Abstract Techniques developed in the past few years enable the derivation of multiscale ion convection and particle precipitation patterns from high‐resolution ground‐based observations, and it has been shown in previous studies that such multiscale geomagnetic forcing can contribute significantly to ionospheric and thermospheric disturbances. In this work, the global ionosphere–thermosphere model (GITM) is utilized to simulate the 27 March 2014 substorm event. Simulations are driven by both the original and spatially smoothed multiscale Super Dual Auroral Radar Network electric potential patterns, and the differences between the two sets of simulations are used to evaluate the effects of mesoscale (<500 km) ion convection. GITM‐simulated neutral winds are compared with scanning Doppler imagers (SDI) wind measurements at Toolik Lake (68.6°N, 149.6°W). Neutral wind variations have been further separated into large‐scale (>500 km) and mesoscale (<500 km) structures. Data‐model comparisons show that, while GITM captures large‐scale wind variations reasonably well, it underestimates the magnitudes of mesoscale winds. Both mesoscale ion convection and precipitation are found to substantially enhance the simulated mesoscale neutral winds. Quantitative analyses at a fixed location and over two vortex regions reveal that F‐region neutral wind variations (at ∼270 km) typically lag mesoscale ion drift enhancements by a few to ∼30 min. The maximum magnitude of mesoscale ion drifts is consistently ∼6 times larger than that of the associated neutral wind changes.

The Kinetic-scale Energy and momentum Transport eXperiment (KiNET-X) sounding rocket explored the coupling of injected barium ions (Ba+) to the ambient ionospheric plasma at two altitudes by releasing two canisters of barium neutrals, each forming an ionized cloud. KiNET-X had instruments on the main payload and on two small deployed subpayloads (Bobs) to measure the Ba+ density (nBa+). The main payload observed nBa+ at a nearly fixed pitch angle to the local geomagnetic field (B→), whereas the Bobs scanned a range of pitch angles. A simple model based on tracing each cloud's particles agreed with the main payload's observed nBa+ profile from the first release; however, for the second release, non-ideal “skidding” motion was required to capture features of the observed nBa+ profile. This paper is a companion to Moses et al. [Phys. Plasmas 32, 042109 (2025)] that analyzes the Bobs' nBa+ profiles. Comparison between the Bobs' measured and modeled nBa+ profiles indicate that pitch angle scattering occurred in each release. Non-idealized pitch angle distributions may arise in the Ba+ clouds from the presence of electric fields parallel to B→ or wave–particle pitch angle diffusion. Pitch angle scattering processes can be proxied in our model by increasing the field-of-view for the density calculation. These different model configurations capture different portions of the measured subpayloads' density and pitch angle profiles. There is perhaps a combination of skidding and scattering that is not easily represented in our model.

Ion-neutral coupling is responsible for dissipating energy deposited into the high-latitude ionosphere during geomagnetically active periods. The neutral wind response time, or the ion-neutral coupling efficiency, is not well characterized, with a wide range of reported response times. Additionally, how this coupling efficiency varies with geomagnetic activity level is not well understood, with few studies addressing the impact of geomagnetic activity level on neutral wind response time. In this study, a statistical analysis of the neutral wind response time during substorm periods is performed. We use data from Scanning Doppler Imagers (SDIs) and the Poker Flat Incoherent Scatter Radar (PFISR) to calculate the neutral wind response time using the new weighted windowed time-lagged correlation method. Substorm events were found using SuperMAG substorm lists and All Sky Imagers (ASIs). This statistical analysis resulted in 23 substorm events, with an average response time of ∼16 min. To determine the controlling factors of this response time, geomagnetic and ionospheric parameters, such as IMF strength and orientation, SYM/H index, AE index, and electron density, are investigated for the statistical substorm set. A superposed epoch analysis of the parameters is performed to determine average geospace conditions required for fast neutral wind responses. It was found that quiet-time conditions in AE and SYM-H indices, a southward turning of IMF around 1.5 h before substorm onset time, and large electron densities lead to faster neutral wind response times. Based on the geomagnetic indices results, it was suggested that thermospheric pre-conditioning may play a role in neutral wind response times.

