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1. Solar Terminator Waves Revealed as Dominant Features of Upper Thermospheric Density

   https://doi.org/10.1029/2025GL115612  

   Fitzpatrick, D J; Sutton, E K; Pilinski, M D; Palo, S E (June 2025, Geophysical Research Letters)

   Observations of solar terminator waves (STWs) in thermospheric mass density (TMD) measurements above 500 km reveal STWs as dominant features of the upper thermosphere. While previous investigations have shown that STWs in the middle‐lower thermosphere have amplitudes on the order of 6%–8% of the background TMD in that region, this study shows that STWs exhibit a striking amplification with altitude, producing density perturbations of up to a factor of two near 500 km. The study analyzes STWs in TMD data across altitude, solar cycle, and both solstices, leveraging a direct comparative methodology with Challenging Minisatellite Payload, Gravity Recovery and Climate Experiment Follow‐On, and High‐Accuracy Satellite Drag Model. The first detection of a dawn STW with a magnitude comparable to dusk is made near 500 km, contrasting the asymmetry seen at lower altitudes. The newfound prominence of STWs highlights the need for further research into their generation mechanisms, role in geophysical variability, and broader implications for thermospheric modeling and spacecraft operations. 

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2. Were Gravity Waves or Lamb Waves Responsible for the Large‐Scale Thermospheric Response to the Tonga Eruption?

   https://doi.org/10.1029/2024av001470  

   Li, Ruoxi; Lei, Jiuhou; Zhang, Shun‐Rong; Liu, Feifan; Chen, Xuetao; Luan, Xiaoli; Meng, Xing (April 2025, AGU Advances)

   Abstract The extraordinary eruption of the Tonga volcano on 15 January 2022 lofted material to heights exceeding 50 km, marking the highest observed since the satellite era. This eruption caused significant disturbances spanning from the hydrosphere up to the thermosphere. Our recent investigation discovered the dramatic thermospheric responses at satellite altitudes. This study, however, provides physical insights into two main possible processes, secondary gravity waves (GWs) and Lamb waves, which may explain those observed large‐scale thermospheric disturbances. The comparison between the simulations and observations suggests that the MESORAC‐HIAMCM secondary GWs are consistent with GRACE‐FO measured global‐propagation thermospheric density disturbances in timing and amplitude. WACCM‐X simulations suggest that the Lamb wave can reach the thermosphere as a sharp, narrow wave packet, and may contribute about 25% to the total disturbances at 510 km. 

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3. A Survey of Atmospheric Gravity Waves Observed by GOLD During Quiet to Moderate Solar and Geomagnetic Activity

   https://doi.org/10.1029/2025JA034068  

   England, Scott L; Zhang, Shun‐Rong; Evans, Scott; Eastes, Richard W (January 2026, Journal of Geophysical Research: Space Physics)

   Abstract Atmospheric gravity waves (GWs) are believed to transport energy and momentum between different regions of the atmosphere. Historically, observations of these waves from both ground and space have been relatively abundant at altitudes up to the lower thermosphere, and somewhat less abundant in the upper thermosphere and F‐region ionosphere altitudes. Much of what is known of the typical properties and occurrence of these waves at thermospheric altitudes has been inferred from their impacts on the ionospheric density and motion, as direct observations of the neutral atmosphere have been less prevalent. Gravity waves in the middle thermosphere, from ∼120–200 km altitude, have rarely been observed directly and as such, their properties at these altitudes are less well documented. NASA's Global‐Scale Observations of the Limb and Disk (GOLD) mission makes observation of the middle thermosphere during daytime. During dedicated campaigns, GOLD has been able to observe GWs in this region. This study leverages 22 such campaigns during quiet geomagnetic conditions and low to moderate solar activity levels. Waves were observed with typical periods ∼2–4 hr. Leveraging ground‐based observations, the wavelengths were identified to be between ∼1,500–5,000 km, with phase speeds ∼150–600 m/s. The waves observed were seen to propagate primarily meridionally, in agreement with prior daytime mid‐latitude observations. Using observations of the background wind, the energy and momentum fluxes carried by these waves were found. During the quiet conditions observed, the waves were seen to transport energy flux over a wide range of latitudes. 

