Tdisk when investigating the effects of dynamical and external forcings in the thermosphere. Plain Language Summary: The thermosphere ionosphere system is influenced by waves from the lower atmosphere and solar and geomagnetic forcing from above. For such a coupled system it is important to decipher the relative influence of the two regimes of forcings. The recently launched Global‐scale Observations of Limb and Disk (GOLD) mission provides daytime thermospheric temperatures with unprecedented local time and spatial coverage. The thermospheric temperature over the Earth's disk visible from geostationary orbit is a first of its kind of measurement, which enables us to investigate the local time behavior over wide latitudinal coverage from 69°S to 69°N. We find that, during active geomagnetic conditions, the thermospheric temperature is enhanced across the whole visible hemisphere, with the largest temperature enhancements at higher mid‐latitudes. The local time behavior shows that the pre‐noon enhancement in temperature is greater compared to the afternoon, which demonstrates, for the first time, a systematic local time varying response of thermosphere to geomagnetic forcing. Key Points: Global‐scale Observations of Limb and Disk (GOLD) thermospheric temperature increases globally in response to geomagnetic activityThe increase in temperature is proportional to the strength of the activity and is greater at higher latitudesTemperature enhancement during active geomagnetic events is greater in the morning than that in the afternoon [ABSTRACT FROM AUTHOR]

Terrestrial gamma ray flash (TGF) observations made by the Atmosphere‐Space Interaction Monitor (ASIM) have demonstrated that these TGFs are accompanied by a prominent optical pulse from a hot leader channel. It is hard to confidently resolve the true sequence of the events in the source region due to temporal proximity of the involved processes. Here we report a bright long duration TGF together with its associated optical recordings showing clear temporal separation between the TGF and the optical pulse. In this observation the optical pulse is clearly distinct and subsequent relative to the TGF. The corresponding lightning discharge occurred at the very end of the TGF. We conclude that the current surge inside the lightning leader channel cannot be responsible for generation of this TGF. The current surge that produced the associated optical pulse can itself be conditioned by the TGF and may be responsible for the TGF termination. Plain Language Summary: TGFs observed from space are found to be associated with current surges in lightning leader channels. These current surges emit radio waves and can be detected with lightning detection networks. They also produce optical pulses which can be observed by the optical sensors on board of the space satellites. The fact that TGFs have usually short duration does not allow to define the real sequence of events in the source region due to timing uncertainties. In this paper we report a unique observation of a rare coincidence of a long duration TGF accompanied by an optical pulse and a high peak current lightning detection. Duration of the TGF is one order of magnitude larger than the overall observational uncertainty, which allows us to reliably discern the TGF and the accompanying current pulse in the leader channel. We could confidently conclude that the TGF was generated first, in the very end of the TGF the current surge in the leader channel occurred, and the optical pulse was produced. The appearance of the current surge close to the end of the TGF can indicate that the current surge is conditioned by the TGF, and, reciprocally, it could condition the TGF termination. Key Points: Accompanying optical pulse is subsequent to TGFTGF precedes the current surge in the leader channel and cannot be generated by this current surgeTGF may be terminated by the current surge in the leader channel [ABSTRACT FROM AUTHOR]

We examined three observations of green emission events (labeled as event A, B and C, respectively) associated with red sprites as captured by amateurs. In all cases, the green emissions were recorded atop of red sprite. Based on the location of causative strokes and background star fields for events A and B, their altitudes are confined between 88 and 100 km, with the maximum brightness at 90.7 and 95.5 km, respectively. Events B and C were lit up for a second time after the recurrence of a sprite element, extending their duration to approximately 1,084 ms and 732.6 ms, much longer than that (about 500 ms) for event A; the intensity of green emissions was also enhanced due to sprite recurrence. It is inferred that the recurrence of sprite elements could affect the ambient condition by further increasing electron density and strengthening the electric field for the ghost production. Plain Language Summary: Ghosts are green emissions appearing in the top part of red sprite. As the most peculiar type of transient luminous events (TLEs) above thunderstorms, ghosts were discovered just recently by amateurs with Sony digital SLR camera with relatively high sensitivity and enhanced color imaging capability. In this paper, we analyzed three green ghost events captured during the ground‐based observations by several amateurs in North America. By analyzing the parent strokes and star field information obtained for two events, ghosts were inferred to occur in the altitude range of 88–100 km, with the brightness peaked at 90.7 and 95.5 km, respectively. After examining the observations, we obtained the time‐resolved brightness of ghost events. The duration for two of these events (about 1,000 ms and 700 ms, respectively) is longer than the third one (about 500 ms), primarily due to the recurrence of a sprite element in these two events, which also resulted in a slight enhancement in the ghost brightness. Consequently, it can be inferred that recurrence of sprite elements, which is usually caused by sustained charge transfer in the parent lightning, may influence the ambient conditions by further increasing the electron density and strengthening the environmental electric field. Key Points: More observations of rare green emissions associated with red sprites near the bottom of lower ionosphere are presentedThe duration of green emission for three ghosts examined ranges from 500 to 1,084 ms, and the altitudes range between 88 and 100 kmRecurrence of sprite elements would elongate the duration of green emission and also increase the brightness of green emissions [ABSTRACT FROM AUTHOR]

This paper aims to extend the understanding of Strong Thermal Emission Velocity Enhancement (STEVE) and the Picket Fence related to strong subauroral ion drifts (SAID). We numerically demonstrated that precipitating energetic electrons are critical for the structuring of the Picket Fence. It is created by feedback‐unstable magnetosphere‐ionosphere interactions driven by the SAID electric field when the Hall conductance created by energetic (≥1 keV) electrons exceeds the Pedersen conductance. We show that thermal excitation of the red‐line emission in STEVE is inhibited by inelastic collisions with molecular nitrogen. Suprathermal (≤500 eV) electrons coming from the turbulent plasmasphere appear to be the major source. We also show that the chemiluminescencent and radiative attachment reactions do not explain the short‐wavelength part of the STEVE continuum and argue that accounting for vibrational excitation may resolve the problem. Atmosphere's upwelling due to enhanced ion‐neutral drag leads to the increased abundance of molecular components relative to atomic oxygen. Key Points: Feedback‐unstable magnetosphere‐ionosphere interaction in the presence of energetic electron precipitation explains the Picket Fence formationThermal excitation of the red‐line emission in the low‐density SAID channel is inhibited by inelastic collisions with molecular nitrogenTo understand the STEVE continuum, transitions between vibrationally excited molecular states in the SAID channel should be taken into account [ABSTRACT FROM AUTHOR]