Winds in the nighttime upper thermosphere are often observed to mimic the ionospheric plasma convection at polar latitudes, and whether the same is true for the daytime winds remains unclear. The dayside sector is subject to large temperature gradient set up by solar irradiance and it also contains the cusp, which is a hotspot of Poynting flux and a region with the strongest soft particle precipitation. We examine daytime winds using a Scanning Doppler Imager (SDI) located at the South Pole, and investigate their distribution under steadily positive and negative IMF By conditions. The results show that daytime winds exhibit significant differences from the plasma convection. Under negative IMF By conditions, winds flow in the same direction as the plasma zonally, but have a meridional component that is strongest in the auroral zone. As a result, winds are more poleward-directed than the plasma convection within the auroral zone, and more westward-directed in the polar cap. Under positive IMF By conditions, winds can flow zonally against the plasma in certain regions. For instance, they flow westward in the polar cap despite the eastward plasma convection there, forming a large angle relative to the plasma convection. The results indicate that ion drag may not be the most dominant force for daytime winds. Although the importance of various forcing terms cannot be resolved with the utilized dataset, we speculate that the pressure gradient force in the presence of cusp heating serves as one important contributor.

Active plasma experiments can be used to strongly perturb the space plasma environment. During the early phase of a chemical release (e.g., few to several seconds), the injected plasma cloud can excite a variety of waves rather than acting as "inert" tracer particles. It is during this early phase of the release that fundamental plasma processes can be studied. For example, the Trigger [Holmgren et al., J. Geophys. Res. 85, 5043 (1980)] and recent KINetic-scale Energy and momentum Transport eXperiment (KiNET-X) missions were both designed to study processes related to auroral electron energization. Early experiments relied primarily on ground-based optics to diagnose the plasma interaction. Advances in optical sensors have dramatically improved imaging capability of both the ion and neutral components of the injected cloud; therefore, optics remain an important part of these types of experiments. However, advances in plasma (fields and particles) instruments have enabled a new generation of possible experiments from the sounding rocket platform. In this article, we discuss previous sounding rocket (and orbital) active experiments, the related science objectives, and an overview of select results from the KiNET-X rocket mission. Specifically, KiNET-X produced an Alfvenic perturbation, a variety of high frequency waves, energized thermal electrons, and produced a field-aligned electron beam of similar to 200 eV. The electron energization indicates non-ideal coupling of the injected barium cloud with the ambient ionospheric plasma. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC-ND) license (https://creativecommons.org/licenses/by-nc-nd/4.0/).

High-latitude neutral winds have a number of drivers, both from solar and magnetospheric origins. Because of this, the neutral wind response to changes in ionospheric convection is not well understood. Previous calculations of response times resulted in a wide range of responses, from tens of minutes to hours. We present a new weighted windowed time-lagged correlation (weighted WTLC) method for calculating the neutral wind response time. This method provides a time evolution of the neutral wind response time and considers the effects of all thermospheric forces, while previous methods were only capable of one or the other. We use data from SDIs, ASIs, and PFISR to calculate the neutral wind response time using this new method in three case studies. The results are visually validated, and the weighted WTLC method was able to correctly calculate the neutral wind response time. The time evolution of the weighted WTLC time is then compared to previous neutral wind response time calculations in order to investigate the role of ion-drag on neutral winds. For the substorm event on 2013 Feb 28, we see a shorter response time from the weighted WTLC method, ranging from 0 to 15 min, than the e-folding time, ranging from 30 to 355 min. The relationship between the two calculation methods and their implications about the ion-drag force is discussed. Using the time-dependent feature of the weighted WTLC method, we observe the neutral wind response time decrease over the course of a substorm event, indicating ion-neutral coupling increased as the substorm progressed. A new methodology for calculating the neutral wind response time to changes in ion convection is introduced The weighted windowed time-lagged correlation method is capable of resolving time-dependent responses of the neutral winds Comparison with previously derived neutral wind response times can shed insight on thermospheric drivers

