Thermospheric Winds Research Articles

Долготная изменчивость ионосферы северного полушария во время магнитных бурь в марте 2012 года по данным ионозондов и GPS/ГЛОНАСС

A comprehensive study of spatio-temporal variations of geomagnetic, ionospheric, and atmospheric parameters in the middle and high latitudes of the Northern Hemisphere during a series of magnetic storms in March 2012 has been expanded by including vertical total electronic content (TEC) data from measurements at the chains of dual-frequency phase receivers GPS/GLONASS in the analysis. The features of longitudinal variations in ionosphere ionization over mid-latitude Eurasia, found earlier from vertical sounding data, are confirmed by vertical TEC data. We emphasize the complex physics of the long magnetically disturbed period in March 2012 with switching between positive and negative effects of an ionospheric storm during the same magnetic storm phases for spaced mid-latitude regions of the Eastern Hemisphere. Such changes in the ionospheric storm effects might have been caused by the superposition of competing processes in the mid-latitude region of the Eastern Hemisphere due to variations in the thermospheric composition, thermospheric winds, and large-scale electric fields affecting ionospheric ionization. We have observed significant differences in the nature of the ionospheric ionization reaction between the Eastern and Western hemispheres to the prolonged geomagnetic disturbance in March 2012. According to TEC data, there was an effect of reduced ionization of the ionosphere at longitudes of the Western Hemisphere, unlike the Eastern one. The effect of a negative ionospheric storm was caused by the formation of vast areas of atmospheric gas with a reduced density ratio [O]/[N2] over the mid-latitude region of the Western Hemisphere in the zone of maximum penetration of geomagnetic disturbances from high latitudes to middle latitudes. According to the INTERMAGNET magnetometer chain data for the analyzed period of magnetic storms on March 7–20, 2012, at midlatitudes of the Northern Hemisphere the maximum geomagnetic field variations were observed in the Western Hemisphere.

A Numerical Investigation on the Formation of the Stalling of the Cross‐Polar Jet in Midnight Thermospheric Winds

AbstractThermospheric winds usually flow from noon to the midnight inside the polar cap and spill further equatorward around midnight, due to the combined effects of solar radiation‐driven pressure gradient and ion drag. Recent observations from the Scanning Doppler Imager at Poker Flat indicated a distinct feature that the cross‐polar jet stalls over short horizontal distances around the magnetic midnight. However, the mechanisms responsible for the occurrence of stalling have not been well understood. In this study, we investigated the physical mechanisms of the stalling of the cross‐polar jet based on the Thermosphere Ionosphere Electrodynamics General Circulation Model simulations. The model reproduced the general stalling features at midnight over a short horizontal distance. The occurrence of stalling is mainly associated with Joule heating, which enhances the neutral temperature and then turns winds poleward by the pressure gradient force, and ion‐neutral coupling, which drags neutral winds westward in the pre magnetic local time midnight sector and contributes to the poleward turning of the meridional winds by the poleward Coriolis force. Meanwhile, the stalling region has a large latitudinal range and depends on the interplanetary magnetic field (IMF). Under southward IMF Bz, thermospheric winds are likely to stall at lower latitudes associated with strong ionospheric convection. The stalling of the cross‐polar jet in the midnight thermospheric winds has UT/longitude dependence, which could be associated with the orientation of the alignment of geomagnetic and geographic poles with respect to ionospheric convection.

Multi‐Instruments Observation of Ionospheric‐Thermospheric Dynamic Coupling Over Mohe (53.5°N, 122.3°E) During the April 2023 Geomagnetic Storm

AbstractThe state and variations of the Ionosphere‐Thermosphere (I‐T) system are strongly influenced by dynamical processes, with these effects being amplified during intense space environment disturbances such as geomagnetic storms, producing multi‐scale disturbance features in the I‐T system. The redistribution of energy and momentum through the dynamical processes is currently not well understood. Unfortunately, the dearth of thermospheric observations has yielded limited studies capable of monitoring and analyzing the I‐T dynamical coupling during storms using co‐located measurements. This work examines the responses of the I‐T system during the April 2023 geomagnetic storm based on high‐resolution continuous I‐T system observations at Mohe observatory, a middle latitude station. Both large‐scale features and short‐period oscillations were identified in multiple key parameters of the I‐T system. Similar disturbed patterns in thermospheric winds occurred during the main and recovery phases of the geomagnetic storm. However, the oscillation features in many parameters of the I‐T system are only observed during the main phase. The results of concurrent I‐T system observations unveil the oscillation features of thermospheric winds, temperatures, the ionospheric F2 peak height, electron density, and total electron content. These findings reveal the important influence of wind divergence on thermospheric temperature disturbances and its modulatory effect on ionospheric disturbances.

