Daytime Thermospheric Wind Transients and Circulation in May 2021

Abstract Changes in the thermospheric wind originating in storm‐time transients in high‐latitude Joule heating and ion circulation are effective in modifying conditions throughout Earth's upper atmosphere and ionosphere. Among the effects these drivers can produce are large‐scale gravity waves (GWs), characterized by significant wind transients that propagate away from the auroral zone, driving transient ion motion during their 1–2 hr passage. Longer period changes in mean winds can develop over the following hours to days, depending on the duration and magnitude of the high latitude heating, and also extend globally. The effectiveness of these processes in modifying the mean density of the daytime ionosphere likely depends on the extent to which these disturbances reach the daytime equatorial region and downward into the E‐region wind dynamo (below 180 km). A study of a month of observations made during the ICON mission reveals the variety of behaviors with both transient effects and longer‐term changes in mean winds. The duration of auroral inputs, as opposed to the average input over time, is identified as important to the development of dynamo‐modifying zonal disturbance winds. During geomagnetic disturbances, we find that the predictive capability of a general circulation model (TIEGCM) for meridional wind transport is good (R  .8) while the storm‐time zonal wind transport is harder to predict (R  .5). This study is the first of its kind, measuring winds and storm responses continuously for a month in both the daytime E‐ and F‐ regions simultaneously with 97 min cadence.Plain Language Summary By heating the upper atmosphere, the aurora can drive increases in winds that expand to points well away from the polar regions. What is often observed are atmospheric waves propagating away from the poles after the sudden increase in auroral activity associated with geomagnetic substorms. These waves occur without bulk transport, like waves breaking on the seashore. The corollary for a tsunami in space comes if continued auroral heating can enforce the bulk transport of gas, initially from high to low latitudes, and then with a potentially westward motion due to the apparent Coriolis force. As energy inputs become larger and longer, the expansion of wind circulation from high to low latitudes and westward wind disturbances are expected to be more likely. However, there exist very few observations of the daytime winds during magnetic storms. In a new set of observations by the Ionospheric Connection Explorer, from May 2021, disturbances in thermospheric winds are observed over 20 days. We find that the magnitude of event as measured by magnetic indices is not necessarily a good predictor of the development of a zonal wind dynamo.Key Points Large‐scale waves and changes to the mean winds are observed in the daytime thermosphere during a series of geomagnetic storms in May 2021 TIEGCM simulations largely recreate signatures of waves propagating from the auroral zone, and changes in the mean meridional circulation The development of zonal disturbance winds at middle latitude is observed to depend on the persistence of auroral inputs