After days of intense solar activity, active region AR3664 launched seven CMEs toward Earth producing an extreme G5 geomagnetic storm commencing at 17:05 UT on 10 May 2024. The storm impacted power grids, disrupted precision navigational systems used by farming equipment, and generated aurora seen around the globe. The storm produced remarkable effects on composition, temperature, and dynamics in the Earth's thermosphere that were observed by NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission and are reported here for the first time. We use synoptic disk images of ΣO/N2 and neutral temperature (at ∼160 km) measured by GOLD to directly link dynamics resulting from the storm with dramatic changes in thermospheric composition and temperature. We observe a heretofore unseen spatial morphology simultaneously in ΣO/N2, neutral temperature, and total electron content. Equator‐to‐pole temperature differences reach 400 K with high latitude peak neutral temperatures near 160 km exceeding 1400 K. Plain Language Summary: On Saturday 10 May 2024, the sun launched a wave of energized plasma toward the Earth. A large disturbance in the Earth's magnetic field associated with the solar wind resulted in an extreme geomagnetic storm. The storm impacted power grids, disrupted navigational systems used by farming equipment, and produced aurora seen around the globe. The storm produced remarkable effects in the Earth's upper atmosphere that were observed by NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission. In this letter, we use images measured by GOLD to directly link atmospheric dynamics resulting from the May 10–12 superstorm with dramatic changes in composition, temperature, and global circulation in the Earth's upper atmosphere. We observe previously unseen structure in the upper atmosphere associated with equator‐to‐pole temperature differences exceeding 400 K. Peak neutral temperatures near 160 km exceed 1400 K at high latitudes. Key Points: GOLD disk images of ΣO/N2 and neutral temperature link storm time dynamics with changes in thermospheric composition and temperatureWe observe a previously unseen spatial morphology in ΣO/N2, neutral temperature, and total electron contentPeak equator‐to‐pole temperature differences exceed 400 K but relax to pre‐storm conditions well before ΣO/N2 [ABSTRACT FROM AUTHOR]

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 and 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. Plain Language Summary: On Earth's nightside during the super geomagnetic storm that occurred on 10 May 2024, NASA's GOLD imager saw something new: a part of Earth's ionosphere, the southern peak of what typically appears as a double‐peaked structure in the ionospheric density at equatorial and low latitudes, merged with the aurora near the southern tip of South America. This has never been reported before. Additionally, the boundary of the aurora expanded further equatorward than usual. These observations of what happened in the Earth's ionosphere during this super storm are reported for the first time in this study. Key Points: EIA crests between ∼70° and 35°W moved poleward, with northern and southern crest reaching ∼38°N and ∼35°S Mlat in the American sectorSouthern EIA crest moved poleward with a speed of ∼450 m/s near ∼55°W Glon during strong IMF Bz and d(Dst)/dtFirst observation of the merging of an EIA crest with the aurora indicating no mid‐latitude ionosphere [ABSTRACT FROM AUTHOR]

This letter reports first simultaneous detection of F‐region plasma‐depleted structure in O(1D) 630.0 and O(1S) 557.7 nm airglow images on a geomagnetically quiet‐night (Ap = 3) of 26 June 2021 from mid‐latitude station (Hanle, India) due to enhanced thermospheric 557.7 nm emission. Since nighttime thermospheric 557.7 nm emission over mid‐latitudes is predominantly masked by significantly larger mesospheric component, F‐region plasma structures are rarely observed in 557.7 nm images. Interestingly, thermospheric 557.7 nm emission was not significant on the following geomagnetically quiet‐night as bands of medium‐scale traveling ionospheric disturbance were only observed in 630.0 nm images. Poleward wind generated by Equatorial Temperature and Wind Anomaly transported plasma from the boundary of equatorial ionization anomaly, causing significant electron density enhancement around 250 km and descent of F‐layer peak over Hanle on 26 June 2021. This amplified the dissociative recombination enabling the simultaneous detection of plasma‐depleted structure in 557.7 and 630.0 nm images. Plain Language Summary: The thermospheric O(1S) 557.7 generated through dissociative recombination of O2+ in the F‐region is significantly lower than its mesospheric counterpart which is generated via the Barth mechanism in the Mesosphere‐Lower‐Thermosphere region. This causes difficulties in the simultaneous detection of mid‐latitude F‐region plasma structures in 630.0 and 557.7 nm airglow images during geomagnetically quiet nights of low solar active years. We report, for the first time, such simultaneous detection of plasma‐depleted structure from mid‐latitude station (Hanle, India) on a geomagnetically quiet night of 26 June 2021. Interestingly, the thermospheric 557.7 nm emission was not significant on the following geomagnetically quiet night as bands of medium‐scale traveling ionospheric disturbance were detected only in the 630.0 nm images. Results from multi‐instrument data sets showed the descent of F‐layer peak and significant electron density enhancement over Hanle. The local poleward wind generated by stronger Equatorial Temperature and Wind Anomaly on 26 June 2021 transported the plasma from the boundary of the equatorial ionization anomaly region to Hanle through geomagnetic field lines. This caused the enhancement of electron density around 250 km and descent of F‐layer peak over Hanle that created suitable condition for the amplification of dissociative recombination reaction. Key Points: Simultaneous observation of mid‐latitude F‐region plasma‐depleted structure in O(1D) 630.0 and O(1S) 557.7 nm airglow imagesSignificantly higher electron density is observed over the region on 26 June 2021 than on the following night at airglow emission altitudeThermospheric O(1S) 557.7 nm emission contributed significantly due to the enhancement in the dissociative recombination reaction [ABSTRACT FROM AUTHOR]