The height range from approximately 80 km to several hundred km in Earth's atmosphere is a transition region from mostly neutral dynamics at the lower altitudes to space-weather driven plasma dynamics at the top.It is an important region for many modern technological systems that either depend on spacecraft orbiting within it, or on radio signals passing through it.Traditional scale analysis of the governing equations for fluid dynamics in this region suggest that flows above about 110 km altitude should be smooth, laminar, and largely horizontal, with little spatial structure at shorter than synoptic length scales.However, when winds in this region are actually measured, such placid behavior is not observed.Understanding and accounting for this greater than expected dynamism requires measurements that are sampled locally and derived with absolute accuracy.Here it is argued that the best-known way to obtain such measurements is by trigonometric tracking of chemical tracer clouds deployed at altitude by a sounding rocket.We present a brief review of the technique, show some recent results, and discuss examples of important physical phenomena that require these measurements in order to advance current understanding.Finally, we note that the rocket-borne chemical tracer program is currently at risk of losing expertise gained over the previous several decades, and we encourage NASA to take steps to ensure that this expertise is preserved.

Multiple years of thermospheric wind and temperature data were examined to study gravity waves in Earth's thermosphere. Winds and temperatures were measured using all‐sky imaging optical Doppler spectrometers deployed at two sites in Alaska, and three in Antarctica. For all sites, oscillatory perturbations were clearly present in high‐pass temporally filtered F‐region line‐of‐sight (LOS) winds for the majority of the clear‐sky nights. Oscillations were also discernible in E‐region LOS wind and F‐region Doppler temperature, albeit less frequently. Oscillation amplitudes correlated strongly with auroral and geomagnetic activity. Observed wave signatures also correlated strongly between geographically nearby observing sites. Amplitudes of LOS wind oscillations were usually small when viewed in the zenith and increased approximately with the sine of the zenith angle—as expected if the underlying motion is predominantly horizontal. Scanning Doppler Imager instruments observe in many look directions simultaneously. Phase relationships between perturbations observed in different look directions were used to identify time intervals when the oscillations were likely to be due to traveling waves. However, a number of instances were noted in which the oscillations had characteristics suggesting geophysical mechanisms other than traveling waves—a recognition that was only possible because of the large number of look directions. Lomb‐Scargle analysis was used on a representative subset of days to resolve the spectral distributions of the wind and temperature oscillations. F‐region wind oscillations on days analyzed this way exhibited periods typically ranging from 60 min and above. By contrast, E‐region wind oscillation periods were as short as 30 min.

This paper investigates the lower‐to‐upper atmosphere coupling at high latitudes (>60°N) during the northern winter months of 2012–2013 years, which includes a period of major Sudden “Stratospheric” Warming (SSW). We perform statistical analysis of thermosphere wind disturbances with periods of 30–70 min, known as the medium scale traveling atmospheric disturbances (MSTADs) in atomic oxygen green line (557.7 nm) near ∼120 km and red line (630.0 nm) emissions near ∼250 km observed from Scanning Doppler Imagers (SDIs) over Alaska. The SDI MSTADs observations (60°–75°N) are interpreted in conjunction with the previous daytime medium‐scale traveling ionospheric disturbance (MSTID) observations by SuperDARN midlatitudes (35°–65°N) radars in the F ‐region ionosphere and western hemisphere, which confirm findings from the SDI instruments. Increases in MSTAD activity from SDIs show correlations with the increasing meridional planetary wave (PW) amplitudes in the stratosphere derived from MERRA2 winds. Furthermore, a detailed study of the lower atmospheric conditions from MERRA2 winds indicates that the lower atmospheric sources of MSTADs are likely due to the stratospheric generated Gravity Waves (GWs) and not orographic GWs. Favorable stratospheric propagation conditions and polar vortex disturbances resulting from the increased PW activity in the stratospheric region both appear to contribute to increased MSTAD activity in the thermosphere. Additionally, the results show that the MSTID activity from SuperDARN HF radars at mid latitudes during the January 2013 SSW is lower than the MSTAD activity in SDI winds at high latitudes.

