Using the horizontal neutral wind observations from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) instrument onboard NASA's Ionospheric Connection Explorer ...(ICON) spacecraft with continuous coverage, we determine the climatology of the mean zonal and meridional winds and the associated mean circulation at low‐to middle latitudes (10°S–40°N) for Northern Hemisphere summer solstice conditions between 90 and 200 km altitudes, specifically on 20 June 2020 solstice as well as for a one‐month period from 8 June–7 July 2020 and for Northern winter season from 16 December 2019–31 January 2020, which spans a 47‐day period, providing full local time coverage. The data are averaged within appropriate altitude, longitude, latitude, solar zenith angle, and local time bins to produce mean wind distributions. The geographical distributions and local time variations of the mean horizontal circulation are evaluated. The instantaneous horizontal winds exhibit a significant degree of spatiotemporal variability often exceeding ±150 m s−1. The daily averaged zonal mean winds demonstrate day‐to‐day variability. Eastward zonal winds and northward (winter‐to‐summer) meridional winds are prevalent in the lower thermosphere, which provides indirect observational evidence of the eastward momentum deposition by small‐scale gravity waves. The mean neutral winds and circulation exhibit smaller scale structures in the lower thermosphere (90–120 km), while they are more homogeneous in the upper thermosphere, indicating the increasingly dissipative nature of the thermosphere. The mean wind and circulation patterns inferred from ICON/MIGHTI measurements can be used to constrain and validate general circulation models, as well as input for numerical wave models.
Plain Language Summary
Atmospheric horizontal winds (i.e., motion of the neutral air), composed of zonal (east‐west) and meridional (north‐south) components, play an important role for the energy and momentum balance of the atmosphere and ionosphere. Due primarily to a lack of observations, winds in the thermosphere are not well sampled. In this study we use the horizontal winds measured from 90 to 200 km altitude by the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard NASA's Ionospheric Connection Explorer spacecraft to generate two‐dimensional maps of zonal and meridional winds, and of the resulting horizontal motion (or circulation) in the thermosphere for Northern Hemisphere solstice conditions. Specifically, winds at solstice (20 June 2020) and a 1 month Northern summer solstitial period (8 June to 7 July 2020) and a 47‐day winter solstitial period (16 December 2019 to 31 January 2020) have been analyzed. Mean winds show significant spatial variation as a function of time, often demonstrating tidal variability.
Key Points
Mean zonal and meridional winds are derived for Northern summer and winter solstice conditions from Ionospheric Connection Explorer/Michelson Interferometer for Global High‐resolution Thermospheric Imaging observations
Horizontal winds exhibit a significant degree of spatiotemporal variability, exceeding ±150 m s−1
Zonal and meridional mean winds are more homogeneous in the upper thermosphere and exhibit reversal in the lower thermosphere
Wind measurements from the Michelson Interferometer for Global High-resolution Ther-23mospheric Imaging (MIGHTI) instrument on the Ionospheric CONnections (ICON) mis-24sion provide new insights into ...the semidiurnal tidal spectrum in the thermosphere, cov-25ering latitudes 9◦S-39◦N and altitudes 100-280 km altitude throughout 2020. Latitude26versus day of year (DOY) variability of solar semidiurnal tides SE2, S0, SW1, SW2, SW327and SW4 at 250 km are presented for the first time, and evaluated relative to similar re-28sults at 106 km. Using daytime-only data, height versus latitude and height versus DOY29variability of SE2, S0, SW1. SW3 and SW4 amplitudes and phases are depicted for the30first time, revealing the effects of a dissipative thermosphere on the vertical evolutions31of these tidal structures. SW2 is absent from these depictions due to potential aliasing32by zonal mean winds. The above results are considered in light of the Climatological Tidal33Model of the Thermosphere (CTMT), which is based on fits to tidal winds and temper-34atures from the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED)35mission between 80 and 120 km during 2002-2008, and extrapolated to an altitude of 40036km based on modeled tidal structures propagating in a dissipative thermosphere, but with-37out in-situ sources of excitation due to tide-tide or tide-ion drag nonlinear interactions.38On the basis of comparisons with the CTMT and other characteristics revealed in the39MIGHTI tidal structures, it is concluded that in-situ sources exist for S0, SW1, SW2 and40SW3 in the thermosphere above about 200 km.
