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Altitude, longitude, latitude, and local time variations of the zonal (upper panels) and meridional (lower panels) winds in m s⁻¹ during 20 June 2020 (i.e., Northern Hemisphere Summer solstice) as observed by Ionospheric Connection Explorer/Michelson Interferometer for Global High‐resolution Thermospheric Imaging. Longitude and latitude variations include data from all altitudes between 88 and 200 km, while the local time variations of the winds are shown for the lower thermosphere 88–114 km and upper thermosphere separately 117–200 km. Mean winds are shown with the red curve. See Figure 1 for the details of the spatiotemporal coverage.
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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 lo...
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Citations
... Luan and Solomon (2008) investigated the longitudinal variations in meridional winds, retrieved from peak heights and density in F 2 layer observed by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites. Yiğit et al. (2022) used ICON/MIGHTI observations to investigate the climatology in the neutral winds at low and middle thermosphere (90-200 km) during solstices. Gasque et al. (2024) presented the LT distribution of the meridional winds in the F region based on ICON/Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) measurements, and focused on the solar terminator wave structures of the thermospheric winds. ...
ICON observations were used to investigate local time (LT) and latitudinal variations of thermospheric meridional winds in the middle‐high thermosphere (160–300 km) during quiet times in 2020 June and December. At middle‐low latitudes (10°S–40°N), meridional winds were predominantly equatorward in the summer hemisphere while mostly poleward in the winter hemisphere. The meridional winds showed that the diurnal variation was dominant between ∼20°N and ∼40°N, but the semi‐diurnal variation played a leading role at lower latitudes (below ∼20°N) during solstice months. Thermosphere‐Ionosphere Electrodynamics General Circulation Model reproduced the ICON observed meridional wind variations qualitatively. A model diagnostic analysis shows that the pressure gradient force dominated the semi‐diurnal variation of the winds, while the Coriolis force played a leading role in the diurnal variation in June. In December, LT variations of meridional winds were primarily driven by pressure gradient and ion drag forces. During both months, the vertical viscosity was important, tending to balance the effects of pressure gradients. Additionally, semi‐diurnal variations of low‐latitude meridional winds in June were more affected by upward propagating tides than those in December.
... Seasonal variability is a prominent and recurrent phenomenon in the thermosphere. Since the pioneering study by Paetzold and Zschörner (1961), who discovered exceptional periodic signatures-semiannual and annual oscillations (SAO and AO)-in thermosphere neutral density, a rich body of research work has been devoted to further delineating the characteristics on the seasonal scale in observations (e.g., Bowman et al., 2008;Dhadly et al., 2020;Emmert & Picone, 2010;Gan et al., 2023;Hedin et al., 1974;Lei et al., 2012;Liu et al., 2023;Matsuo & Forbes, 2010;Qian et al., 2009Qian et al., , 2022Yamazaki et al., 2023;Yiğit et al., 2022;Yue et al., 2019). It is now well understood that the SAO is the dominant component in global averaged neutral density, which exhibits solstice lows and equinox highs. ...
... As seen by Challenging Minisatellite Payload and Gravity Recovery and Climate Experiment, the SAO in neutral density is more salient at low latitudes, while the AO becomes increasingly important at mid to high latitudes (Lei et al., 2012). Apart from neutral density, similar SAO and AO morphologies have been observed in thermospheric winds (e.g., Dhadly et al., 2020;Yamazaki et al., 2023;Yiğit et al., 2022) with Gravity Field and Steady-State Ocean Circulation Explorer and Ionospheric Connection Explorer (ICON) and in composition (e.g., Gan et al., 2023;Qian et al., 2022;Yue et al., 2019) with Global Ultraviolet Imager/Thermosphere Ionosphere Mesosphere Energetics and Dynamics (GUVI/TIMED) and Global-scale Observations of the Limb and Disk (GOLD). Meanwhile, simulations by first principles thermosphere-ionosphere models and whole atmosphere models have revealed that multiple physical mechanisms, including internal processes in the thermosphere and processes associated with atmospheric waves propagating from the lower atmosphere, play critical roles in the thermosphere seasonal variability (e.g., Fuller-Rowell, 1998;Jones et al., 2017Jones et al., , 2018Jones et al., , 2021Pilinski & Crowley, 2015;Qian & Yue, 2017;Qian et al., 2009Qian et al., , 2016. ...
Leveraging the unique perspective enabled by Global‐scale Observations of the Limb and Disk, we examined the characteristics of equinox transitions in the thermospheric column integrated ratio of atomic oxygen to molecular nitrogen (O/N2) in the Northern Hemisphere. We found that the timing of the O/N2 equinox transition from winter to summer or vice versa exhibits a progression with latitude, particularly, near spring equinox. The O/N2 equinox transition is far slower during spring compared to fall, leading to a remarkable seasonal asymmetry. Ionospheric Connection Explorer observed a prominent asymmetry in the summer‐to‐winter circulation in the middle to upper thermosphere, implying that the inter‐hemispheric circulation plays a crucial role in the O/N2 equinox transition. Additionally, since the wave‐driven meridional circulation in the lower thermosphere displays a seasonal asymmetry between the northward‐to‐southward and southward‐to‐northward transitions, we would anticipate that the O/N2 equinox transition is also influenced by the lower atmospheric forcing.