Each year, society becomes ever more reliant on spacecraft in low Earth orbit, and on systems that transmit radio signals through Earth's ionosphere.However, both spacecraft orbits and radio propagation are strongly affected by natural variations or "weather" occurring in the height range from 80 km to several hundred kilometers altitude -the so-called "Space Atmosphere Interaction Region" (SAIR).Weather in this region is described by a coupled system of three-dimensional time-dependent partial differential equations that can only be evaluated using numerical models, which are limited because: (1) available computational resources only allow coarse grid resolutions, (2) solutions require specifications of drivers and boundary conditions that are not adequately described by observations, and (3) accurate evaluation of some terms may not be computationally feasible.These limitations create an urgent need for comprehensive real-time observations of the actual state of the SAIR, and the drivers of its weather -to constrain the models, to validate their physics, and to provide timely, actionable data to end-users of technological systems.Unfortunately, however, existing infrastructure for observing SAIR weather is astonishingly inadequate.For example, even as early as the mid-1800s, routine meteorological observations of the troposphere had already exceeded today's typical resolution for the corresponding measurements in the SAIR.The proposed solution is to establish a large array of roughly thirty relocatable ground-based flagship observatories, and a similar number of simpler and lower-cost instrument outposts.These would be deployed from the Canadian arctic down through the Americas to southern Argentina, with observational coverage spanning SAIR altitudes.Observatory instruments would characterize winds, temperatures and electrodynamics within the SAIR, as well as the drivers that force these fields -both from above, due to solar radiation and geomagnetic activity, and from below, due to upward propagating waves and tides.Cyber infrastructure would be developed to host and freely distribute observational data, assimilate observations contributed by others, and provide higher-level data products derived from assimilation and modeling.The array of observatories proposed here is essentially an exact realization of one of the major recommendations of the previous (2013) Heliophysics Decadal Survey, that has yet to be implemented.

This white paper describes a new set of instruments that will be deployed in northern Scandinavia during 2023 to support the EISCAT-3D radar facility.These instruments include two types of Fabry-Perot spectrometers to measure thermospheric winds and temperatures, multispectral filtered all-sky cameras, GNSS receivers, and 3-component fluxgate magnetometers.The instruments will be housed in three observatories and will view a common volume within the EISCAT study region.They will provide a range of geophysical products that can be used synergistically with EISCAT-3D observations.Attention is drawn to one particular data product, which will be a 4-dimensional (longitude, latitude, altitude, & time) fit to the 3-component thermospheric wind field inside the EISCAT-3D measurement volume.This is a new research product that is generated by an "evolutionary" algorithm that adjusts a 4D forward model to best match the line-ofsight wind components observed by the Fabry-Perot instruments.The principal recommendation given here is to encourage the research community to use these data for new research products.The data will be freely available to all interested users via the web, once the observatories come online.

Understanding the effects of multiple scales in the dynamics of the mesosphere, ionosphere, and thermosphere (MIT) regions and their interactions remains a key challenge in geospace science.These pertain to spatial scales at which phenomena occur (e.g., horizontal/vertical scale sizes of TID structures; horizontal scales at which thermospheric winds decorrelate; scale sizes of auroral structures; horizontal extent, variability and location of stable auroral red arcs).They also apply to energies that are associated with these physical phenomena (e.g., energetic events in the lower atmosphere creating thermospheric signatures; precipitating energy and fluxes associated with auroral structures; etc).Much of the work done over the past decades in investigating various scales by necessity used standalone instruments, often co-located with major facilities.Our ability to achieve an in-depth understanding of the coupling and transport of energy across these scales is hampered by our inability to make coordinated observations of the necessary physical drivers, which can often be separated by large distances between cause and effect.In order to understand how drivers of energy and momentum at multiple scales and over large distances manifest as variability in the geospace system, observations at various spatial scales spanning large distances need to be obtained.This white paper makes the case for building new and maintaining existing networks of homogeneous instruments in order to sample various scale sizes across large distances.