Exospheric temperature is one of the key parameters in constructing thermospheric models and has been extensively studied with in situ observations and remote sensing. The Global‐scale Observations ...of the Limb and Disk (GOLD) at a geosynchronous vantage point provides dayglow limb images for two longitude sectors, from which we can estimate the terrestrial exospheric temperature since 2018. In this paper, we investigate climatological behavior of the exospheric temperature measured by GOLD. The temperature has positive correlations with solar and geomagnetic activity and exhibits a morning‐afternoon asymmetry, both of which agree with previous studies. We have found that the arithmetic sum of F10.7 (solar) and Ap (geomagnetic) indices is highly correlated with the exospheric temperature, explaining ∼64% of the day‐to‐day variability. Furthermore, the exospheric temperature has good correlation with thermospheric parameters (e.g., neutral temperature, O2 density, and NO emission index) sampled at various heights above ∼130 km, in spite of the well‐known thermal gradient below ∼200 km. However, thermospheric temperature at altitudes around 100 km is not well correlated with the GOLD exospheric temperature. The result implies that effects other than thermospheric heating by solar Extreme Ultraviolet and geomagnetic activity take control below a threshold altitude that exists between ∼100 and ∼130 km.
Plain Language Summary
In the terrestrial thermosphere, which is the topmost layer of the collisional atmosphere surrounding the Earth, temperature generally increases with altitude unlike in the troposphere near the surface. However, the thermospheric temperature does not increase indefinitely, but approaches an asymptote, which is referred to as the exospheric temperature, the temperature of the outermost layer of the atmosphere. The Global‐scale Observations of the Limb and Disk (GOLD) instrument, launched into a geosynchronous orbit in 2018, regularly observes atmospheric glow from the Earth's limb, from which we can estimate the terrestrial exospheric temperature (Texo). The GOLD Texo is higher in the afternoon than in the morning and generally increases as solar radiation strength or geomagnetic field variability increases. In this paper, we show that a simple arithmetic sum of the solar and geomagnetic indices can explain a large part of the day‐to‐day Texo variability. Also, Texo has good (poor) correlation with thermospheric parameters such as temperature and density above ∼130 km (at ∼100 km). The behavior suggests that factors other than solar radiation and geomagnetic energy deposition, both of which are correlated well with Texo, take control below a threshold altitude that is between ∼100 and ∼130 km.
Key Points
We report on the climatology of exospheric temperature (Texo) measured by NASA‐GOLD in 2018–2021
Texo is highly correlated with Ap and F10.7, whose linear combination can explain ∼64% of the Texo day‐to‐day variability in 2018–2021
Texo has positive correlation with Swarm thermospheric mass density, GOLD disk temperature, GOLD O2 density, and TIMED/SABER TCI
Coincident Ionospheric Connections Explorer (ICON) measurements of neutral winds, plasma drifts and total ion densities (:=Ne, electron density) are analyzed during January 1–21, 2020 to reveal the ...relationship between neutral winds and ionospheric variability on a day‐to‐day basis. Atmosphere‐ionosphere (A‐I) connectivity inevitably involves a spectrum of planetary waves (PWs), tides and secondary waves due to wave‐wave nonlinear interactions. To provide a definitive attribution of dynamical origins, the current study focuses on a time interval when the longitudinal wave‐4 component of the E‐region winds is dominated by the eastward‐propagating diurnal tide with zonal wavenumber s = −3 (DE3). DE3 is identified in winds and ionospheric parameters through its characteristic dependence on local solar time and longitude as ICON's orbit precesses. Superimposed on this trend are large variations in low‐latitude DE3 wave‐4 zonal winds (±40 ms−1) and topside F‐region equatorial vertical drifts at periods consistent with 2‐days and 6‐days PWs, and a ∼3‐day ultra‐fast Kelvin wave (UFKW), coexisting during this time interval; the DE3 winds, dynamo electric fields, and drifts are modulated by these waves. Wave‐4 variability in Ne is of order 25%–35%, but the origins are more complex, likely additionally reflecting transport by ∼20–25 ms−1 wave‐4 in‐situ winds containing strong signatures of DE3 interactions with ambient diurnal Sun‐synchronous winds and ion drag. These results are the first to show a direct link between day‐to‐day wave‐4 variability in contemporaneously measured E‐region neutral winds and F‐region ionospheric drifts and electron densities.
