Journals
2026 EN
England Scott L. · Zhang ShunRong · Evans Scott
+1 more
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.
Journals
2026 EN
Smith C. B. · Pontius D. H.
Abstract Brooks et al. (2019), https://doi.org/10.1002/2019ja026870 developed a model to explain the observed dual‐periodicity observed in Saturn's magnetosphere. They posited that the northern and southern neutral thermospheres have different angular velocities because of unequal conductances leading to unequal coupling with the magnetosphere. In particular, currents to the hemisphere with stronger conductance should be higher than those to the other hemisphere. To do so, they applied a model developed previously to explain the relative insensitivity of the Io plasma torus at Jupiter to short‐period temporal fluctuations in the magnetosphere. In response, Cowley et al. (2020), https://doi.org/10.1029/2020ja028247 provided a valuable analysis of Cassini magnetometer data that calculated the torques exerted in each hemisphere as a function of latitude. In contrast to the predictions of Brooks et al., Cowley et al. found that the torque imbalance in the auroral zone was quite modest. In this paper we resolve the matter by solving time‐dependent versions of the original equations and arrive at results that are quite similar to the data presented by Cowley et al. A key step is removing the influence of currents that cross the auroral zone without diverging.
Journals
2026 EN
Sadler F. Brent · Lessard Marc · Otto Antonius
+1 more
Abstract Observations of neutral structuring in the cusp region have been obtained in several satellite and rocket missions. Observations include altitude‐dependent neutral density enhancements (CHAllenging Minisatellite Payload and Gravity Recovery And Climate Experiment satellites) and slight neutral density depletion at lower altitudes (Streak satellite and Rocket Experiment for Neutral Upwelling 2 (RENU2) mission). Soft electron precipitation (< a few hundred eV), ion upwelling, and ion‐neutral momentum exchange are among the processes theorized to drive neutral enhancement. We investigate these processes as they relate to neutral structuring using a high‐resolution ionosphere‐thermosphere numerical model (Alfvén wave scale) that includes inertial ion and neutral terms. A detailed study of a soft electron precipitation event is performed, and the resulting thermospheric response is assessed. Ion‐neutral coupling is found to span vertically for hundreds of kilometers for 15–20 min, enhancing the neutral density at higher altitudes. A slight depletion is found near the F region, driven by divergent ion flow near the region of the most intense electron heating. Temporal effects include an upward traveling compression wave and high‐altitude gravity waves. Thermospheric structuring occurs on a 10‐min time scale.
Journals
2026 EN
Espa S. · Lacorata G.
Abstract Self‐organization of planetary turbulence in persistent and geometrically well‐defined flow features, has been attracting, recently, great scientific interest, thanks also to the spectacular images of Saturn's hexagon provided by the Voyager and Cassini missions. These flow patterns can be replicated in laboratory experiments with shallow rotating fluids, provided some characteristic non‐dimensional parameters are suitably set up. In particular, we consider here prograde and retrograde (with respect to the rotation of the tank) hexagonal‐shaped jets. Both Eulerian and Lagrangian data, directly reconstructed from the experiments, were analyzed and discussed. Our results are consistent with the conjecture that barotropic instability plays a key role in the genesis and maintenance of the hexagonal shape. We also show that the cross‐analysis of experimental results and kinematic simulations on a simplified six‐node meandering jet model allows, in principle, to formulate a scenario about the Lagrangian dispersion properties of the hexagon on Saturn in relation to its spatial and temporal characteristic scales.