In this work, we study the trajectories of the dust particles ejected in the geysers of the moon Enceladus, inside its Hill sphere, where its gravitational influence dominates. Our goal is to determine the properties of the particles that escape from the moon or of those that eventually impact on its surface. The analysis considers the gravity of Enceladus, the Lorentz force due to Saturn's magnetic field and the gas drag from the geysers. The local magnetic field around Enceladus is derived from an interpolation of Cassini data from six flybys. In general, we obtain that grain sizes and launching speeds define whether grains escape, though the launching angle may also play an important role in some cases. Plain Language Summary: Dust grains expelled from the geysers of the small Saturnian moon Enceladus follow particular trajectories determined by the different forces like the gravity of the moon, the gas drag of the geysers and the magnetic field of Saturn. In this work we study the dynamics of these grains inside a region around the moon, called the Hill sphere, where its gravitational influence dominates, in order to determine the effects of the different forces on the grains, as well as the properties of grains, like their size, mass and electric charge. Key Points: We study the trajectories of dust grains from the geysers of Enceladus inside the Hill sphere of the moonWe implement an interpolated magnetic field background to study the trajectories of dust grainsWe study the dynamic properties of dust grains that are able to escape from Enceladus [ABSTRACT FROM AUTHOR]

Abstract The balance of processes affecting electron density drives the dynamics of upper‐atmospheric electrical events, such as sprites. We examine the detachment of electrons from negatively charged atomic oxygen (O−) via collisions with neutral molecular nitrogen (N2) leading to the formation of nitrous oxide (N2O). Past research posited that this process, even without significant vibrational excitation of N2, strongly impacts the dynamics of sprites. We introduce updated rate coefficients derived from recent experimental measurements which suggest a negligible influence of this reaction on sprite dynamics. Given that previous rates were incompatible with the observed decay of the light emissions from sprite glows, our findings support that glows actually result from electron depletion in sprite columns.

Recent studies suggest that, despite its aurora‐like appearance, the picket fence may not be driven by magnetospheric particle precipitation but instead by local electric fields parallel to Earth's magnetic field. Here, we evaluate the parallel electric fields hypothesis by quantitatively comparing picket fence spectra with the emissions generated in a kinetic model driven by local parallel electric fields energizing ambient electrons in a realistic neutral atmosphere. We find that, at a typical picket fence altitude of 110 km, parallel electric fields between 40 and 70 Td (∼80–150 mV/m at 110 km) energize ambient electrons sufficiently so that, when they collide with neutrals, they reproduce the observed ratio of N2 first positive to atomic oxygen green line emissions, without producing N2+ ${\mathrm{N}}_{2}^{+}$ first negative emissions. These findings establish a quantitative connection between ionospheric electrodynamics and observable picket fence emissions, offering verifiable targets for future models and experiments. Plain Language Summary: The "picket fence" is a captivating visual phenomenon featuring vibrant green streaks often observed with and at lower altitudes than the rare purpleish‐white arc called STEVE (Strong Thermal Emission Velocity Enhancement). It occurs in the subauroral sky, at lower latitudes than the auroral oval, raising questions about whether it is a type of aurora or a separate phenomenon. A recent hypothesis proposes that electric fields aligned with Earth's magnetic field in the dense part of the atmosphere where the picket fence forms might energize local electrons, which collide with the neutral atmosphere to create picket fence emissions. This distinguishes the picket fence from traditional auroras caused by energetic particles accelerated higher up in space which stream down and collide with the upper atmosphere. In this study, we compare optical observations of the picket fence to a detailed calculation of the emissions produced by ambient electrons energized by parallel electric fields in the upper atmosphere. The results show that large parallel electric fields can indeed replicate the observed picket fence phenomenon. These findings offer important targets for future picket fence models and experiments. This research demonstrates that the picket fence serves as a valuable testing ground for understanding the chemistry and electrodynamics of Earth's upper atmosphere. Key Points: Observations of picket fence spectra differ quantitatively from green aurora spectral observations, suggesting different originsKinetic modeling driven by local parallel electric fields replicates picket fence spectra without requiring particle precipitationAt 110 km, parallel electric fields between 40 and 70 Townsend (∼80–150 mV/m at 110 km) reproduce observed picket fence spectra [ABSTRACT FROM AUTHOR]

The 2022 Tongan volcanic eruption released significant energy into the atmosphere. Tropospheric satellite images show that the eruption generated pressure waves that traveled globally. The Global Observation of the Limb and Disk (GOLD) mission observed significant wave‐like thermospheric temperature perturbations (>100 K) from 12 to 16 UT. These temperature perturbations' spatial curvatures and arrival times are initially similar to the tropospheric wave‐fronts but differ significantly with eastward propagation. The perturbations had a phase speed of ∼300–400 m/s and wavelengths greater than 2,400 km. Near‐concurrent Ionospheric Connection Explorer neutral wind measurements suggest that the eruption's effects reversed the direction of the prevailing thermospheric zonal winds around the perturbed regions. The eruption's global and whole atmospheric effects provide a unique opportunity to study how different atmospheric layers exchange energy and momentum during explosive events. GOLD's synoptic observations are uniquely positioned to study these effects in the middle thermosphere. Plain Language Summary: The powerful Hunga Tonga‐Hunga Ha'apai volcanic eruption on 15 January 2022, at 4:15 UT was visible from space. Satellite images in the troposphere (near 10 km altitude) show concentric pressure waves emanating from the eruption center that travel globally. Wave signatures were seen across multiple atmospheric layers. NASA's Global‐scale Observations of Limb and Disk mission observed its signatures near 150 km altitude as neutral temperature perturbations. Explosive events that affect multiple atmospheric layers allow us to observe how the different atmospheric layers exchange energy and momentum in response to an impulsive source. In this paper, we present an analysis of the eruption induced temperature perturbation in the middle thermosphere. Key Points: Global‐scale Observations of Limb and Disk mission observed global‐scale thermospheric neutral temperature perturbations (>100 K) induced by the Tongan volcanic eruptionThe phase speeds of the perturbations are around 300–400 m/s with a large wavelength (>2,400 km)The zonal winds near the peak temperature perturbation reverse from eastward to westward [ABSTRACT FROM AUTHOR]