Key Points
Coincident Ionospheric Connections Explorer measurements of neutral winds, plasma drifts and total ion densities (:=Ne) are analyzed during January 1–21, 2020
We show for the first time that planetary wave winds modulate DE3 and produce longitudinal wave‐4 variations in F‐region vertical drifts ∼±10 ms−1, Ne ∼ ±30%
Measured F‐region wave‐4 winds suggest that SPW4, SE2 & DW5 arising from tide‐tide and tide‐ion drag interactions also contribute to Ne variability
Vertical shears of horizontal winds play an important role in the dynamics of the upper atmosphere. Prior observations have indicated that these shears predominantly occur in the lower thermosphere. ...MIGHTI observations from the Ionospheric Connection Explorer indicate that strong wind shears are a common feature of the lower thermosphere between 100–130 km, varying greatly between orbits. This work focuses on these strong shears, and examines their occurrences, horizontal scales and underlying organization. The wind shears can persist for 1000s km horizontally. Over a large data set, no preferred direction for the strong wind shears is found. The shears that persist for a short horizontal extent are slightly larger in amplitude and more numerous than those that persist across large horizontal scales. The altitude at which the strongest shears occur, regardless of the horizontal extent, show a downward progression with local time, following the climatological winds and upward propagating tides.
Plain Language Summary
Understanding wind patterns is a key component to understanding our atmosphere. Gradients in these winds, seen as regions where slow and fast moving air are in close proximity, or air moving in opposite directions is in close proximity, can play especially important roles in atmospheric motion and mixing of the air. At altitudes near the edge of space, such gradients are known to be especially large. New observations from the Ionospheric Connection Explorer (ICON) spacecraft have shown that these gradients are often present in this region, but can vary greatly from one orbit of the spacecraft to the next. This work examines the characteristics of the gradients observed with ICON. The analysis presented shows the direction and magnitude of these gradients, at what time of day and what altitude they are observed. The altitude where the strongest gradients are observed is shown to change with time of day.
Key Points
Strong horizontal wind shears with spatial scales from 100s km to over 10,000 km are observed with ICON MIGHTI
Patterns in the direction, horizontal scale length and altitude of the strongest shears observed in the lower thermosphere are examined
The strongest shears display an altitude variation in local time that reflects the downward phase progression in the tidal winds
The design, principles of operation, calibration, and data analysis approaches of the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) on the NASA Ionospheric ...Connection (ICON) satellite have been documented prior to the ICON launch. Here we update and expand on the MIGHTI wind data analysis and discuss the on-orbit instrument performance. In particular, we show typical raw data and we describe key processing steps, including the correction of a “signal-intensity dependent phase shift,” which is necessitated by unexpected detector behavior. We describe a new zero-wind calibration approach that is preferred over the originally planned approach due to its higher precision. Similar to the original approach, the new approach is independent of any a priori data. A detailed update on the wind uncertainties is provided and compared to the mission requirements, showing that MIGHTI has met the ICON mission requirements. While MIGHTI observations are not required to produce absolute airglow brightness profiles, we describe a relative brightness profile product, which is included in the published data. We briefly review the spatial resolution of the MIGHTI wind data in addition to the data coverage and data gaps that occurred during the nominal mission. Finally, we include comparisons of the MIGHTI wind data with ground-based Fabry-Perot interferometer observations and meteor radar observations, updating previous studies with more recent data, again showing good agreement. The data processing steps covered in this work and all the derived wind data correspond to the MIGHTI data release Version 5 (v05).
