Thermospheric Horizontal Winds Research Articles

Effect of Vertical Shear in the Zonal Wind on Equatorial Electrojet Sidebands: An Observational Perspective Using Swarm and ICON Data

AbstractThe wind dynamo in the ionosphere leads to differential motion of ions and electrons, which in turn sets up electric fields and currents. Observations show that daytime lower thermospheric horizontal winds have large vertical gradients. Numerical modeling conducted approximately 50 years ago demonstrated that the zonal wind shears in the ∼130–180 km altitude range can generate off‐equatorial relative minima (dips) in the daytime height‐integrated eastward current density, appearing as westward sidebands north and south of the equatorial electrojet (EEJ). This study observationally confirms this connection for the first time by combining Ionospheric CONnection explorer zonal wind profiles and Swarm latitudinal zonal currents. We demonstrate observationally that the magnitude of the EEJ sideband current is proportional to the strength of westward turning winds with altitude in the Pedersen conductivity dominated region. Additional numerical experiments explain the importance of wind shear in different altitude regions in generating the sideband current. This study contributes to the better understanding of the neutral wind effect on the local current generation.

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.

Pronounced Suppression and X-Pattern Merging of Equatorial Ionization Anomalies After the 2022 Tonga Volcano Eruption.

Following the 2022 Tonga Volcano eruption, dramatic suppression and deformation of the equatorial ionization anomaly (EIA) crests occurred in the American sector ∼14,000 km away from the epicenter. The EIA crests variations and associated ionosphere‐thermosphere disturbances were investigated using Global Navigation Satellite System total electron content data, Global‐scale Observations of the Limb and Disk ultraviolet images, Ionospheric Connection Explorer wind data, and ionosonde observations. The main results are as follows: (a) Following the eastward passage of expected eruption‐induced atmospheric disturbances, daytime EIA crests, especially the southern one, showed severe suppression of more than 10 TEC Unit and collapsed equatorward over 10° latitudes, forming a single band of enhanced density near the geomagnetic equator around 14–17 UT, (b) Evening EIA crests experienced a drastic deformation around 22 UT, forming a unique X‐pattern in a limited longitudinal area between 20 and 40°W. (c) Thermospheric horizontal winds, especially the zonal winds, showed long‐lasting quasi‐periodic fluctuations between ±200 m/s for 7–8 hr after the passage of volcano‐induced Lamb waves. The EIA suppression and X‐pattern merging was consistent with a westward equatorial zonal dynamo electric field induced by the strong zonal wind oscillation with a westward reversal.

Statistical Characterization of GITM Thermospheric Horizontal Winds in Comparison to GOCE Estimations

AbstractCharacterization of the thermospheric horizontal wind is an important challenge in atmospheric modeling due to its vital role in the transport of densities and energy, associations with the diurnal tide, and interplay with vertical winds that drive changes in the thermosphere neutral composition. The mechanisms and drivers that underlie the physics of thermospheric horizontal winds remain under investigation and, to date, no comprehensive statistical study between thermospheric winds generated by a physics‐based atmospheric model and those retrieved from satellite measurements has been performed. Comparisons between cross‐track horizontal winds from a 10‐month run of Global Ionosphere‐Thermosphere (GITM) and those derived from the Gravity field and steady‐state Ocean Circulation Explorer (GOCE) satellite show that GITM did best modeling horizontal winds in the polar zone but overestimated horizontal wind speeds at midlatitudes just outside of the equatorial ionization anomaly region. GITM's wind response to AE was best at polar noon and worst in the midnight auroral zone, its ability to capture seasonality was best in the northern high latitudes and worst in the southern high latitudes, and GITM displayed less wind variability as a function of F10.7 than GOCE, matching it best for F10.7 >∼150. Discrepancies in GITM's performance may be explained by inaccurate modeling of ion drift, ion drag, and electron densities.