Using observations from the Global‐scale Observations of the Limb and Disk (GOLD) mission, we investigate post‐sunset ionospheric responses to the September 2019 Antarctic sudden stratospheric warming –first ever from a synoptic perspective. Observations reveal a prevalent quasi‐6‐day periodicity in the equatorial ionization anomaly region over South America and the Atlantic, coincident with enhanced quasi‐6‐day wave (Q6DW) activity in the mesosphere (Liu et al., 2021, https://doi.org/10.1029/2020JA028909). The atmosphere‐ionosphere coupling via large‐scale waves is rarely studied over the ocean due to the lack of observations. More importantly, further analyses suggest that multiple pathways are involved in transmitting the quasi‐6‐day periodicity from the middle atmosphere into the post‐sunset F‐region ionosphere, including modulation of F‐region field aligned winds and pre‐reversal enhancements by the tides and or Q6DW. A remarkable depletion in electron density, attributable to the overall change in thermosphere composition driven by the dissipative tides and or Q6DWs, is also seen during the period of enhanced Q6DW activity. Plain Language Summary: The equatorial ionization anomaly (EIA) owes an appreciable amount of variability to the lower/middle atmosphere wave forcing of various scales. The 2019 September Antarctic sudden stratospheric warming (SSW)—rarely occurs in the southern hemisphere—provides a great opportunity to explore the effects atmospheric waves have on the EIA, as a bunch of waves gets significantly reinforced following the polar vortex disruption. Combining the observations made by the NASA GOLD and TIMED missions reveals a coincident quasi‐6‐day modulation of the post‐sunset EIA crests and mesospheric temperatures during the course of this SSW event, demonstrating a solid case that the ionosphere couples with the atmosphere via planetary‐scale waves. Key Points: A prominent quasi‐6‐day periodicity in the post‐sunset equatorial ionization anomaly (EIA) is observed during the 2019 Antarctic sudden stratospheric warming, coincident with mesospheric quasi‐6‐day waves (Q6DWs)The EIA displays an overall depletion over South America and the Atlantic when the Q6DWs are quite activeMultiple mechanisms play roles in coupling of the Q6DW with the post‐sunset EIA [ABSTRACT FROM AUTHOR]

Terrestrial gamma‐ray flashes are linked to growth of long bidirectional lightning leader system consisting of positive and stepping negative leaders. The spatial extent of streamer zones of a typical lightning leader with tip potential exceeding several tens of megavolts is on the order of 10–100 m. The photoelectric absorption of bremsstrahlung radiation generated by avalanching relativistic runaway electrons occurs efficiently on the same spatial scales. The intense multiplication of these electrons is triggered when the size of the negative leader streamer zone crosses a threshold of approximately 100 m (for sea‐level air pressure conditions) allowing self‐replication of these avalanches due to the upstream relativistic electron seeds generated by the photoelectric absorption. The model results also highlight importance of electrode effects in interpretation of X‐ray emissions from centimeter to meter long laboratory discharges, in particular, a similar feedback effect produced by generation of runaway electrons from the cathode material. Plain Language Summary: We propose a physical mechanism that explains spectacular naturally occurring bursts of X‐rays that are observed in association with lightning activity in the Earth's atmosphere. These events are commonly referred to as terrestrial gamma ray flashes (TGFs). The mechanism is based on a feedback process allowing amplification of relativistic electron avalanches when X‐rays emitted by these electrons travel backwards with respect to the electron motion and generate new relativistic electron seeds due to the photoelectric absorption in air. The presented model results agree with the observational and experimental evidence indicating that TGFs are associated with steps of negative lightning leaders and originate from relatively compact regions of space with spatial extent on the order of 10–100 m. The mechanism is not sensitive to the origin and amount of the initial runaway electrons and identical results are obtained whether the initial seeds are provided by the natural background as cosmic ray secondaries or generated by the streamer discharges. We also provide quantitative evidence that in the presence of electrodes the same amplification mechanism and X‐ray production may involve generation of runaway electrons from the cathode material. These effects may be relevant to development of new X‐ray sources. Key Points: The onset of terrestrial gamma ray flashes is linked to the spatial extent of the lightning leader streamer zoneThe photoelectric absorption is the dominant feedback factor defining inception of relativistic runaway discharges in airThe photoelectric absorption generated runaway electrons from the cathode facilitate X‐rays from laboratory sparks [ABSTRACT FROM AUTHOR]

We investigate the launch of negative upward streamers from sprite glows. This phenomenon is readily observed in high‐speed observations of sprites and underlies the classification of sprites into carrot or column types. First, we describe how an attachment instability leads to a sharply defined region in the upper part of the streamer channel. This region has an enhanced electric field, low conductivity and strongly emits in the first positive system of molecular nitrogen. We identify it as the sprite glow. We then show how, in the most common configuration of a carrot sprite, several upward streamers emerge close to the lower boundary of the glow, where negative charge gets trapped and the lateral electric field is high enough. These streamers cut off the current flowing toward the glow and lead to the optical deactivation of the glow above. Finally, we discuss how our results naturally explain angel sprites. Plain Language Summary: Sprites are large electric discharges that develop in the mesosphere. Sprites often start as one channel that later branches, leading to an intricate structure of hundreds of filaments. According to their morphology, most sprites are classified into column and carrot types. The most noticeable difference is that electrical discharge fronts in column sprites propagate exclusively downward, whereas carrot sprites also shoot upward propagating channels. Observations reveal that these upward channels emerge from glowing structures in the main channel. In this work we study the evolution of a sprite channel and propose a mechanism for the emergence of carrot sprites and angel sprites, a rare sprite variety that presents an inverted v‐shape accompanying the main downward channel. Key Points: Sprite glows emerge as bright and sharply defined regions with low conductivity and an enhanced electric fieldUpward negative streamers in carrot sprites emerge predominantly close to the lower boundary of the glowThe upward streamer channel develops a secondary glow with its upper boundary positively charged [ABSTRACT FROM AUTHOR]