... Our focus on ICON/MIGHTI version 5 zonal and meridional winds from both green and red line measurements during the Hurricane Grace period aims to explain the spatiotemporal variability in instantaneous horizontal winds. Previous ICON observations provided indirect evidence of momentum deposition by small-scale gravity waves, contributing to our understanding of the thermosphere's dynamics during such meteorological phenomena (Yiğit et al., 2022). The inclusion of a complementary analysis for August 2020, without a hurricane event, offers a better view of the hurricane-induced variations in mean winds and thermospheric circulation. ...
Effects of Hurricane Grace in August 2021 are studied in the thermosphere and ionosphere, using data from the COSMIC‐2, ICON, and GOLD satellites. Significant impacts on electron density, thermospheric winds, and temperature are observed after the onset of the hurricane, compared to the pre‐hurricane phase. Comparison of the observations during the hurricane with the ones during a non‐hurricane year clearly provides further evidence for substantial hurricane‐induced thermospheric and ionospheric changes. We reveal an enhancement in electron density during the hurricane's rapid intensification and pronounced changes in thermospheric winds. Additionally, the low‐latitude thermosphere exhibits considerable warming of up to 70 K around 150 km during this period. These changes highlight the long‐range vertical coupling mechanisms between hurricanes and the upper atmosphere, and provide valuable insights into the profound influence of meteorological events on upper atmospheric dynamics, emphasizing the need for further exploration.
... Recently, validation of the ICON/MIGHTI green line winds have been successfully performed (Makela et al., 2021) and the mean horizontal winds and the associated circulation patterns in the Northern Hemisphere during solstice were characterized (Yiğit et al., 2022). ...
The response of the thermospheric daytime longitudinally averaged zonal and meridional winds and neutral temperature to the 2020/2021 major sudden stratospheric warming (SSW) is studied at low-to middle latitudes (0 ◦ - 40 ◦ N) using observations by NASA’s ICON and GOLD satellites. The major SSW commenced on 1 January 2021 and lasted for several days. Results are compared with the non-SSW winter of 2019/2020 and pre-SSW period of December 2020. Major changes in winds and temperature are observed during the SSW. The northward and westward winds are enhanced in the thermosphere especially above ∼140 km during the warming event, while temperature around 150 km drops up to 50 K compared to the pre-SSW phase. Changes in the zonal and meridional winds are likely caused by the SSW-induced changes in the propagation and dissipation conditions of internal atmospheric waves. Changes in the horizontal circulation during the SSW can generate upwelling at low-latitudes, which can contribute to the adiabatic cooling of the low-latitude thermosphere. The observed changes during the major SSW are a manifestation of long-range vertical coupling in the atmosphere.
... The influences of the E-region on the much more abundant plasma at the F-peak clearly depend on a range of inputs from 100-150 km altitude. A remarkable finding by ICON is that the previously identified strong and highly variable wind shears observed in this altitude range by means of chemiluminescent trail injections at night (Larsen 2002) are similarly prevalent in the daytime E-region Yigit et al. 2022). The different effects of strong winds at the altitudes of peak Hall conductivity vs. higher altitudes of greater Pedersen conductivity lead to an overall variability in the driving electric fields. ...
The two-year prime mission of the NASA Ionospheric Connection Explorer (ICON) is complete. The baseline operational and scientific objectives have been met and exceeded, as detailed in this report. In October of 2019, ICON was launched into an orbit that provides its instruments the capability to deliver near-continuous measurements of the densest plasma in Earth’s space environment. Through collection of a key set of in-situ and remote sensing measurements that are, by virtue of a detailed mission design, uniquely synergistic, ICON enables completely new investigations of the mechanisms that control the behavior of the ionosphere-thermosphere system under both geomagnetically quiet and active conditions. In a two-year period that included a deep solar minimum, ICON has elucidated a number of remarkable effects in the ionosphere attributable to energetic inputs from the lower and middle atmosphere, and shown how these are transmitted from the edge of space to the peak of plasma density above. The observatory operated in a period of low activity for 2 years and then for a year with increasing solar activity, observing the changing balance of the impacts of lower and upper atmospheric drivers on the ionosphere.
... The red-line wind data cover the height range approximately 160-300 km during day and 200-300 km at night. These wind data are useful not only for studying the neutral dynamics of the thermosphere (e.g., Cullens et al., 2020;Englert et al., 2017;Forbes et al., 2022;He et al., 2021;Triplett et al., 2023;Yiğit et al., 2022) but also for investigating atmosphere-ionosphere coupling processes, which can be realized by combining the ICON/MIGHTI wind data with ionospheric measurements made by ICON (e.g., England et al., 2021;Immel et al., 2021;Forbes et al., 2021;Park et al., 2021;Heelis et al., 2022; or by other missions (e.g., Aa et al., 2022;Gasperini et al., 2021;Gasperini et al., 2022;Harding et al., 2022;Le et al., 2022;Oberheide, 2022;Yamazaki et al., 2021;Yamazaki, Arras, et al., 2022). ...