Thermospheric gravity wave characteristics in the daytime over low-latitudes during geomagnetic quiet and disturbed conditions

The importance of gravity waves (GWs) in the redistribution of energy and momentum in the Earth's atmosphere is well known. It is extremely important to understand the nature of variation in the GW characteristics to address issues of coupling in the Earth's atmosphere. We have used digisonde measurements to investigate the variation in thermospheric wave dynamics in the daytime over low-latitudes (Ahmedabad, India) during geomagnetic quiet and disturbed times. Vertical phase speeds and wavelengths of GWs are calculated by monitoring the phase offsets in the digisonde derived height variations of constant electron densities throughout the day. During geomagnetic quiet times, the magnitudes of vertical propagation speeds of GWs and their vertical wavelengths show seasonal variation and vary in the range of 10–70 ms−1 and 70–520 km, respectively. The distributions of vertical propagation parameters of GWs for geomagnetic quiet days show two peaks annually and are anti-correlated with the thermospheric horizontal winds. As these winds over a given location flow in different directions in different seasons, this information can be used to infer the season dependent preferential direction of propagation of GWs. Changes in the vertical propagation characteristics of GWs during geomagnetic disturbed days as compared to their average quiet time values show good semblance with the integrated values of the AE index. A linear relation has been obtained between the changes in vertical phase speeds of GWs and the integrated values of AE index during geomagnetic disturbed days. This linear relation, in combination with empirical formulation arrived at for averaged values of vertical propagation speeds during geomagnetic quiet times, yields comprehensive information on propagation characteristics of GWs as a function of day of the year that includes geomagnetic disturbance conditions as well. Such systematic information on the GW characteristics in the daytime thermosphere and their quantification during geomagnetic quiet times and their changes due to enhancement in energy input at high-latitudes (as characterized by the AE index) also provides a strong and direct information on high-to low-latitude coupling.

A Comparison of Quiet Time Thermospheric Winds Between FPI Observations and Model Calculations

AbstractWe investigate local time, seasonal, and longitudinal variations of thermospheric horizontal winds at midlatitudes during geomagnetically quiet times by comparing winds from observations, an empirical wind model, and a numerical model. The observations are from three Fabry‐Perot interferometers (FPIs), the empirical model is the most recent version of the Horizontal Wind Model 14 (HWM14), and the numerical model is the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model of the National Center for Atmospheric Research. The three FPI stations are located at Xinglong (geographically 40.2°N, 117.4°E; geomagnetically 35°N) and Kelan (geographically 38.7°N, 111.6°E; geomagnetically 34°N) in China and at Millstone Hill (geographically 42.6°N, 71.5°W; geomagnetically 52°N) in United States. The results show that the winds at Millstone Hill are more southward and more westward than those at Xinglong and Kelan in all seasons; the directional reversal time of zonal winds is also earlier at Millstone Hill than at the other two stations. Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model shows better agreement with FPI observations in the winter months compared to summer, and in general best replicates those measurements taken at Millstone Hill. HWM14 generally produces agreement with quiet time midlatitude neutral wind measurements, with HWM14 meridional winds comparing very well throughout most of a year. HWM14 model‐data discrepancies occur mainly in the zonal winds during the winter season. Overall, HWM14 predicts MH FPI observations slightly better than it does for the data from the two Asian stations.

Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI): Instrument Design and Calibration.

The Michelson Interferometer for Global High-resolution imaging of the Thermosphere and Ionosphere (MIGHTI) instrument was built for launch and operation on the NASA Ionospheric Connection Explorer (ICON) mission. The instrument was designed to measure thermospheric horizontal wind velocity profiles and thermospheric temperature in altitude regions between 90km and 300km, during day and night. For the wind measurements it uses two perpendicular fields of view pointed at the Earth's limb, observing the Doppler shift of the atomic oxygen red and green lines at 630.0nm and 557.7nm wavelength. The wavelength shift is measured using field-widened, temperature compensated Doppler Asymmetric Spatial Heterodyne (DASH) spectrometers, employing low order échelle gratings operating at two different orders for the different atmospheric lines. The temperature measurement is accomplished by a multichannel photometric measurement of the spectral shape of the molecular oxygen A-band around 762nm wavelength. For each field of view, the signals of the two oxygen lines and the A-band are detected on different regions of a single, cooled, frame transfer charge coupled device (CCD) detector. On-board calibration sources are used to periodically quantify thermal drifts, simultaneously with observing the atmosphere. The MIGHTI requirements, the resulting instrument design and the calibration are described.

