Abstract The Daily Atmospheric Ionospheric Limb Imager (DAILI) (Daily Atmospheric and Ionospheric Limb Imager) mission utilized a 6U CubeSat, deployed at about 410 km altitude in a 51‐degree orbit, to make simultaneous daytime limb measurements in two colors; at 630 nm to measure the OI (630 nm) redline and at 765 nm to measure the Atmospheric (0,0) band (O2A band). The ratio of the O2A band to the redline allows a retrieval of the absolute daytime density from ∼140–200 km altitude using a technique demonstrated on the Remote Atmosphere and Ionosphere Detection System (RAIDS) experiment in 2009 (Hecht et al., 2015, https://doi.org/10.1002/2014gl062355 ). Daily Atmospheric Ionospheric Limb Imager required the development of a new Earthshade that would fit into the 6U form factor and would, while observing the limb at 140 km and above, suppress scattering by a factor at or above 1 million from both the daylit Earth's surface and from clouds, and this capability is demonstrated on‐orbit. Over 7200 limb profiles were obtained, many of which were geomagnetically quiet but several of which occurred during a Kp geomagnetic storm where the NASA GUVI (Global UV Imager) instrument obtained O/ data. This allowed a determination of the extent of variations of compared to the GUVI O/ data during a large storm at altitudes below 200 km. The DAILI data for this storm are not reproduced by current models. Both the DAILI and GUVI data are consistent with significant equatorward transport of heated air from the auroral zone compared to model predictions.
Abstract A distinct echo structure associated with E‐region field‐aligned irregularities (FAIs) was observed over Qujing (25.6°N, 103.7°E) on 19 May 2023 by a VHF coherent scatter radar. In the altitude–time–intensity (ATI) plot, the echoes exhibited pronounced quasi‐periodic features, and two different echo morphologies appeared after 23:30 LT on 19 May. The higher‐altitude echoes retained clear striation‐like structures, whereas the lower‐altitude echoes gradually evolved into a continuous descending layer. Simultaneously, ionosonde observations showed obvious increases in the Es‐layer critical frequency (foEs) and quasi‐periodic variations in the frequency spread Δf (ftEs − fbEs). Meteor radar observations showed strong and time‐evolving zonal wind shear in the 80–90 km altitude range, and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature perturbation analysis indicated significantly enhanced gravity wave activity above 70 km on the same day. Furthermore, the Atmospheric Infrared Sounder (AIRS) satellite observations revealed intense convective activity in the troposphere southwest of Qujing, accompanied by a northeastward‐propagating brightness temperature perturbation pattern. These results suggest that the background winds modulated by the gravity waves generated by the deep convection in the troposphere played an important role in driving Es layer structuring and E‐region FAI generation in the ionosphere.
Abstract The plasmapause (PP) marks the transition from a dense and cold plasma to a tenuous and hot plasma in the inner magnetosphere. Several electrodynamic processes are taking place at this steep electron density gradient. Here we make use of the high‐resolution magnetic field recordings by the Swarm A and C spacecraft for investigating broad‐band signal bursts in the 4–10 Hz frequency range. In more than 80% of the cases, when the Swarm passes the L‐shell bounding the plasmasphere on its low‐Earth orbit, an enhanced broad‐band signal is observed in the transverse field components. The actual location of the plasmapause is derived from in situ plasma measurements of the Arase satellite in the magnetosphere. The broad‐band signals around midnight are different from those near noon. During the dark hours in ∼90% of cases, a broad‐band signal is found right outside the PP continuing far into the magnetosphere. Its characteristic agrees well with the large‐amplitude kilometer‐scale FACs described recently by Zhou and Lühr (2025), https://doi.org/10.5194/angeo‐43‐667‐2025 . Different from that, around the noon sector broad‐band bursts appear rather confined in the L range (∼0.03 Re) and about 0.5 Re inward from the PP. The bursts observed on the dayside seem to represent resonances of preexisting waves, which suffer a chaotic breakdown after running into saturation. Opposed to the nightside, daytime broad‐band signal bursts are encountered only on about 80% of the passes. This implies that the resonance does not exist everywhere along the dayside PP. On the dawn and dusk sides, both types of broad‐band signatures can be found.