Abstract This study investigates thermospheric seasonal variations composed of annual and semiannual oscillations (AO and SAO) in the column density ratio of atomic oxygen to molecular nitrogen observed by Global‐scale Observations of Limb and Disk. On the basis of multiple analysis approaches, it is revealed that the AO is the predominant component at middle latitudes, while the SAO tends to be increasingly important toward the lower latitudes. The SAO is also evident in the southern mid‐latitudes. Quantitative analyses demonstrate that AO (SAO) peaks around winter solstices (equinoxes). Additionally, the seasonal variation at low‐latitudes exhibits greater amplitudes in the early morning and around noon, whereas it gradually weakens from early morning toward late afternoon at mid‐latitudes. A hypothesis is that seasonal variations are likely modulated by atmospheric tides, including those generated in the thermosphere and those propagating upward from the lower atmosphere.

Carbon monoxide (CO) is a sensitive tracer of the thermal profile and winds in Mars' middle atmosphere and the chemistry that balances CO2 in the whole atmosphere of Mars. The Emirates Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission Hope probe images Mars at ultraviolet wavelengths from approximately 100 to 170 nm. ΣCO/CO2, the column density ratio of CO to carbon dioxide, provides a sensitive measure of CO relative variability within the Martian thermosphere. Derived from the heritage of ΣO/N2 used at Earth, the ΣCO/CO2 algorithm uses emission from the CO Fourth Positive Group band system to derive the relative column abundance of CO above ∼70 km. We describe the EMUS ΣCO/CO2 algorithm, review the Level 3 data product, and discuss preliminary validation of the algorithm. The ΣCO/CO2 algorithm produces column density ratios that characterize the spatial structure and relative variability of CO abundance in the Martian thermosphere. Plain Language Summary: The Emirates Mars Ultraviolet Spectrometer onboard the Emirates Mars Mission observes light in a wavelength region referred to as far ultraviolet. This study focuses on far ultraviolet light that is produced by carbon monoxide (CO). Observations of far ultraviolet light can be used to determine the relative abundance of CO in the upper atmosphere of Mars. Variations in the relative abundance of CO over time and across different locations can provide valuable information regarding the temperatures and winds in the middle atmosphere of Mars, as well as the chemistry that balances the whole atmosphere of Mars. This study presents a method for converting observations of far ultraviolet light into relative abundances of CO in the upper atmosphere of Mars. Key Points: Emirates Ultraviolet Spectrometer (EMUS) observations provide large scale, high cadence, synoptic views of carbon monoxide (CO) relative column abundance in the lower thermosphere of MarsSignificant variability in CO brightness and relative column abundance is regularly observed in EMUS disk imagesObserved variability of CO relative column abundance in local time, latitude, and solar longitude is inconsistent with model predictions [ABSTRACT FROM AUTHOR]

Modeling of the electrical conductivity (EC) of icy moon oceans has previously assumed that chloride, sulfate, and other ions released from rock leaching are the main solutes and carriers of EC. Here, we show that accreted volatiles, such as carbon dioxide and ammonia, can add a significant fraction of solutes in bodies whose volatile content was in part supplied from cometary materials. These volatiles can increase the EC of aqueous solutions above 1 S/m. Our salinity and EC estimates can serve as a basis for planning future magnetometer investigations at icy moons and dwarf planets. In particular, oceans expected in some of the Uranian satellites and Neptune's satellite Triton could have EC above 3 S/m as a result of accretion of both carbon dioxide and ammonia, even if rock leaching during water‐rock separation was limited, and if chlorine and sulfur abundances may be at CI carbonaceous chondritic levels. Plain Language Summary: Searching for deep oceans in icy bodies is a major driver of planetary exploration. Magnetometry is used to detect electric currents in deep oceans generated by a varying magnetic field, from a giant planet or solar wind, as was done at Jupiter's moon Europa during the Galileo mission. Previous studies have assumed that the salinity of deep oceans is determined by the leaching of major elements from accreted rock, which may yield brackish and low electrical conductivity solutions. We show that carbon dioxide, an abundant ice in the outer solar system, could contribute a significant fraction of solutes (carbonate and bicarbonate ions) in ocean worlds, regulated in particular by the presence of ammonia. The latest solar system dynamical models predict that dwarf planets and most icy moons accreted ices rich in carbon dioxide and ammonia. If oceans exist today inside these bodies, the resulting ion concentrations would produce a stronger magnetic signature than previously expected, perhaps detectable by future spacecraft missions. Key Points: Accreted CO2 and NH3 ices can significantly increase the salinity and electrical conductivity (EC) of deep oceansWater to rock ratio and ammonia abundance drive the concentrations of carbonate and bicarbonate ions in solutionPredicted EC estimates can be used to plan future ocean search missions at icy moons and dwarf planets [ABSTRACT FROM AUTHOR]