Journals
2026 EN
DuBois Ami M. · Crabtree Chris · Lichko Emily
+1 more
Abstract Non‐gyrotropic distribution functions are often observed in thin current sheets prior to magnetic reconnection. This study uses NASA's Magnetospheric Multiscale mission data to confirm a novel source of agyrotropy in compressed current sheets and highlights its significance in reconnection. Data analysis reveals a strong correlation between agyrotropy at the current sheet center and the perpendicular ambipolar electric field, which develops to maintain quasi‐neutrality as the current sheet is compressed to sub‐ion gyro‐radius scales. This agyrotropy is consistent with theory that includes the effect of a localized transverse electric field on the distribution function. The electric field affects the gyro‐plane asymmetrically through the termη = 1 + V E × B ′ / Ω $\eta =1+{V}_{E B mathit{ Omega }$ , whereV E × B ′${V}_{E B mathit is the spatial gradient of theE × B $E B$ velocity andΩ ${\Omega }$ is the cyclotron frequency. This asymmetry causes the agyrotropy, which is confirmed by data analysis. For compression such thatΩ i < V E × B ′ < Ω e Omega }}_{i}< {V}_{E B mathit lt; {{\Omega }}_{e}$ , the electron distribution function is stretched in the direction of theE × B $E B$ drift. Conventional methods for quantifying agyrotropy, based on pressure tensor contributions that are uncorrelated to the electric field, are inadequate in the current sheet center where reconnection is likely to be initiated. The perpendicular ambipolar electric field provides a measure of agyrotropy in distribution functions, which may be the most relevant indicator of reconnection.
Journals
2026 EN
Zhou Hao · Yang Jipeng · Li Yaozong
+4 more
Abstract Fluctuations in thermospheric neutral density affect the operational stability and lifetime of low Earth orbit (LEO) satellites. Solar activity and geomagnetic disturbances induce substantial density variations in the thermosphere, thereby impacting critical satellite operations such as orbit control, attitude maneuvers, and collision avoidance. However, existing empirical models fail to accurately capture these localized thermospheric density oscillations. To date, effective methods for the high‐precision prediction of LEO satellite orbital decay under varying geomagnetic conditions remain underdeveloped. This study proposes a machine learning‐enhanced method for predicting orbital decay at specified altitudes within the LEO region by making use of Gravity Recovery and Climate Experiment Level‐1B observations and integrating along‐track high‐precision thermospheric density, aerodynamic coefficients, and satellite mass parameters. During the 9–11 May 2024 storm event, along‐track thermospheric density surged, resulting in a 48‐hr semi‐major‐axis decay of approximately 168 m before stabilizing at around 83 m thereafter. For the 24 August 2005 interplanetary coronal mass ejection (ICME) case, the cumulative decay (−45.4 m) showed close alignment with the observed orbital data (−40.4 m). When independently tested across 113 ICME events, the random forest model accounted for 85% of the variance in orbital decay, achieving a test R 2 of 0.749 during all geomagnetically periods in 2005. The results demonstrate that our proposed approach delivers significantly improved prediction accuracy of satellite orbital decay across varying geomagnetic conditions compared with empirical models. This work provides novel insights into thermospheric disturbance impacts on satellite orbits and offers essential theoretical support for LEO mission planning and orbital management.
Journals
2026 EN
RomeroMinaya Jorge · Blum Lauren W. · Cantilina Jared
+4 more
Abstract The precipitation of particles into the Earth's atmosphere is a primary loss mechanism of the radiation belt for which many unknowns remain. With the growing number of space mission concepts to solve them, less expensive alternatives such as CubeSats are being used. Here we present the Double Aperture Relativistic Telescope (DART), a miniaturized solid‐state charged particle telescope whose objective is to measure differential energy particles at 180° separation. Designed for low altitude orbit, this instrument will provide measurements of energetic electrons streaming in opposite directions along Earth's magnetic field, enabling determination of precipitating particles as well as those backscattered and returning up the magnetic field line. This paper presents a description of the DART design, including a Geant4‐based analysis to characterize the instrument response and test results from a radioactive Sr‐90/Y‐90 source and atmospheric muons. Our Geant4 analysis shows that DART can measure electrons from∼ 0.6 M e V ${\sim} 0.6$ – 3.9 M e V $3.9$ and the results from the radioactive source and muon tests have verified the performance of the instrument. These measurements will allow a new scientific understanding of energy deposition by radiation belt particle precipitation to the atmosphere, which is crucial to better understanding this particle loss mechanism.