Abstract Anomalous electron heating (AEH) in the auroral E region is widely attributed to Farley–Buneman turbulence, yet long‐standing inconsistencies remain among observed and modeled electron temperature–electric field relationships. We develop an AEH model that links (a) a heuristic nonlinear saturation theory for the parallel turbulent electric field, constrained by rocket measurements, with (b) a Boltzmann‐equation calculation of the resulting non‐Maxwellian electron distribution and electron temperature, including realistic inelastic electron–neutral cooling. The model is evaluated using 505 AEH events from a 14‐year Poker Flat Incoherent Scatter Radar database and is consistent with the observed electron‐temperature response over a broad range of driving electric fields. By combining an observationally motivated saturation constraint with kinetic treatment of non‐Maxwellian effects, the model narrows discrepancies between prior observations and theories, and provides a practical framework for predicting AEH in space weather applications.
Abstract Projection‐matrix generation is an essential prerequisite for ionospheric tomographic inversion, and its computational speed determines the practicality of high‐resolution tomography. We developed and tested an algorithm that combines a fast ray‐voxel intersection test with the spherical‐triangle method. The algorithm was evaluated against the conventional formulation, the parametric‐equation approach, and the segmentation intersection and voxel tracking (SIVT) approach using two European GNSS epochs and voxel grids at different resolutions. Within these tested configurations, the spherical‐triangle method alone performed well for coarse grid configurations, whereas the combined “fast intersection test scheme II + spherical‐triangle method” gave the shortest runtime for the finest grid resolution. At a resolution of 0.1 (longitude) × 0.1 (latitude) × 1 km (altitude), the combined method completed the computations in approximately 3.88 and 8.45 s for 9,225 and 17,081 valid rays, respectively, corresponding to a 40%–50% reduction relative to using parametric equations alone in the same test setting. With five‐thread parallelization, the processing time was further reduced to 1.31 and 2.62 s. These results indicate that the proposed workflow can reduce the cost of projection‐matrix generation for the tested ionospheric tomography configurations without altering the numerical results.
Abstract We develop a semi‐empirical model for high latitude EUV‐generated conductance that combines first‐principles results from GLOW with empirical calibration using strictly screened incoherent scatter radar‐satellite (DMSP) conjunctions (with both electron and ion precipitation energy fluxes less than 0.1 milliwatt per square meter). GLOW conductances are emulated with a compact neural network driven by solar elevation angle, F10.7, Ap, day of year, and geographic latitude. A uniform correction of 0.8 S is applied to both Pedersen and Hall conductances to bring the model into agreement with radar‐satellite conjunction measurements. Evaluation against a multi‐decade, six‐radar data set shows close agreement with observations across sites and time, and substantial reduction of the high‐latitude bias present in Moen and Brekke (1993, https://doi.org/10.1029/92GL02109 ) model (MB93). Further analysis of MB93 shows that the database used in that study does not fully remove precipitation effects. In comparison, our approach uses radar‐satellite conjunctions to fully separate EUV from precipitation events. We provide open‐source code to facilitate straightforward implementation and adaptation of the model in other studies.
Abstract Magnetospheric chorus waves often accompany a gap in intensity near half of the electron cyclotron frequency, , whose generation process is still debated. One mechanism involving a nonlinear damping process proposed by Omura et al. (2009, https://doi.org/10.1029/2009ja014206 ) predicts a characteristic spectral signature where the lower cutoff of upper band waves, , follows local . Here, we statistically analyze spectral features of banded chorus waves from the Van Allen Probes and Arase spacecraft. Observationally, is almost one‐to‐one correlated with up to magnetic latitude for the banded chorus events in the midnight‐to‐dawn sector, but becomes nearly independent of for the events found on the dayside. By contrast, the upper cutoff of lower band waves, , depends only weakly on magnetic latitude and remains at approximately (where is the equatorial ) near the equatorial region, meaning that the power gap should have been already formed there. These results indicate that while the good correlation between and lends strong support for the nonlinear damping mechanism as a power gap enhancer during poleward propagation of chorus waves, its formation near the equatorial source region and/or on the dayside likely needs more concrete explanation.