The seminal Miller‐Urey experiment suggests that lightning may have contributed to the origin of life on Earth through the formation of amino acids and carbon acids. We here focus on the early stages of lightning in the atmosphere of Primordial Earth, so‐called streamer discharges. We discuss rate coefficients for electrons and study electron avalanches and avalanche‐to‐streamer transitions by modeling the motion of electrons with a 2.5 D particle‐in‐cell Monte Carlo code in the strongly reducing atmosphere used by Miller and Urey (MU) and the weakly reducing atmospheric suggested more recently (by Kasting (1993), https://doi.org/10.1126/science.11536547) for Earth 3.8 Ga ago and compare results with conditions on Modern Earth. Our simulations show that streamers incept at fields of 140–180 Td in Kasting's mixture and at fields of ≈114 Td in the MU mixture, thus their inception is more difficult in Kasting's mixture. Conclusively, discharges on Primordial Earth might have been more challenging to incept. Plain Language Summary: In the 1950s Miller and Urey performed discharge experiments in a gas mixture resembling the atmosphere of Ancient Earth and showed that a significant amount of prebiotic material was produced, possibly laying the foundation for the further synthesis of the first biomolecules. We perform numerical computer simulations of electron avalanches in the gas mixture used by Miller and Urey as well as in a mixture suggested more recently for the composition of Ancient Earth's atmosphere 3.8 Ga ago and study the conditions needed for the inception of filamentary discharges. We calculate electron and discharge properties and compare them with results for discharges on Modern Earth. We provide a table summarizing the electric fields needed for discharge inception in these different atmospheres. Our simulations show that discharges in the Miller‐Urey mixture incept at lower fields than in Kasting's mixture and partly on Modern Earth which implies that discharges in the atmosphere of Ancient Earth might have been more challenging to incept than previously thought. Key Points: We perform simulations of electron avalanches and streamers in different gas mixtures for Primordial and Modern EarthIonization rates are higher in strongly reducing gas mixtures, such as used by Miller and Urey, than in weakly reducing mixturesIn the Miller and Urey gas mixture streamers incept at ≈114 Td; in weakly reducing atmospheres, suggested by Kasting, at 140–180 Td [ABSTRACT FROM AUTHOR]

We quantify the temperature‐dependence of the clear‐sky climate sensitivity in a one‐dimensional radiative‐convective equilibrium model. The atmosphere is adjusted to fixed surface temperatures between 280 and 330 K while preserving other boundary conditions in particular the relative humidity and the CO2 concentration. We show that an out‐of‐bounds usage of the radiation scheme rapid radiative transfer model for GCMs (RRTMG) can lead to an erroneous decrease of the feedback parameter and an associated "bump" in climate sensitivity as found in other modeling studies. Using a line‐by‐line radiative transfer model, we find no evidence for a strengthening of the longwave radiative feedback for surface temperatures between 305 and 320 K. However, the line‐by‐line simulations also show a slight decrease in climate sensitivity when surface temperatures exceed 310 K. This decrease is caused by water‐vapor masking the radiative forcing at the flanks of the CO2 absorption band, which reduces the total radiative forcing by about 18%. Plain Language Summary: The climate feedback parameter describes how the net radiative balance at the top of the atmosphere changes with surface temperature. The magnitude of the feedback parameter here depends on the current state of the climate system. For example, a warmer climate state is accompanied by a moister atmosphere which limits the climate feedback and hence increase climate sensitivity—which is the surface warming due to a doubling of CO2. Other modeling studies have shown that the climate sensitivity will first increase in a warmer reference climate, but decrease again when surface temperatures exceed 310 K. In this study, we are using a reference radiative transfer model to show how the misuse of a simplified radiation scheme can lead to this spurious signal in the estimation of the climate feedback parameter. In addition, we explain how changes in the H2O and CO2 concentrations influence the spectral distribution of both the feedback parameter and the radiative forcing. Key Points: The (negative) clear‐sky radiative feedback monotonically increases for surface temperatures between 280 and 330 KMasking effects by water‐vapor at the flanks of the CO2 band weaken the radiative forcing at high column water vaporAt present‐day CO2 concentrations Earth's climate is stable for surface temperatures up to at least 330 K [ABSTRACT FROM AUTHOR]

Blue LUminous Events (BLUEs) are transient corona discharges occurring in thunderclouds and characterized by strong 337.0 nm light flashes with absent (or weak) 777.4 nm component. We present the first nighttime climatology of BLUEs as detected by the Modular Multispectral Imaging Array of the Atmosphere‐Space Interaction Monitor showing their worldwide geographical and seasonal distribution. A total (land and ocean) of ∼11 BLUEs occur around the globe every second at local midnight and the average BLUE land/sea ratio is ∼7:4. The northwest region of Colombia shows an annual nighttime peak. Globally, BLUEs are maximized during the boreal summer‐autumn, contrary to lightning which is maximed in the boreal summer. The geographical distribution of nighttime BLUEs shows three main regions in, by order of importance, the Americas, Europe/Africa and Asia/Australia. Plain Language Summary: Blue LUminous Events (BLUEs) are transient corona discharges occurring in thunderclouds and characterized by their distinct 337.0 nm light flashes with absent (or negligible) 777.4 nm component. We present the first two year nighttime climatology of BLUEs as detected by the Modular Multispectral Imaging Array of the Atmosphere‐Space Interaction Monitor on board the International Space Station that shows distinct worldwide geographical and seasonal distributions. Key Points: The first nighttime two‐year climatology of streamer corona discharges (blue luminous events) in thunderclouds is presentedGlobally, the rate of blue luminous events at local midnight is ∼11 per secondZonal and meridional distributions of blue luminous events peak in the northern tropic and the Americas, respectively [ABSTRACT FROM AUTHOR]