Journals
2026 EN
Yearby K. H. · Walker S. N. · Pickett J. S.
Abstract The Wideband Data instrument is part of the Cluster spacecraft Wave Experiment Consortium. Its primary data path is a direct connection to the spacecraft data handling system providing real time downlink to the ground stations of the Deep Space Network and Panska Ves Observatory. However, it was recognized during the mission design phase that this link may not always be available, especially given that simultaneous data acquisition from the four Cluster spacecraft required the use of four ground stations. Therefore, a secondary data path at reduced bit rate was included whereby the data was transferred to the Digital Wave Processor instrument and then to the spacecraft Solid State Recorder. Given that available resources were limited, both for onboard hardware and within the spacecraft assembly, integration and testing program, the design of this backup data path was less than optimal. Although it was verified during ground testing that data could be acquired via this route, the design did not make the best use of the available telemetry bandwidth, and the timing accuracy was too limited to support some multi‐spacecraft observations. This paper describes work around solutions to optimize bandwidth utilization and timing accuracy. These involve patches to the onboard software of the Digital Wave Processor instrument and ingenious signal processing on the ground.
Journals
2026 EN
Hadid L. Z. · Chust T. · Wahlund J.E.
+26 more
Abstract We report in situ evidence for Enceladus' Alfvén wing system and its coupling with Saturn's ionosphere, based on multi‐instrument observations from the Cassini spacecraft. Analysis of 36 events, including 13 from non‐flyby paths, confirms the existence of a Main Alfvén Wing (MAW) current system generated at Enceladus, and associated Reflected Alfvén Wings (RAWs) occurring both at Saturn's ionosphere and on the density gradient of Enceladus' plasma torus, extending longitudinally to at least∼ 120 ° ${\sim} 120 circ}$ ( ∼ ${\sim} $ 2,000 moon radii) downstream of the moon. Additionally, the observations reveal the systematic existence of a filamentation process of these large‐scale Alfvénic perturbations (MAW and RAWs) during their propagation at any distance from their source. These findings demonstrate a more extensive electrodynamic coupling than previously reported for Enceladus and more generally for any moon‐magnetosphere interaction. Moreover, the observation of energetic electron depletions and water‐group ion signatures at longitudes even further from the moon supports the interpretation of an extended and persistent interaction region. These results highlight Enceladus' role in shaping Saturn's magnetospheric environment and underscore the importance of future missions to exhaustively analyze this type of complex interaction between a moon and a planet.
Journals
2026 EN
Dahani Souhail · Turc Lucile · Lipsanen Veera
+8 more
Abstract Foreshock Bubbles (FBs) are large‐scale transient structures found in Earth's foreshock region and are associated with foreshock‐discontinuity interaction. FBs play a significant role in accelerating and energizing plasma through various mechanisms. In this study, we investigate the contribution of FBs to ion acceleration and energization by analyzing the key energy terms found in the equations that describe the temporal evolution of the kinetic and internal energy densities, namely, the pressure gradient term, the electromagnetic term and the pressure‐strain term. To carry out this study, we employ the global hybrid‐Vlasov simulation Vlasiator and compare our results with in situ observations from the Magnetospheric MultiScale mission. We find that FBs exhibit distinct signatures in the energy terms throughout their life cycles, from formation to decay as they interact with the bow shock. We show that the evolution of FBs involves complex energy conversions between electromagnetic, kinetic, and thermal energies. Notably, the energy term magnitudes increase during the initial phase of the FB, reach a peak, and subsequently decline as the FB dissipates, in agreement with previous studies. We find also strong energy conversion at the interface between the FB core and compressed edge due to the presence of a current sheet highlighting the complex contributions of the FB in accelerating and energizing ions.