Abstract The ionospheric recovery responses to two G5 geomagnetic storms over China in May and October 2024 were compared using GNSS‐derived total electron content (TEC), detrended TEC, the rate of TEC index (ROTI), magnetometer data, and GUVI/TIMED O/N 2 observations. The results show that the May recovery phase was dominated by widespread and persistent negative TEC anomalies, with low‐latitude depletion reaching approximately 60 TECU. In contrast, the October event exhibited a pronounced latitudinal structure, including a C‐shaped positive enhancement at mid‐latitudes and low‐latitude depletion of approximately 50 TECU. The elevated O/N 2 ratio may have favored the mid‐latitude enhancement. Although substantial low‐latitude TEC depletion occurred on both May 11 and October 11, no pronounced ROTI enhancement was observed in October, suggesting that the depletion was mainly associated with weakened electrodynamic forcing, contraction of the EIA, and reduced electron content in the upper ionosphere. Six groups of large‐scale traveling ionospheric disturbances (LSTIDs) and two types of ionospheric irregularities were identified in May, compared with three LSTID groups and one type of brief, localized irregularity in October. These differences suggest that high‐latitude energy input, low‐latitude electrodynamics, and thermospheric composition jointly controlled the contrasting ionospheric recovery responses.
Abstract Electromagnetic ion cyclotron (EMIC) waves contribute to magnetospheric loss cone scattering, but the fundamental processes during EMIC wave formation and evolution have not been systematically studied. We examine the formation of EMIC waves, driven by temperature anisotropy, in a uniform plasma using one‐dimensional (1D) and two‐dimensional (2D) hybrid simulations in proton‐only and hydrogen‐helium mixed plasmas. In a 1D plasma, a hot proton population with sufficient temperature anisotropy leads to the growth of parallel‐propagating EMIC waves in line with the predictions of the linear‐fit dispersion relation. The changing plasma temperatures and wave structures during EMIC wave evolution are illustrated. Additionally, a standing structure with parallel wave‐vector was found to act as a mechanism for parallel heating of the cold background plasma. This electrostatic structure emerges with half the EMIC wavelength, and is generated by nonlinear EMIC self‐interaction. In the 2D case, we observe the formation of structures possessing perpendicular wavenumbers and a broad‐spectrum overlap between the EMIC and electrostatic waves leading to locally enhanced amplitudes. In simulations with a cold helium population, we note specific contributions of each ion species to the thermal evolution of the system; hot protons carry cross‐field thermal energy and cold protons and helium respond to the electrostatic mode and its harmonics respectively. Harmonics of the electrostatic mode are nontrivial in the mixed plasma and contribute substantially to heating in the cold plasma background. Overall, results show evidence of nonlinear processes contributing to particle heating and modulation of excited EMIC waves.
Abstract We report MMS observations of electrostatic solitary waves (ESWs) in two magnetotail earthward ion‐flow events. In the first event, the ion beam is nearly antiparallel to the background magnetic field, and the associated ESWs exhibit predominantly parallel bipolar electric fields with negligible perpendicular components. In the second event, the ion beam has a significant perpendicular velocity component, and the associated ESWs show bipolar variations in both parallel and perpendicular electric field components. Variance analysis of the three‐component electric field defines the maximum‐variance direction L , which represents the principal electric field orientation of each ESW. The statistical distributions of the L directions show that they are not randomly distributed but are closely aligned with the local ion‐beam direction. These observations reveal a systematic association between the local ion‐beam direction and the electric field orientation of slow electron holes, suggesting that ion beams may play an important role in organizing oblique electrostatic structures in magnetotail earthward ion flows.
Abstract Disappearing solar wind events (DSWEs), intervals when the solar wind proton density <1 , represent one extreme of the Mars solar wind interaction. Using data from NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission from February 2016 to May 2024, 154 orbits across 90 unique days were identified. During these events, the solar wind kinetic energy flux had a median value of ∼0.08 and never exceeded 0.37 , whereas normal times have a median value twice as large (∼0.16 ) and maximum values exceeding 8 . In contrast, electromagnetic energy fluxes and solar ionizing irradiance remained near normal levels. Observations of and density and fluxes from MAVEN's SupraThermal and Thermal Ion Composition (STATIC) instrument were analyzed. During DSWEs, ion density and ion flux distributions are significantly restructured, consistent with expansion of Mars' ionosphere and induced magnetosphere. Ion escape rates are significantly reduced during DSWEs compared to normal solar wind conditions, with falling to ∼75% and to ∼88% of normal values (9.45 and 1.30 ions/s). DSWE ion fluxes binned by solar wind kinetic and solar ionizing irradiance followed the same empirical relations as under normal conditions, whereas DSWE ion fluxes were depleted relative to the background trend for solar wind electromagnetic energy. Thus, the observed decrease in ion escape is likely attributable to the reduced solar wind kinetic energy flux. These results suggest that even under extreme solar wind density reductions, Mars' ion escape remains mostly governed by the same background drivers as during normal times.