The Earth’s upper atmosphere (85–550 km) is the nearest region of geospace and is highly dynamic in nature. Neutral winds impact a large portion of the dynamics in this region. They play a critical role in determining the state of the ionosphere-thermosphere system at almost all latitudes and altitudes. Their influences range from wave breaking/dissipation in the mesosphere and lower thermosphere to global redistribution of energy and momentum deposited at high latitudes by the magnetosphere. Despite their known importance, global geospace neutral winds have remained one of the least sampled state parameters of the Earth’s upper atmosphere and are still poorly characterized even after multiple decades of observations. This paper presents an overview of historical neutral wind measurements and the critical need for their global height-resolved measurements. Some satellite missions are still operational and deliver valuable information on the contribution of neutral winds in global atmospheric dynamics. However, many significant gaps remain in their global monitoring, and our current understanding of the drivers of neutral winds is incomplete. We discuss the challenges posed by these measurement gaps in understanding geospace physics and weather. Further, we propose some wind observation solutions, including the simultaneous operations of upcoming NASA DYNAMIC and GDC missions as well as support for the development of ground-based observing methodologies, that will lead to fundamental advances in geospace science and address humanity’s emerging space needs.

Intense sunward (westward) plasma flows, named Subauroral Polarization Stream (SAPS), have been known to occur equatorward of the electron auroras for decades, yet their effect on the upper thermosphere has not been well understood. On the one hand, the large velocity of SAPS results in large momentum exchange upon each ion‐neutral collision. On the other hand, the low plasma density associated with SAPS implies a low ion‐neutral collision frequency. We investigate the SAPS effect during non‐storm time by utilizing a Scanning Doppler Imager (SDI) for monitoring the upper thermosphere, SuperDARN radars for SAPS, all‐sky imagers and DMSP Spectrographic Imager for the auroral oval, and GPS receivers for the total electron content. Our observations suggest that SAPS at times drives substantial (>50 m/s) westward winds at subauroral latitudes in the dusk‐midnight sector, but not always. The occurrence of the westward winds varies with AE index, plasma content in the trough, and local time. The latitudinally averaged wind speed varies from 60 to 160 m/s, and is statistically 21% of the plasma. These westward winds also shift to lower latitude with increasing AE and increasing MLT. We do not observe SAPS driving poleward wind surges, neutral temperature enhancements, or acoustic‐gravity waves, likely due to the somewhat weak forcing of SAPS during the non‐storm time.

This study presents multi-instrument observations of persistent large-scale traveling ionosphere/atmospheric disturbances (LSTIDs/LSTADs) observed during moderately increased auroral electrojet activity and a sudden stratospheric warming in the polar winter hemisphere. The Global Ultraviolet Imager (GUVI), Gravity field and steady-state Ocean Circulation Explorer, Scanning Doppler Imaging Fabry-Perot Interferometers, and the Poker Flat Incoherent Scatter Radar are used to demonstrate the presence of LSTIDs/LSTADs between 19 UT and 5 UT on 18-19 January 2013 over the Alaska region down to lower midlatitudes. This study showcases the first use of GUVI for the study of LSTADs. These novel GUVI observations demonstrate the potential for the GUVI far ultraviolet emissions to be used for global-scale studies of waves and atmospheric disturbances in the thermosphere, a region lacking in long-term global measurements. These observations typify changes in the radiance from around 140 to 180 km, opening a new window into the behavior of the thermosphere.

Simple scaling analysis of terms in the Navier‐Stokes momentum equation for Earth's atmosphere suggests that winds at heights above 120 km should be smooth and laminar, with little spatial variation over horizontal scale lengths smaller than several hundred kilometers. However, there is increasing evidence that this traditional understanding may fail to account for several important processes, including both waves and small‐scale ion‐neutral momentum coupling. Here, we examine the thermospheric neutral wind field over Alaska in unprecedented detail using observations from an array of four ground‐based all‐sky imaging Fabry‐Perot interferometers, processed using a new geophysical inverse algorithm, to derive high‐resolution maps of all three wind components, with a temporal cadence of 30 seconds. The reconstructed high‐resolution neutral winds showed synoptic‐scale agreement with prior observations and previously validated techniques, with all results exhibiting behavior in agreement with basic physics. However, stacked time‐series plots of vector wind components reveal significantly more spatial and temporal structure than previously reported. In particular, the observed responses included complex wave‐like behavior and highly geographically variable vertical winds. Local flow features were observed at spatial scales as small as 100 km at times, with temporal scales as short as a few tens of minutes. Instances of close spatial and temporal correlations were observed between the wind fields reconstructed from green‐line spectra and ionospheric flows observed independently by SuperDARN.