Asymmetric Ionospheric Jets in Jupiter's Aurora

AbstractSimultaneous infrared observations of and H2 emissions from Jupiter's northern aurora using the Near Infrared Spectrograph at Keck Observatory were used to measure the ionospheric and thermospheric wind velocities. ions supercorotate near the dawn auroral oval and subcorotate across the dusk sector and in the dawn polar region relative to the planetary rotation rate, broadly in agreement with past observations and models. An anticyclonic vortex is discovered in H2 flows, closely matching the mean magnetospheric subcorotation when the observed magnetospheric flows are averaged azimuthally. In comparing ion and neutral winds, we measure the line‐of‐sight effective ion drift in the neutral reference frame for the first time, revealing two blue‐shifted sunward flows of ∼2 km/s. Observed and H2 emissions overlap with predictions of the Pedersen conductivity layer, suggesting two different regions of the ionosphere: (a) a deep layer, where neutral forces dominate the thermosphere and symmetric breakdown‐in‐corotation currents can close, and (b) a higher layer, where the observed effective ion drift allows dawn‐to‐dusk Pedersen currents within the upper atmosphere, in turn closing asymmetric currents within the magnetosphere. This ionospheric structure aligns well with recent Juno observations of Jupiter's aurora. The detected thermospheric vortex implies the driving of neutral flows by the momentum from the magnetosphere within the thermosphere and deeper in the atmosphere to potentially 20 mbar. Jovian neutral thermosphere might bridge the gap between current observations and modelings and perhaps be significant to the dynamics of aurora on Earth and other outer planets.

Unveiling the combined effects of neutral dynamics and electrodynamic forcing on dayside ionosphere during the 3–4 February 2022 “SpaceX” geomagnetic storms

Geomagnetic storms of G1-class were observed on 3 and 4 February 2022, which caused the loss of 38 out of 49 SpaceX satellites during their launch due to enhanced neutral density. The effects of storm-time neutral dynamics and electrodynamics over the American sector during this minor storm have been investigated using Global Positioning System—total electron content (TEC) and Global‐scale Observations of the Limb and Disk (GOLD) mission measured thermospheric composition and temperature. Results revealed an unexpected feature in terms of increase in O/N2 and depletion in TEC over the American low-latitudes. This feature is in addition to the classic storm time ionospheric variations of enhancement in ionospheric electron density in presence of enhanced O/N2 and an intense equatorial electrojet (EEJ). Further, significant morning-noon electron density reductions were observed over the southern mid-high latitudes along the American longitudes. Results from Multiscale Atmosphere-Geospace Environment (MAGE) model simulations elucidated storm-induced equatorward thermospheric wind which caused the strong morning counter electrojet by generating the disturbance dynamo electric field. This further explains the morning TEC depletion at low-latitudes despite an increase in O/N2. Sub-storm related magnetospheric convection resulted in significant noon-time peak in EEJ on 4 February. Observation and modelling approaches together suggested that combined effects of storm-time neutral dynamic and electrodynamic forcing resulted in significant ionospheric variations over the American sector during minor geomagnetic storms.

Equatorial Plasma Bubbles: Zonal Thermosphere Wind Influence

There are a number of the theoretical studies pointing to the key role of the zonal thermosphere winds in the equatorial plasma bubble (EPB) generation and evolution. However, there are insufficient observational data to confirm the relationship between these phenomena. To study this relationship, the detailed comparative and correlation analysis of the LT-variations of the EPB occurrence probability and the zonal thermosphere wind velocity was carried out. The data from the EPB observations recorded aboard the ISS-b satellite (~972–1220 km) during the solstice and equinox periods were used. The data from the zonal thermosphere wind velocity observations obtained aboard the CHAMP satellite (~380–450 km) were also used. It was found that these characteristics are similar, when they are compared, and have very strong correlation (R 0.9) in summer, strong correlation (R 0.8) in winter and (R 0.79) at equinox. It was found that in all seasons the delay in the development of the EPB occurrence probability maxima with respect to the maxima of the west wind velocity is 1–3 h. It is in good agreement with the estimation of the time of the seeding perturbation development and the EPB rise to the topside ionosphere altitudes. The results can be considered a new confirmation of the theoretical conclusion (Kudeki model) about the key influence of the zonal west thermosphere winds on the equatorial plasma bubble generation.