Abstract The Martian atmospheric temperature profile is a core parameter for studying the Martian climate environment and atmospheric thermodynamics. However, its observational data remain severely limited. This study achieves temperature profile retrieval in the altitude range of 0–80 km by using the O 2 (a 1 Δ g ) airglow spectral data acquired by the infrared channel of the Spectroscopy Spectrograph for the Investigation of Characteristics of the Atmosphere of Mars (SPICAM) in limb observation mode and combining photochemical reaction mechanisms with a radiative transfer model. Comparisons are made with the simulation data from the Mars Climate Database (MCD) and the observation data from the Mars Climate Sounder (MCS) to validate the feasibility and accuracy of this study. Results indicate that within the altitude range of 20–60 km, the errors of the retrieved temperature profiles are controlled within ±20 K, exhibiting high precision. The collisional annihilation of O 2 (a 1 Δ g ) and reduction in atmospheric density diminish retrieval accuracies at altitudes below 20 km and above 60 km, respectively. This study realizes Martian atmospheric temperature profile retrieval on the basis of 1.27 μm O 2 (a 1 Δ g ) airglow obtained from SPICAM limb observations, providing technical approaches and data support for the in‐depth exploration of Martian atmospheric dynamics and climate characteristics.
Abstract Microchannel plates (MCP) detectors have been extensively used in space to measure particles and photons. The primary radiation causes electron emission inside the MCP small pores that transforms into an electron avalanche. This is due to a bias voltage accelerating the secondary electrons down the pores creating more secondary electrons. The typical gain of 10 7 at the beginning of life decreases with the charge extracted from the MCP. The gain can be recovered by increasing the bias voltage until the charge extracted reaches the end‐of‐life of the MCP (typically several C/cm 2 ). The gain decrease with charge extracted experienced by MCP detectors in space instruments affects their overall sensitivity. Since the sensitivity is used to convert the individual counts from the MCP detector into physical quantities of the primary radiation, it is necessary to quantify and track these changes in detector gain. This study shows how in‐flight data from a plasma ion instrument are used to evaluate the gain over time and how to recover the gain loss with bias voltage increase. Since the bias voltage adjustment is occasional, we also show how to estimate the gain as a function of time. Finally, we relate the gain loss or the bias voltage increase required for constant gain with an estimate of the charge extracted from the MCP. These results can be used to evaluate the voltage bias required at the end of the mission, which is useful when designing the detector (e.g., HV standoffs) and the power supplies that bias them.
Abstract The 1962 Starfish Prime high‐altitude 1.4‐megaton nuclear detonation injected large amounts of fission‐produced energetic electrons into the inner magnetosphere ( L = 1.12), creating an intense artificial radiation belt that persisted for years. In this study, we revisit historical data from this unprecedented event to investigate the potential role of Kennel‐Petschek (K‐P) self‐limiting processes in capping the electron radiation fluxes arising from a nuclear explosion. Using digitized data from the Injun and STARAD satellites, we reconstruct an estimate for the spectral, spatial, and temporal evolution of the Starfish‐related electron flux. We show that by the time of the earliest global satellite measurements (∼10 hr post‐detonation), the flux spectrum appears to potentially be capped at levels consistent with the theoretical K‐P limit for energies below ∼2 MeV. We also attempt to place these observations in the context of the burst timeline: ground‐based measurements of immediately occurring broadband whistlers were likely associated with the initial electromagnetic pulse, whereas the later evolution of trapped particles over several hours could have been dominated by K‐P pitch angle scattering from a newly created trapped electron population. Acting alongside longitudinal drift equilibration and very slow Coulomb scattering on atmospheric neutrals, these processes could have reduced the excess energetic electron population until the trapped distribution approached marginal stability at the K‐P flux limit. Our results provide evidence that suggests that the intensity of the artificial radiation belts could be constrained by naturally occurring K‐P‐like self‐limiting plasma processes, resulting in the creation of an upper limit to even artificially create trapped electron space radiation.