... An eastward jet can be seen at 30°N during the Northern Hemisphere (N.H.) summer. The reversal of the zonal-mean zonal wind is often seen around 105 km, which was also noted by Yiğit et al. (2022) based on the analysis of the v04 ICON/MIGHTI data. The zonal-mean meridional wind is generally weak with little seasonal variation. ...
Version 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.
... More information on MIGHTI wind, error analyses, and validation can be found in Englert et al. (2017), Harding et al. (2017Harding et al. ( , 2021, and Makela et al. (2021). Recently, Yiğit et al. (2022) examined the climatology of MIGHTI mean zonal and meridional winds and associated mean circulation finding the prevalence of eastward zonal winds and northward meridional winds and general agreement with middle thermospheric wind climatologies, validating the use of MIGHTI to study mean winds. ...
Sudden stratospheric warmings (SSWs) are large‐scale phenomena characterized by dramatic dynamic disruptions in the stratospheric winter polar regions. Previous studies, especially those employing whole atmosphere models, indicate that SSWs have strong impacts on the circulation of the mesosphere lower thermosphere (MLT) and drive a reversal in the mean meridional circulation (MMC) near 90–125 km altitude. However, the robustness of these effects and the roles of SSW‐induced changes in global‐scale wave activity to drive the reversal have been difficult to observe simultaneously. This work employs horizontal lower thermospheric (∼93–106 km altitude) winds near 10°S‐40°N latitude from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer (ICON) to present observational evidence of a prominent MLT MMC reversal associated with the January 2021 major SSW event and to demonstrate connections to semidiurnal tidal activity and possible associations with a ∼3‐day ultra‐fast Kevin wave.
... Our results have broader impact concerning the observed interpretation of upper atmosphere variability. Recent satellite measurements demonstrate that the horizontal winds exhibit a substantial degree of variability in direction and magnitude, which rapidly evolve as a function of altitude (Yiğit et al., 2022). The observed degree of wind variability cannot be explained considering only space weather processes. ...
Gravity waves (GWs) are generated at all altitudes in the atmosphere, but sources above the lower stratosphere are rarely considered by parameterizations employed in general circulation models. This study assesses the potential impact on the thermosphere produced by small‐scale waves originating at different heights. Within the proposed numerical framework, GW sources are represented by wave momentum forcing, whose values are expressed relative to the forcing required to obtain typical wave spectra around the tropopause. The relative importance of tropospheric and extra‐tropospheric sources and the response in the thermosphere are studied in a series of sensitivity experiments. They demonstrate that the accumulation of wave momentum steeply drops with height as a consequence of decreasing density, even when the forcing is maintained at a uniform level throughout the middle atmosphere. When a broad spectrum is forced at twice the tropospheric rate, the thermospheric drag is increased by only a factor of two, and that increase is produced by waves that were forced in the lower stratosphere. With increasing altitude, vertically localized sources contribute progressively less. For example, for GWs excited near the mesopause to produce an impact comparable with that due to waves propagating from below, the forcing must be orders of magnitude stronger than in the troposphere. The estimated forcing of the so‐called secondary harmonics by breaking primary waves is much weaker, such that the systematic dynamical effect of secondary waves in the thermosphere is negligible compared to that of the primary GWs generated in the troposphere.
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.
Growing evidence indicates that a selected group of global-scale waves from the lower 3 atmosphere constitute a significant source of ionosphere-thermosphere (IT, 100-600 km) 4 variability. Due to the geometry of the magnetic field lines, this IT coupling occurs mainly at low 5 latitudes (< 30 •) and is driven by waves originating in the tropical troposphere such as the diurnal 6 eastward propagating tide with zonal wave number s =-3 (DE3) and the quasi-3-day ultra-fast 7 Kelvin wave with s =-1 (UFKW1). In this work, over 2 years of simultaneous in situ ion densities 8 from Ion Velocity Meters (IVMs) onboard the Ionospheric Connection Explorer (ICON) near 9 590 km and the Scintillation Observations and Response of the Ionosphere to Electrodynamics 10 (SORTIE) CubeSat near 420 km, along with remotely-sensed lower (ca. 105 km) and middle 11 (ca. 220 km) thermospheric horizontal winds from ICON’s Michelson Interferometer for Global 12 High-resolution Thermospheric Imaging (MIGHTI) are employed to demonstrate a rich spectrum 13 of waves coupling these IT regions. Strong DE3 and UFKW1 topside ionospheric variations are 14 traced to lower thermospheric zonal winds, while large diurnal s = 2 (DW2) and zonally symmetric 15 (D0) variations are traced to middle thermospheric winds generated in situ. Analyses of diurnal 16 tides from the Climatological Tidal Model of the Thermosphere (CTMT) reveal general agreement 17 near 105 km, with larger discrepancies near 220 km due to in situ tidal generation not captured 18 by CTMT. This study highlights the utility of simultaneous satellite measurements for studies of IT 19 coupling via global-scale waves. 20