The polar summer mesosphere is the Earth's coldest region, allowing the formation of mesospheric ice clouds. These ice clouds produce strong polar mesospheric summer echoes (PMSE) that are used as tracers of mesospheric dynamics. Here, we report the first observations of extreme vertical drafts (±50 ms−1) in the mesosphere obtained from PMSE, characterized by velocities more than five standard deviations larger than the observed vertical wind variability. Using aperture synthesis radar imaging, the observed PMSE morphology resembles a solitary wave in a varicose mode, narrow along propagation (3–4 km) and elongated (>10 km) transverse to propagation direction, with a relatively large vertical extent (∼13 km). These spatial features are similar to previously observed mesospheric bores, but we observe only one crest with much larger vertical extent and higher vertical velocities. Plain Language Summary: Extreme events are ubiquitous in geophysical flows. Examples of these events are tornadoes and rogue waves in the lower atmosphere and oceans, respectively. In the mesosphere, the boundary of Earth's atmosphere and outer space, extreme events can also occur, although this region is poorly observed. Here, we present the first observations of extreme vertical velocities (±50 ms−1) in the mesosphere, that are more than five times their expected standard deviation. These observations are possible by tracking and imaging strong mesospheric radar echoes that occur in the summer at polar latitudes, with a radar used in a radio camera mode. The morphology of our observations resembles previously observed instabilities called bores or wave walls, but with much larger vertical velocities and vertical extents. Key Points: First observations of extreme vertical velocities in the polar summer mesosphereThe observed solitary wave in a varicose mode resembles a mesospheric bore, with large vertical extent and vertical velocitiesSuch extreme events might have been missed or ignored in previous observations of vertical velocities or other mesospheric observations [ABSTRACT FROM AUTHOR]

We conducted observational and modeling studies of thermospheric composition and ionospheric total electron content (TEC) variations during two geomagnetically quiet periods (maximum Kp = 1.7) at solar minimum. Daytime thermospheric O and N2 column density ratio (SO/N2) observed by Global-scale Observations of the Limb and Disk and TEC from a network of ground-based Global Navigation Satellites System receivers both exhibited large (~30% of reference values) and long-lived (5-11 h) day-to-day variations in roughly the same mid-latitude geographic regions. Numerical simulations replicated the observed variability, though not perfectly. Analysis of the simulations suggested that the variations were mainly generated in the high-latitudes and were subsequently advected equatorward and westward. When high-latitudes input was turned off in simulations, the variations were negligible. This suggested the potentially important role of high-latitude geomagnetic forcing in thermospheric composition and ionospheric density variations at mid-latitudes even during some "geomagnetically quiet" periods at solar-minimum. Plain Language Summary This study presents two cases when geomagnetic forcing can be a plausible source of mid-latitude thermospheric composition and ionosphere density variations even during what is typically considered as geomagnetically quiet times (magnetic activity index Kp < 2). The column density ratio of thermospheric O and N2 (SO/N2) plays a major role in the daytime ionospheric F-region plasma density at mid-latitudes. In this study, thermospheric and ionospheric variations during geomagnetically quiet times are investigated with the two-dimensional images of SO/N2 provided by a satellite located in geostationary orbit and the ground-based total electron content (TEC) maps. Both SO/N2 and TEC displayed similar strong, long-lived and localized depletions and enhancements at mid-latitudes. Numerical simulations driven by an empirical model of geomagnetic activity, but with a climatological tide in the lower boundary, qualitatively produced the patterns of observed variations. Analysis of simulations revealed that SO/N2 variations were initially formed at high-latitudes and then transported equatorward and westward. When geomagnetic forcing was turned off in simulations, the modeled SO/N2 and TEC variations were negligible. This study suggests the potentially important roles of high-latitude forcing in thermosphere and ionosphere variations at mid-latitudes even during some "geomagnetically quiet" periods at solar-minimum. [ABSTRACT FROM AUTHOR]

We report on the first observations of atmospheric occultations at Mars of ~10 keV X‐rays from an extrasolar source identified as Scorpius X‐1. The measurements are taken by the SEP (Solar Energetic Particle) instrument on the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft. The detected X‐ray photons from Scorpius X‐1 penetrate the Martian atmosphere down to ~70 km altitude, before being attenuated by the neutrals in the atmosphere. The occultation altitude varies by a few km depending on the source X‐ray energy and the atmospheric neutral density. In this work, we study the detection response of SEP to Scorpius X‐1 X‐rays and demonstrate that X‐ray occultation data can be used in conjunction with a model of the light extinction curve in order to gain insights into the neutral density of the Mars atmosphere in the 50–100 km altitude range, an important and largely unexplored altitude range at Mars. Key Points: MAVEN/SEP at Mars detects 10–20 keV hard X‐rays from Scorpius X‐1, an extrasolar source ~9,000 light years away from the solar systemThe energy and field of view response of SEP to the measured X‐rays is studied and an X‐ray occultation case‐study is presentedThe X‐ray occultations can be used to gain insights into the neutral density of the Mars atmosphere in the 50–100 km altitude range [ABSTRACT FROM AUTHOR]

The Rocket Experiment for Neutral Upwelling 2 (RENU2) sounding rocket launched from the Andøya Space Center on 13 December 2015 into the dayside polar cusp. An ultraviolet photomultiplier tube (UV PMT) on the RENU2 payload was oriented to look up along the spin axis for emissions of neutral atomic oxygen above the payload. Data from the UV PMT have been compared to predicted auroral emissions calculated by the Global Airglow (GLOW) model. The comparison between GLOW calculations driven by RENU2 electron precipitation measurements and UV PMT data suggest enhanced neutral density in the cusp at altitudes above the RENU2 trajectory. Key Points: Elevated UV PMT signal relative to GLOW prediction indicates an enhanced neutral density population above the RENU2 trajectorySimple, low‐resource instruments like the RENU2 UV PMT can be a powerful diagnostic tool for thermospheric structurePhysics‐based models like GLOW are useful for understanding dynamic behavior in the cusp [ABSTRACT FROM AUTHOR]