Abstract On the occasion of the 50 years since the launch of Helios 1 and Helios 2, we proposed a special collection related to the solar wind, its origin, evolution, and space weather related effects. In addition to old missions like ULYSSES that explored the solar wind outside the ecliptic plane, recent missions like the Parker Solar Probe (PSP) and Solar Orbiter (SolO), as well as established ones such as the Solar Dynamics Observatory (SDO) and the Solar Terrestrial Relations Observatories (STEREOs), offer extensive new measurements that help to refine existing knowledge of slow and fast solar wind in the heliosphere and the development of new models. This collection addressed different solar wind topics, which included but were not limited to the mechanisms of solar wind acceleration and outflow, dynamics of stream interaction, the configuration of the magnetic field and plasma topology at the source surface and within the inner heliosphere.
Abstract We investigated the spatial distributions and pitch‐angle distributions of 30–300 eV H + and O + ion fluxes using data obtained by the low‐energy particle experiments‐ion mass analyzer (LEPi) onboard the Arase satellite. A statistical analysis over 3.7 years reveals that the H + ion flux is enhanced at L > 4 from the premidnight sector through dawn to noon, with the peak location shifting inward during geomagnetic disturbances. The pitch‐angle distribution of H + ions varies with both L and geomagnetic activity, which can be interpreted as the influence of transport processes and the ring current effect. In contrast, the O + ion flux exhibits a pronounced enhancement in a confined region at L = 3–5 and 19–9 magnetic local time, with a strong dawn‐dusk asymmetry. The O + flux increases with geomagnetic activity at L = 3–5, while remaining nearly unchanged at L = 6. Its pitch‐angle distribution is consistently bidirectional and field‐aligned, with little dependence on geomagnetic conditions. These distinct spatial and pitch‐angle characteristics indicate that the observed H + ions correspond to the inner part of the warm plasma cloak, whereas the observed O + ions represent the high‐energy tail of the oxygen torus. The results further suggest that low‐energy O + ions are supplied directly from the nightside ionosphere along geomagnetic field lines, rather than being transported inward from the outer magnetosphere. Our findings demonstrate that the warm plasma cloak and the oxygen torus constitute likely independent plasma populations in the inner magnetosphere.
Abstract The coupling between inner‐belt energetic electron precipitation (EEP) and ionospheric disturbances at low‐to‐mid latitudes during extreme geomagnetic storms remains an unresolved aspect of magnetosphere‐ionosphere coupling. We investigated this coupling using high‐resolution observations from the medium‐energy electron spectrometer (MES) aboard the Macao Science Satellite‐1A (MSS‐1A), during the “Gannon Storm” of 10–11 May 2024 (minimum Dst nT; minimum SYM‐H nT), integrated with global vertical total electron content (VTEC) maps from the CAS Global Ionosphere Map (GIM) and magnetic field line mapping using IGRF‐13 and T96. MSS‐1A recorded intense flux enhancements (40–754 keV) penetrating to unusually low ‐shells during the storm main phase, with significant spectral hardening (power‐law index from 1.89 to 1.6), indicating deep injection and adiabatic transport into the inner magnetosphere and slot region. Concurrently, GIM observations revealed large‐scale VTEC restructuring, including storm‐enhanced density (SED) exceeding 80 TECU and poleward displacement of the equatorial ionization anomaly (EIA). While MSS‐1A confirms bounce loss cone filling at 55–507 keV, three independent lines of evidence—temporal precedence, global spatial scale, and multi‐hour persistence—demonstrate that the large‐scale VTEC response was driven by prompt penetration electric fields (PPEF) and the super‐fountain effect rather than direct EEP. Our findings show that, although EEP may enhance localized ionization in the E‐region under extreme storm conditions, its contribution to large‐scale VTEC perturbations is secondary to storm‐time electrodynamic forcing, remaining below the detection capability of current global ionospheric maps due to altitude decoupling from the F‐region and spatial‐temporal smoothing in the gridded data.