Polar and Equatorial Ionospheric Electrodynamical Coupling Under a Prolonged Northward Bz Interval

AbstractThe interplanetary magnetic field (IMF) significantly influences the global ionospheric electrodynamics, but it is still largely unknown the high‐ and low‐latitude ionospheric electrodynamical coupling under long‐duration northward IMF Bz (NBz) and By conditions. During the long‐duration NBz and duskward By conditions (Kp < 2, AE < 100 nT, SYM‐H > −25 nT) on 20 August 2014, six pairs of magnetometer data showed that daytime equatorial electrojet (EEJ) underwent strong decreases (reach up to 90%) at wide longitudes. Thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) captured well the decrease and the control simulations revealed that both effects of the NBz and By significantly change the plasma convection, Joule heating, and thermospheric winds at high latitudes; and penetration electric field (PEF) due to the NBz plays an important role in the decrease of the equatorial electric field. This study indicates the significant NBz effects on the equatorial ionospheric electrodynamics even during weak geomagnetic conditions, apart from the widely believed meteorological effects. Further study should be taken to disclose the relative contribution between geomagnetic and meteorological effects on the decrease of the equatorial electric field.

Longitudinal variability of thermospheric zonal winds near dawn and dusk

Understanding the morphology and dynamics of the thermosphere is key to understanding the Earth’s upper atmosphere as a whole. Thermospheric winds play an important role in this process by transporting momentum and energy and affecting the composition, dynamics and morphology of not only the thermosphere but also of the ionosphere. The general morphology of the winds has been well established over the past decades, but we are only starting to understand its variability. In this process the lower atmosphere plays an important role due to direct penetration of waves from the lower atmosphere into the ionosphere/thermosphere, secondary waves generated on the way, or internal feedback mechanisms in the coupled ionosphere-thermosphere system. Therefore, knowledge about thermospheric variability and its causes is critical for an improved understanding of the global ionosphere-thermosphere system and its coupling to the lower atmosphere. We have used low-to mid-latitude zonal wind observations obtained by the Gravity Field and Steady-State Ocean Explorer (GOCE) satellite near 260 km altitude during geomagnetically quiet times to investigate the interannual and spatial zonal wind variability near dawn and dusk, during December solstice. The temporal and spatial variability is presented as a variation about the zonal mean values and decomposed into its underlying wavenumbers using a Fourier analysis. The obtained wave features are compared between different years and clear interannual changes are observed in the individual wave components, which appear to align with changes in the solar flux but do not correlate with variations in either El Niño Southern Oscillation or the Quasi Biennial Oscillation. The obtained wave features are compared and contrasted with results from the Climatological Tidal Model of the Thermosphere (CTMT) and revealed a very good agreement between CTMT and the 2009 and 2010 December GOCE zonal wind perturbations at dawn. However, during dusk, the CTMT zonal wind perturbations and in particular the zonal wave-1 component show significant differences with those observed by GOCE.

Ionosphere-thermosphere coupling via global-scale waves: new insights from two-years of concurrent in situ and remotely-sensed satellite observations

Growing evidence indicates that a selected group of global-scale waves from the lower atmosphere constitute a significant source of ionosphere-thermosphere (IT, 100–600 km) variability. Due to the geometry of the magnetic field lines, this IT coupling occurs mainly at low latitudes (< 30°) and is driven by waves originating in the tropical troposphere such as the diurnal eastward propagating tide with zonal wave number s = −3 (DE3) and the quasi-3-day ultra-fast Kelvin wave with s = −1 (UFKW1). In this work, over 2 years of simultaneous in situ ion densities from Ion Velocity Meters (IVMs) onboard the Ionospheric Connection Explorer (ICON) near 590 km and the Scintillation Observations and Response of the Ionosphere to Electrodynamics (SORTIE) CubeSat near 420 km, along with remotely-sensed lower (ca. 105 km) and middle (ca. 220 km) thermospheric horizontal winds from ICON’s Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) are employed to demonstrate a rich spectrum of waves coupling these IT regions. Strong DE3 and UFKW1 topside ionospheric variations are traced to lower thermospheric zonal winds, while large diurnal s = 2 (DW2) and zonally symmetric (D0) variations are traced to middle thermospheric winds generated in situ. Analyses of diurnal tides from the Climatological Tidal Model of the Thermosphere (CTMT) reveal general agreement near 105 km, with larger discrepancies near 220 km due to in situ tidal generation not captured by CTMT. This study highlights the utility of simultaneous satellite measurements for studies of IT coupling via global-scale waves.