Terrestrial gamma ray flashes (TGFs) are beams of high‐energy photons associated to lightning. These photons are the bremsstrahlung of energetic electrons whose origin is currently explained by two mechanisms: energizing electrons in weak, but large‐scale thundercloud fields or the acceleration of low‐energy electrons in strong, but localized fields of lightning leaders. Contemporary measurements by the Atmosphere‐Space Interactions Monitor suggest that the production of TGFs is related to the leader step and associated streamer coronae when upward moving intracloud lightning illuminates. Based on these observations, we apply a particle‐in‐cell Monte Carlo code tracing electrons in the superposed electric field of two encountering streamer coronae and modeling the subsequent photon emission. We also perform a parameter study by solving the deterministic equations of motion for one electron. We find that this mechanism can explain the occurrence of TGFs with photons energies of several MeV lasting for tens to hundreds of μs, in agreement with observations. Plain Language Summary: For more than two decades, it has been known that thunderstorms emit high‐energy X‐rays and γ rays, the so‐called terrestrial gamma ray flashes (TGFs) lasting for tens to hundreds of μs, which are the bremsstrahlung ("braking radiation") of energetic electrons and are the most energetic natural phenomena on Earth. Within the last years, two theories have been crystallized out to explain the origin of energetic electrons: the acceleration and multiplication of energetic electrons as remnants of cosmic rays in the large‐scale electric fields of thunderclouds or the acceleration of thermal electrons in high electric fields in the vicinity of the tips of lightning leaders. Contemporary measurements of the Atmosphere‐Space Interactions Monitor (ASIM) show that TGFs are produced at the onset of the main optical lightning pulse, indicating that the electron acceleration is related to the upward pointing lighting leader tip. We have performed computational simulations of the electron acceleration in the superposed electric field of two encountering streamer coronae, a compilation of small plasma channels with high‐field tips, arising in the proximity of the lightning leader tip and the upper charge layer. We find that this scenario can explain the occurrence of TGFs with energies and durations compatible with previous and contemporary measurements. Key Points: Relativistic electrons are produced during the breakdown of ICs during a current surge when two streamer coronae approach each otherThe acceleration of electrons between two streamer coronae leads to TGFs lasting for tens to hundreds of μs with photon energies of O(10 MeV)The maximum photon energy in TGFs is determined by the electric field of the upper cloud charge layer [ABSTRACT FROM AUTHOR]

We conduct observational and modeling studies of thermospheric composition responses to weak geomagnetic activity (nongeomagnetic storms). We found that the thermospheric O and N2 column density ratio (∑O/N2) in part of the Northern Hemisphere measured by Global‐scale Observations of the Limb and Disk (GOLD) exhibited large and long‐lived depletions during weak geomagnetic activity in May and June 2019. The depletions reached 30% of quiet time values, extended equatorward to 10°N and lasted more than 10 hr. Furthermore, numerical simulation results are similar to these observations and indicate that the ∑O/N2 depletions were pushed westward by zonal winds. The ∑O/N2 evolution during weak geomagnetic activity suggests that the formation mechanism of the ∑O/N2 depletions is similar to that during a geomagnetic storm. The effects of weak geomagnetic activity are often ignored but, in fact, are important for understanding thermosphere neutral composition variability and hence the state of the thermosphere‐ionosphere system. Plain Language Summary: The column density ratio of O and N2 (∑O/N2) has been used to monitor geomagnetic storm effects in the thermosphere, as well as providing valuable information about the ionosphere. This triggers an important question: Can weak geomagnetic activities cause changes in thermospheric composition too? Here, we conduct studies based on geostationary orbit observations and numerical simulations. Model outputs replicate the general morphology of this variability for the cases examined. This made it possible to understand the cause of the composition response to weak geomagnetic forcing. We found that the ∑O/N2 depletion observed was pushed westward by the zonal wind. During weak geomagnetic activity, the ∑O/N2 response is similar to the response during a geomagnetic storm, albeit it is weaker. In summary, our study suggests that weak geomagnetic activity can also generate strong and long‐lived responses in thermosphere composition during solar minimum and that this response can be important to understanding the thermosphere and ionosphere variability during the so‐called quiet times. Key Points: The observed ∑O/N2 exhibits strong and long‐lived response to weak geomagnetic activityThe numerical simulation results resemble the observed ∑O/N2 responses during weak geomagnetic activityWeak geomagnetic activity may have important effects on thermosphere‐ionosphere variability that cannot be simply ignored [ABSTRACT FROM AUTHOR]

Submicrosecond (0.476 μs per frame with an exposure time of 160 ns) high‐resolution (0.38 nm) time‐resolved spectra of laboratory‐produced lightning‐like electrical discharges have been recorded for the first time within the visible spectral range (645–665 nm). The spectra were recorded with the GrAnada LIghtning Ultrafast Spectrograph (GALIUS), a high‐speed imaging spectrograph recently developed for lightning research in the IAA‐CSIC. Unprecedented spectral time dynamics are explored for meter long laboratory electrical discharges produced with a 2.0 MV Marx generator. The maximum electron density and gas temperature measured in a timescale of ≤0.50 μs (160 ns) were, respectively, ≃1018 cm−3 and ≃32,000 K. Overpressure in the lightning‐like plasma channel, black‐body dynamics, and self‐absorption in spectral lines were investigated. Plain Language Summary: Lightning are extremely rapid and violent atmospheric electricity events taking place inside thunderclouds and between thunderclouds and the ground. Investigation of the fundamental properties of different types of lightning and lightning leaders can help to deepen our understanding on lightning propagation dynamics (stepping) and how lightning can produce high‐energy pulses of X‐ray or pulses of even higher energy (the so‐called terrestrial gamma‐ray flashes). Time‐resolved optical emission spectroscopy is an ideal diagnostic technique to remotely study the fast temporal dynamics of lightning. The fastest spectroscopic techniques used to date were able to achieve microsecond timescales. We present the first study exploring submicrosecond time regimes using the GrAnada LIghtning Ultrafast Spectrograph (GALIUS), a high‐speed imaging spectrograph recently developed for lightning research at the IAA‐CSIC. Key Points: Submicrosecond time‐resolved spectra of lightning‐like discharges are presented for the first timeSome evidence points to slight non‐LTE conditions right behind the shock front in posttrigger submicrosecond timesOptical thickness goes from thin (pretrigger times) to thick (submicrosecond) and back to thin (few microseconds) [ABSTRACT FROM AUTHOR]