Abstract This study investigates whether Global Navigation Satellite System Reflectometry (GNSS‐R) measurements from low‐Earth‐orbit (LEO) satellites can improve three‐dimensional (3D) ionospheric electron density imaging, with a focus on high‐latitude regions. Conventional ionospheric tomography is limited by sparse ground‐based receiver coverage over oceans and high latitudes, as well as the restricted viewing geometry of Global Navigation Satellite System radio occultation (GNSS‐RO) measurements. GNSS‐R measurements provide grazing‐angle ray paths over polar regions that have not previously been incorporated into ionospheric tomographic imaging. Using simulated observations, we incorporate total electron content estimates from reflected GNSS‐R ray‐paths and low‐elevation (less than ) ground‐based signals into a voxel‐based tomographic inversion. These measurements are combined with conventional data sources, including ground‐based receivers, GNSS‐RO, and precise orbit determination links. Reconstruction performance is evaluated with and without the inclusion of GNSS‐R and low‐elevation measurements. Reconstructed electron density fields are evaluated using voxel‐intersection statistics across signal geometries and by quantifying reconstruction error relative to the simulated truth. Results show that GNSS‐R and low‐elevation signals substantially increase the number and spatial diversity of ray‐paths, particularly at high latitudes. This increased ray‐path diversity improves spatial resolution and reduces reconstruction error relative to inversions using only ground‐based and GNSS‐RO observations. To explore future mission capabilities, we test conceptual LEO constellation configurations to examine how satellite number and orbital configuration influence imaging performance. These findings highlight GNSS‐R as a valuable complementary data source for next‐generation 3D ionospheric imaging and provide a framework for incorporating real GNSS‐R measurements into future space‐weather monitoring systems.
Abstract We examine the response of the ionosphere to the 10–13 May 2024 and 10–13 October 2024 geomagnetic storms, the two recent great geomagnetic storms of solar cycle 25. We identified the occurrence of ionospheric irregularities during the two geomagnetic storms. We used percentage change in vertical total electron content (VTEC%) computed using VTEC data from three equatorial ground‐based global navigation satellite system (GNSS) receivers in the American, African, and Asian sectors, to study the ionospheric response. In order to determine the occurrence of ionospheric irregularities, we employed the rate of change of electron density (Ne) index (RODI) from Swarm‐A satellite (Sw‐A) and the rate of change of TEC index (ROTI) from the GNSS TEC. Our results revealed a positive ionospheric storm over the African sector during the main phase of May storm; whereas a negative ionospheric response was recorded over the American sector. In contrast, during the October storm, a positive ionospheric storm effect was observed over the American and African sectors and a negative over the Asian sector. Additionally, we identified that the October storm had a higher percentage irregularity occurrence rate than the May storm. During the main phases of the two storms, irregularities were triggered over the African sector and inhibited over the American and Asian sectors. Sw‐A showed a longitudinal trend in the occurrence rate of irregularities during the May storm, with America having the lowest rate, followed by Africa and Asia. During the October storm, this longitudinal dependence is reversed.
Abstract Neutral winds play a critical role in transporting mass, momentum, and energy throughout the upper atmosphere. During geomagnetic storms, momentum is transferred to the thermosphere primarily through ion–neutral collisions, along with other forces. This study investigates high‐latitude neutral wind dynamics during the 17 March 2013 storm using the Global Ionosphere Thermosphere Model. We analyze the five acceleration terms—pressure gradient, viscosity, ion drag, Coriolis, and advection—across altitudes, at selected locations, and statistically across defined high‐latitude regions. These regions are categorized based on electron energy flux into auroral oval, polar cap, and subauroral boundaries using the Feature Tracking Empirical Model of Auroral Precipitation (FTA). It was found that above 300 km, pressure gradient dominated across all regions, followed by viscosity. The latter was driven by vertical shear in horizontal winds and amplified by decreasing mass density. Below 300 km, acceleration profiles varied by region: advection dominated in the polar cap, while pressure gradient was the leading driver in subauroral region, and dominated alongside ion drag in the auroral oval. Temporally, ion drag was most pronounced during the early storm main phase, then declined as the storm progressed due to O/N 2 depletion, which reduced TEC and consequently ion‐neutral collision frequency. Spatially, ion drag was strongest within the auroral oval and increased with enhanced electron energy flux. These results revealed an underestimated pathway in which pressure gradients and ion drag in E and F1 layers generate horizontal shear, driving a viscous acceleration response at higher altitudes.