Lower Atmospheric Sources of Observed Thermosphere Medium Scale Traveling Atmospheric Disturbances Over Alaska During the 2012–2013 Winter Months

AbstractThis 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.

Wavelike Oscillations in High Latitude Thermospheric Doppler Temperature and Line‐Of‐Sight Wind Observed Using All‐Sky Imaging Fabry‐Perot Spectrometers

AbstractMultiple 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.

Climatology of quiet time geomagnetic field variations at two locations in Antarctica

During geomagnetic quiet conditions, Indian Antarctic stations are considered to be located outside the auroral oval: Maitri (CGM coordinates: 63.3°S, 54.2°E) is equatorward and Bharati (CGM coordinates: 74.8°S, 98.4°E) is poleward of the auroral oval. Simultaneous observations of magnetic field variations at these two locations for 10-years (2013–2022) provide an opportunity to study quiet-time magnetic field patterns, if any. Geomagnetic quiet days with ΣKp ≤ 3 are selected, during which the lower values of solar wind and interplanetary parameters are also confirmed. Maitri station exhibits clear southern hemispheric solar quiet (Sq) type of magnetic field variation on geomagnetic quiet days in all seasons, indicating the influence of ionospheric dynamo due to thermospheric winds. Interestingly, Bharati station also displays regular and systematic magnetic field variations in all three components. The D-component at Bharati exhibits very strong variation at early morning hours (7–8 MLT), which is ∼2–4 times stronger than that of H-component, driving strong equatorward/northward currents during all seasons. Both stations show annual type of seasonal variation with peak amplitude during summer and least during winter. The schematic illustration of global and polar Sqs proposed here explains the results obtained through 10-years’ statistical study.

Ionospheric scintillation characteristics over Indian region from latitudinally-aligned geodetic GPS observations

Ionospheric scintillations pose one of the biggest threats to Satellite-Earth communication links in the communication and navigation systems services whose occurrence characteristics could be explained through the strength of fading (amplitude scintillation index; S4) and the rapidity of the fades (decorrelation time; \U0001d70f0). In the present work, we analyzed the S4 index from a latitudinal array of three geodetic global positioning system (GPS) stations along the Indian longitude sector during the descending phase of the 24th solar cycle. The results show predominant occurrences of scintillations during the post-sunset and nighttime periods indicating the level of scintillations close to equatorial, beyond anomaly crest, and near mid-latitude locations. The strong amplitude scintillations during 2014 and 2015 and their lowest magnitudes during 2016 and 2017 reflect the solar activity dependence of the scintillation occurrences. The directional distribution and 2-dimensional surface sky plots of the S4 index substantiate the occurrence of intense scintillation being more prevalent towards the equatorial location, whereas the weak and moderate scintillations are perceived towards the higher low latitude stations. The occurrences of intense scintillations are confirmed in high solar active years and equinox season at all stations, while the equatorial station presents relatively higher occurrences in winter followed by summer. The summer season witnessed intense occurrences even under moderate to low solar activity years (2016–2017), reflecting the post-midnight occurrences due to meridional thermospheric winds. Moreover, maximum percentage occurrences of weak scintillations in all seasons are evident towards the poleward side, far away from the anomaly crest. The amplitude scintillation frequency during the period is further compared with the solar and geomagnetic indices to substantiate the analysis drawn from the variations. The correlation coefficients between the scintillation index occurrence rate with the solar and geomagnetic in-dices at three GNSS stations provide valuable insights into the relationship between solar activity and scintillation. The results show that the strength and direction of the correlation can vary significantly depending on the specific solar index and location. Thus, emphasize further studies on scintillation occurrences for developing effective forecasting and mitigation models over the Indian low-latitude region.

IMF Dependence of Midnight Bifurcation in the Thermospheric Wind at an Auroral Latitude Based on Nine Winter Measurements in Tromsø, Norway

AbstractA thermospheric wind data set from a Fabry‐Perot interferometer (630 nm) and the ion velocity from a Dynasonde in Tromsø, Norway, was analyzed for nine winter seasons to study the dynamics of the thermosphere and F‐region ionosphere at an auroral latitude. This study focused on bifurcation in the zonal component of the neutral wind and ion velocity at midnight and its dependence on the Y component of the interplanetary magnetic field (IMF). Ionospheric plasma convection patterns are evidently imprinted on the thermospheric wind variations as aspects of the westward and eastward accelerations at dusk and late morning, respectively. The zonal wind bifurcates immediately before midnight for IMF By < 0, but for By > 0, it inverts gradually into the postmidnight sector. Neutral wind streams, originating from higher latitudes, may result in the dependence because of anti‐sunward plasma flow distorted in the polar cap.