First observations of the midlatitude evening anomaly using Super Dual Auroral Radar Network (SuperDARN) radars

[1] Under geomagnetically quiet conditions, the daytime midlatitude ionosphere is mainly influenced by solar radiation: typically, electron densities in the ionosphere peak around solar noon. Previous observations from the Millstone Hill incoherent scatter radar (ISR) have evidenced the presence of evening electron densities higher than daytime densities during the summer. The recent development of midlatitude Super Dual Auroral Radar Network (SuperDARN) radars over North America and Japan has revealed an evening enhancement in ground backscatter during the summer. SuperDARN observations are compared to data from the Millstone Hill ISR, confirming a direct relation between the observed evening enhancements in electron densities and ground backscatter. Statistics over a year of data from the Blackstone radar show that the enhancement occurs during sunset for a few hours from April to September. The evening enhancement observed by both SuperDARN and the Millstone Hill ISR is shown to be related to recent satellite observations reporting an enhancement in electron densities over a wide range of longitudes in the Northern Hemisphere midlatitude sector during summer time. Finally, global results from the International Reference Ionosphere (IRI) and the horizontal wind model (HWM07) are presented in relation with previously published experimental results and proposed mechanisms of the evening enhancement, namely, thermospheric horizontal winds and geomagnetic field configuration. It is shown that the IRI captures the features of the evening enhancement as observed by SuperDARN radars and satellites.

Horizontal structures and dynamics of Titan's thermosphere

The Cassini Ion Neutral Mass Spectrometer (INMS) measures densities of gases including N2 and CH4 in situ in Titan's upper atmosphere. We have used data from 13 targeted flybys of Titan (T5–T32) to construct an empirical model that describes the mean state of the thermosphere in the northern hemisphere, giving the N2 and CH4 densities between 1000 and 1600 km as a function of latitude and height. The principal features in the INMS data are well reproduced by this simple model. We find a pronounced oblateness in the thermosphere, with densities above 1100 km altitude increasing by around 70% from the northern (winter) pole to the equator, resulting in isobaric surfaces being ≃45 km higher over the equator than at the northern pole. Thermospheric temperatures derived from the densities tend to decrease with height from 149 ± 10 K to 140 ± 13 K near 1600 km. Considerable latitude differences are present in the temperatures below 1200 km. Near 1000 km altitude, temperatures reach 164 ± 6 K at 20°N and 131 ± 6 K near 80°N. Using our Thermosphere General Circulation Model with this thermal structure imposed, we derive thermospheric horizontal wind speeds reaching ∼150 m s−1, with primarily poleward flow at equatorial latitudes which, northward of around 60°N, is accompanied by a band of prograde zonal winds of up to 50 m s−1. At high latitudes, converging horizontal winds generate regions of strong subsistence. We find thermospheric dynamics to be sensitive to coupling from below. CH4 abundances are enhanced in the northern polar region, which may result from transport by thermospheric winds.

Spatial structure in the thermospheric horizontal wind above Poker Flat, Alaska, during solar minimum

A new all‐sky imaging, wavelength scanning Fabry‐Perot spectrometer was used to record high‐resolution (R ≃ 200,000) spectra of the λ630 nm thermospheric optical emission above Poker Flat, Alaska. These spectra were used to derive spatially resolved maps of the horizontal wind vector at approximately 250 km altitude. We describe the procedure used to infer vector winds from line‐of‐sight Doppler shifts, along with its limitations. We present the time evolution of the vector wind fields obtained from this method for 6 nights of observation. Five of the 6 nights contained periods when we inferred the existence of significant curvature, divergence or shear in the thermospheric wind across our instrument's ∼1000 km diameter field of view. The sixth night exhibited little spatial structure and is shown for comparison. We compare these results with a “generic” solar minimum winter time run of the National Center for Atmospheric Research's Thermosphere, Ionosphere, and Electrodynamics General Circulation Model. While agreement was good at the start and end of the night, considerable differences were found in the late evening and midnight sectors. Some possible origins for these discrepancies are proposed. In particular, we suggest that the F region horizontal wind may be deflected by upwelling vertical winds, which are in turn driven by E region heating in the auroral zone. We note that both the instrument used and our high‐latitude implementation of the analysis procedure are new experimental techniques. Thus the data presented here should be regarded as preliminary and, if possible, be validated by comparison with results from other techniques.

F-region neutral winds from ionosonde measurements of hmF2 at low latitude magnetic conjugate regions

The behavior of the F2-peak height difference Δh m F2, between low latitude magnetic conjugate points, is known to be governed by thermospheric winds blowing along the magnetic meridian. Ground based ionosonde measurements of h m F2, at two pairs of magnetic conjugate stations, have been analysed in conjunction with the results of a realistic dynamic computer model of the tropical ionospheric F-region, to determine thermospheric wind velocities. The behavior of monthly average values of the sum, at conjugate points, of the thermospheric horizontal wind velocity component in the magnetic meridian, at low latitudes, has been inferred for months of solstice and equinox, as well as for periods of low and high solar activity.