Few remote sensing or in-situ techniques can measure winds in Earth's thermosphere between altitudes of 120 and 200 km. One possible approach within this region uses Doppler spectroscopy of the optical emission from atomic oxygen at 558 nm, although historical approaches have been hindered in the auroral zone because the emission altitude varies dramatically, both across the sky and over time, as a result of changing characteristic energy of auroral precipitation. Thus, a new approach is presented that instead uses this variation as an advantage, to resolve height profiles of the horizontal wind. Emission heights are estimated using the Doppler temperature derived from the 558 nm emission. During periods when the resulting estimates span a wide enough height interval, it is possible to use low order polynomial functions of altitude to model the Doppler shifts observed across the sky and over time, and thus reconstruct height profiles of the horizontal wind components. The technique introduced here is shown to work well provided there are no strong horizontal gradients in the wind field. Conditions satisfying these caveats do occur frequently and the resulting wind profiles validate well when compared to absolute in-situ wind measurements from a rocket-borne chemical release. While both the optical and chemical tracer techniques agreed with each other, they did not agree with the HWM-14 horizontal wind model. Applying this technique to wind measurements near the geomagnetic cusp footprint indicated that cusp-region forcing did not penetrate to atmospheric heights of 240 km or lower.

Strong Thermal Emission Velocity Enhancement (STEVE) is a nightsky optical phenomenon of great research interest in recent years. Recent findings indicated that STEVE likely represents certain extremely intensified chemiluminescence airglow instead of traditional aurora. In this study, we investigate the patterns and variations of the neutral wind and temperature before the STEVE emergence using joint scanning Doppler imager (SDI 630 nm) and optical all‐sky imager (ASI) observations, and make an initial effort to explore the potential preconditioning role of neutral winds in the STEVE production. Neutral winds enhance in westward and southward directions following substorm auroral intensification, and show an equatorward propagating trend from auroral latitudes. However, in STEVE events the enhanced equatorward winds feature a steep stop/reversal at certain subauroral latitude, and strong wind convergence is developed there. This pattern sustains for ∼15–20 min, and then STEVE arises at about this stop latitude. The strength of the southward wind intensification and wind convergence is in general weaker or absent in nonSTEVE substorm events. We propose that enhanced equatorward winds may transport relevant neutrals species that are key to the STEVE airglow production to subauroral latitudes, and pile up at the stop latitude of the equatorward winds due to the strong convergence there. Such a transport/pileup effect led by the neutral winds may prepare a reservoir of neutral constituent which, when further aided by subauroral ion drift, leads to a dramatic increase of the airglow production and the STEVE occurrence.

We have measured auroral zone thermospheric neutral winds in the midnight local time sector, using ground-based optical Doppler spectroscopy of the 630.0 nm emission from atomic oxygen, originating at around 240 km altitude over Alaska. One of the most prominent features seen in winds at these latitudes is the cross-polar jet emerging from the polar cap at local times around magnetic midnight. The standard view is that wind flows anti-sunward in the midnight sector and spills equatorward over magnetic latitudes extending well below those of the auroral zone. The purpose of this paper is to show that this view is too simplistic. From our observatory at Poker Flat, Alaska (similar to 65.12oN), the anti-sunward flow is frequently seen to stall over surprisingly short horizontal distances (100-200 km), without spilling further equatorward. This behavior is most prevalent during a low solar activity at mid-winter when the combination of pressure gradient established by solar heating and the ion drag is not enough to allow the jet to push through the background atmosphere on the nightside. At higher latitudes, by contrast, the flow is relatively uniformly anti-sunward around magnetic midnight even during quiet conditions. During periods of high solar and magnetic activity, the expected spilling of the midnight sector cross-polar jet to lower latitudes often is indeed observed over Alaska. Our observation of abrupt stalling during quiet solar and geomagnetic conditions is a very significant difference from the model predictions, with potentially important ramifications- which is the motivation for the present study.