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4. Solar Flare‐Induced Gradient Drift Instability Observed by SuperDARN HF Radars

   https://doi.org/10.1029/2025JA033824  

   Chakraborty, S; Nishitani, N; Shi, X; Ponomarenko, P; Ruohoniemi, J M; Baker, J_B H; Coster, A J; Häggström, I (October 2025, Journal of Geophysical Research: Space Physics)

   Abstract Solar flares are a rapid increase in solar irradiance, specifically in X‐ray and Extreme Ultraviolet spectra, which enhances the ionization in the dayside ionosphere and creates Sudden Ionospheric Disturbances (SIDs). SIDs are known to create space weather impacts on traveling high frequency (HF: 3–30 MHz) radio waves, by disrupting the communication channels. In this study, we examine ionospheric scatters at dawn terminator, which stems from a severe X9.3 flare on 6 September 2017 peaked at 12:02 UT, utilizing SuperDARN HF coherent scatter radars and Global Navigation Satellite System (GNSS) Total Electron Content (TEC) observations. Specifically, we are interested in the transients in the ionospheric electrodynamics at the sub‐auroral latitude near the terminator stemming from the flare effect. Observations suggest that flare‐induced density gradient likely favors the formation of gradient‐drift instability near the dawn terminator, leading to the irregularities observed by the SuperDARN radars with line‐of‐sight (LoS) Doppler velocity reaching nearly 300 m/s. The flare amplifies the eastward TEC gradient near the dawn terminator by approximately 2–3 times compared to a geomagnetically quiet and non‐flare day. The observed irregularities, attributed to flare‐driven instabilities, exhibit a velocity consistent with the equatorial return flow of ionospheric Hall convection. In contrast to prior studies indicating decreased cross‐polar‐cap potential and associated ionospheric convection flow, our findings show the flare is followed by an increase in localized electric field near the dawn terminator, as depicted in radar LoS velocity. 

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5. Atmospheric Tides in the Thermosphere: Forcing From Above Versus Forcing From Below

   https://doi.org/10.1029/2025JA034254  

   Neogi, Mukta; Oberheide, Jens; Martinez, Benjamin; Lu, Xian (August 2025, Journal of Geophysical Research: Space Physics)

   Abstract This study investigates the relative importance of lower atmospheric versus in situ forcing in driving migrating and nonmigrating diurnal and semidiurnal tides in the thermosphere, as well as the impact of solar and geomagnetic activity on these tides. Our focus is the “thermospheric gap” region 110–250 km, where few observations are available but the tidal spectrum is known to change dramatically. We employ the Hough Mode Extension (HME) fitting approach to the Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (SD‐WACCM‐X) to separate upward‐propagating tides originating from tropospheric and stratospheric sources from those generated in situ within the thermosphere and calculate their ratio to quantify their relative importance. To assess the extent to which the solar and geomagnetic activity impact these thermospheric tides, we compare SD‐WACCM‐X simulations with control simulations in which geomagnetic (Kp) and solar (F10.7) activity are held at lower fixed values. Focusing on the year 2014, a period characterized by elevated solar and geomagnetic variability, we analyze how these external drivers modify tidal amplitudes as a function of latitude and season. The model simulations are validated by comparison with independent neutral density tide observations from the CHAllenging Minisatellite Payload data at approximately 390 km altitude from 2002 to 2004 (F10.7 = 140 sfu). The results highlight the critical role of both thermospheric and lower atmospheric forcings in shaping tidal dynamics, which has implications for forthcoming satellite missions targeting the space weather of the ionosphere‐thermosphere system and improving space operations in the very low Earth orbit. 

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