The Effects of a Small Geomagnetic Storm on Earth's Thermosphere and Ionosphere: ICON Observations of the 25 January 2021 Disturbance

AbstractIsolated geomagnetic storms during solar minimum offer the opportunity to observe the effects of solar wind on Earth's atmosphere without interference from other background changes in the ionosphere‐thermosphere system. Here, we observe the effects of a minor storm on the ionosphere at middle and low latitudes, where disturbances in thermospheric winds and composition can produce storm‐time changes in ionospheric densities. Using data from the NASA Ionospheric Connection Explorer (ICON), we investigate the competing effects of composition changes and neutral wind disturbances in modifying the ionosphere during the 25 January 2021 storm. Large enhancements in plasma densities are seen across the dayside late on 25 January, associated with large changes in meridional winds and significant high‐latitude electric current enhancements as measured by the auroral electrojet index. At the same time, we find evidence of rapid transport of heated air from the southern auroral zone to the equator in the East‐Asian sector with the appearance of deeply reduced O/N2 in the morning sector and storm‐time northward winds prior to its appearance. We observe significant, rapid erosion of the high plasma densities in the sector where thermospheric disturbance appears over 5 hr of observation. The noon‐time plasma density enhancements are short‐lived and we observe no westward perturbation in the low‐middle latitude, dayside zonal winds over a 5‐day period including the storm. Thus, we attribute the ionospheric changes to prompt penetration and meridional wind forcing, while ruling out the presence of a storm‐time disturbance dynamo and its related ionospheric effects.

Average Ionospheric Mid‐ and Low‐Latitude Currents From E‐ and F‐Region Altitudes, Derived by the Swarm Constellation

AbstractBy utilizing magnetic field measurements from the Swarm constellation during the years 2014–2019, average characteristics of the height‐integrated ionospheric zonal and meridional currents at middle and low latitudes are investigated. We provide for the first time the detailed dependencies of these currents on location, local time, and season. The main results obtained are: (a) Large parts of the ionospheric currents flow in the F‐region, particularly on the nightside and in the winter hemisphere. (b) Clearly outstanding zonal current is the equatorial electrojet (EEJ) around noon, accompanied by currents in opposite direction at middle latitudes. At night, eastward currents are prevailing in the summer hemisphere, and westward in winter. During equinox the nighttime zonal currents show a lot of hemispheric similarity, with westward currents around pre‐midnight and eastward currents post‐midnight at mid latitudes. These features can be attributed to the meridional thermospheric wind effects. (c) The meridional currents are weak at night, exhibiting a preferred direction toward the summer hemisphere. At daytime the meridional currents flow toward equator in the pre‐noon sector and opposite current directions prevail in the afternoon, which is consistent with the closing of the EEJ current circuit. Large hemispheric differences in meridional currents are found during December solstice, suggesting that most of the EEJ current is closed through the winter, northern hemisphere. (d) The zonal and meridional currents at middle latitude exhibit clear signatures of solar tides at daytime. Most prominent are the non‐migrating tidal components DE3 in the zonal currents and SW5 in the meridional.

Monthly Climatologies of Zonal‐Mean and Tidal Winds in the Thermosphere as Observed by ICON/MIGHTI During April 2020–March 2022

AbstractVersion 5 (v05) of the thermospheric wind data from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) instrument on the Ionospheric Connection Explorer (ICON) mission has been recently released, which largely avoids local‐time dependent artificial baseline drifts that are found in previous versions of the ICON/MIGHTI wind data. This paper describes monthly climatologies of zonal‐mean winds and tides based on the v05 ICON/MIGHTI data under geomagnetically quiet conditions (Hp30 < 3o) during April 2020–March 2022. Green‐line winds in the lower thermosphere (90–110 km) and red‐line winds in the middle thermosphere (200–300 km) are analyzed, as these data cover both daytime and nighttime. The latitude and height structures of zonal‐mean winds and tides are presented for each month, and the results are compared with the widely used empirical model, Horizontal Wind Model 2014 (HWM14). The ICON/MIGHTI and HWM14 results are in general agreement, providing a validation of the v05 ICON/MIGHTI data. The agreement is especially good for the zonal‐mean winds. Amplitudes of lower thermospheric tides from ICON/MIGHTI tend to be larger than those from HWM14 as well as from an empirical model, Climatological Tidal Model of the Thermosphere (CTMT). This could be due to the influence of interannual variability of the tides. The amplitude structure of lower thermospheric tides in HWM14 does not match those from ICON/MIGHTI and CTMT in some months. Also, HWM14 underestimates the meridional‐wind amplitude of the migrating diurnal tide in the middle thermosphere. These results highlight the need for improved tidal representation in HWM14.