We report for the first time the day‐to‐day variation of the longitudinal structure in height of the F2 layer (hmF2) in the equatorial ionosphere using multi‐satellite observations of electron ...density profiles by the Constellation Observing System for Meteorology, Ionosphere and Climate‐2 (COSMIC‐2). These observations reveal a ∼3‐day modulation of the hmF2 wavenumber‐4 structure viewed in a fixed local time frame during January 30–February 14, 2021. Simultaneously, ∼3‐day planetary wave activity is discerned from zonal wind observations at ∼100 km by the Ionospheric Connection Explorer (ICON) Michelson Interferometer for Global High‐Resolution Thermospheric Imaging (MIGHTI). This signature is not observed at ∼180–250 km altitudes, suggesting the dissipation of this wave below the F‐region. We propose that the 3‐day variation identified in hmF2 is likely caused by the planetary wave‐tide interaction through the E‐region dynamo.
Plain Language Summary
The F‐region ionosphere at ∼200–400 km altitudes often shows global‐scale structures and variations that are attributed to neutral atmospheric tides and planetary waves propagating from the lower atmosphere. Previous observations have identified 3‐day planetary waves at E‐region altitudes (∼100 km), but the vertical extent of these waves has not yet been determined due to lack of high altitude observations. For this study, we use newly obtained concurrent atmospheric and ionospheric observations that provide the necessary coverage. Our study provides the first observational evidence of the vertical propagation of a 3‐day planetary wave across both E‐ and F‐region altitudes. The results suggest that this wave causes the day‐to‐day variation of the four‐peaked longitudinal structure in the equatorial ionosphere through modulating the E‐region dynamo rather than direct propagation into the F‐region.
Key Points
First examination of 3‐day wave activity across ∼100–250 km altitudes and the ionospheric peak height change using coordinated satellites
Ionospheric longitudinal structure shows a ∼3‐day variation coinciding the planetary wave observed in atmospheric winds at ∼100 km altitude
The neutral atmospheric 3‐day wave signature is not observed at ∼180–250 km altitudes, suggesting the modulation of tides in the E‐region
A quasi‐2‐day wave (Q2DW) event during January‐February, 2020, is investigated in terms of its propagation from 96 to 250 km as a function of latitude (10°S to 30°N), its nonlinear interactions with ...migrating tides to produce 16 and 9.6‐h secondary waves (SWs), and the plasma drift and density perturbations that it produces in the topside F‐region (590–607 km) between magnetic latitudes 18°S and 18°N. This is accomplished through analysis of coincident Ionospheric Connections Explorer (ICON) measurements of neutral winds, plasma drifts and ion densities, and wind measurements from four low‐latitude (±15°) specular meteor radars (SMRs). The Q2DW westward‐propagating components that existed during this period consist of zonal wavenumbers s = 2 and s = 3, that is, Q2DW+2 and Q2DW+3 (e.g., He, Chau et al., 2021, https://doi.org/10.1029/93jd00380). SWs in the ICON measurements are inferred from Q2DW+2 and Q2DW+3 characteristics derived from traditional longitude‐UT fits that potentially contain aliasing contributions from SWs (“apparent” Q2DWs), from fits to space‐based zonal wavenumbers that each reflect the aggregate signature of either Q2DW+2 or Q2DW+3 and its SWs combined (“effective” Q2DWs), and based on information contained in published numerical simulations. The total Q2DW ionospheric responses consists of F‐region field‐aligned and meridional drifts of order ±25 ms−1 and ±5–7 ms−1, respectively, and total ion density perturbations of order (±10%–25%). It is shown that the SWs can sometimes make substantial contributions to the Q2DW winds, drifts, and plasma densities.
Key Points
The first contemporaneous measurements of Q2DW winds and the topside ionospheric response are reported based on ICON data
Q2DW topside F‐region field‐aligned and meridional drifts of ∼25 m/s and ∼6 m/s, and electron density perturbations of ∼10%–25% occurred
ICON winds, drifts, Ne, and ground‐based radar‐measured winds contain signatures of Q2DW‐tide nonlinear interactions
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