History of EISCAT – Part 6: the participation of Japan in the EISCAT Scientific Association

Abstract. In Sect. 1, the original planning of Japanese Svalbard IS (incoherent scatter) radar with phased-array antennas is described. In 1988, this plan was proposed as one of the major projects for the forthcoming Solar–Terrestrial Environment Laboratory, Nagoya University, Japan, to be reorganized by the Research Institute of Atmospherics at Nagoya University. On the other hand, in 1989, UK scientists proposed a plan of polar cap radar with parabolic dish antennas in Longyearbyen to the EISCAT (European incoherent scatter) Council. In Sect. 2, the circumstances leading to Japan's participation in the EISCAT Scientific Association, with details of its processes with strong collaborations with Norwegian scientists and the EISCAT Scientific Association are described. In 1995, Japan participated EISCAT Scientific Association as the seventh member country with funds contributing to the second dish antenna of the EISCAT Svalbard Radar. In Sect. 3, a summary of the EISCAT-related achievement by Japanese scientists is described, where major interests are the lower thermosphere wind dynamics, the magnetosphere–ionosphere–thermosphere coupling, characteristics, and driving mechanisms of ion upflow, electrodynamics of current, electric field and particles, characteristics and production mechanisms of auroras, such as pulsating aurora, and aurora tomography. In Sect. 4, a summary of the scientific collaborations between Japan and Europe, particularly those between Japan and Norway, and hopes for the forthcoming EISCAT_3D and further collaboration with EISCAT community are described.

Understanding Nighttime Ionospheric Depletions Associated With Sudden Stratospheric Warmings in the American Sector

AbstractThis study focuses on understanding what drives the previously observed deep nighttime ionospheric hole in the American sector during the January 2013 sudden stratospheric warming (SSW). Performing a set of numerical experiments with the thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) constrained by a high‐altitude version of the Navy Global Environmental Model, we demonstrate that this nighttime ionospheric hole was the result of increased poleward and down magnetic field line plasma motion at low and midlatitudes in response to altered F‐region neutral meridional winds. Thermospheric meridional wind modifications that produced this nighttime depletion resulted from the well‐known enhancements in semidiurnal tidal amplitudes associated with stratospheric warming (SSWs) in the upper mesosphere and thermosphere. Investigations into other deep nighttime ionospheric depletions and their cause were also considered. Measurements of total electron content from Global Navigation Satellite System receivers and additional constrained TIME‐GCM simulations showed that nighttime ionospheric depletions were also observed on several nights during the January‐February 2010 SSW, which resulted from the same forcing mechanisms as those observed in January 2013. Lastly, the recent January 2021 SSW was examined using Modern‐Era Retrospective Analysis for Research and Applications, Version 2, COSMIC‐2 Global Ionospheric Specification electron density, and ICON Michelson Interferometer for Global High‐Resolution Thermospheric Imaging horizontal wind data and revealed a deep nighttime ionospheric depletion in the American sector was likely driven by modified meridional winds in the thermosphere. The results shown herein highlight the importance of thermospheric winds in driving nighttime ionospheric variability over a wide latitude range.

Atmospheric and Ionospheric Responses to Hunga‐Tonga Volcano Eruption Simulated by WACCM‐X

AbstractHigh‐resolution Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension is used to simulate the responses to the Hunga‐Tonga volcano eruption on 15 January 2022. Global propagation of the Lamb wave L’0 and L’1 pseudomodes are reproduced in the simulation, with the exponential growth of wave amplitudes with altitudes. The wavefront is vertical up to the lower thermosphere, and tilts outward above. These features are consistent with theoretical results. With simulated surface pressure perturbation agreeing with observations (∼100–250 Pa), thermospheric wind perturbations over 100 ms−1 are comparable with reported satellite and ground‐based observations. Traveling ionospheric disturbances in the total electron contents from the simulation show good agreement with observations, including magnitude and propagating speed and evidence of conjugacy in the first 1–2 hr after eruption. Conjugacy in E × B drift, on the other hand, is more persistent.