Quiet-Time Day-to-Day Variability of Equatorial Vertical E x B Drift From Atmosphere Perturbations at Dawn

Ionospheric day-to-day variability is ubiquitous, even under undisturbed geomagnetic and solar conditions. In this paper, quiet-time day-to-day variability of equatorial vertical E x B drift is investigated using observations from ROCSAT-1 satellite and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM-X) v2.1 simulations. Both observations and model simulations illustrate that the day-to-day variability reaches the maximum at dawn, and the variability of dawn drift is largest around June solstice at similar to 90-180 degrees W. However, there are significant challenges to reproduce the observed magnitude of the variability and the longitude distributions at other seasons. Using a standalone electro-dynamo model, we find that the day-to-day variability of neutral winds in the E-region (<=similar to 130 km) is the primary driver of the day-to-day variability of dawn drift. Ionospheric conductivity modulates the drift variability responses to the E-region wind variability, thereby determining its strength as well as its seasonal and longitudinal variations. Further, the day-to-day variability of dawn drift induced by individual tidal components of winds in June are examined: DW1, SW2, D0, and SW1 are the most important contributors. Plain Language Summary The ionosphere is different from one day to the next, even under geomagnetic and solar quiet condition. The vertical E x B drift at the geomagnetic equator is a key parameter that influences the state of the ionosphere and atmosphere. In this paper, we study the quiet-time day-to-day variability of the equatorial vertical E x B drift by ROCSAT-1 observations and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM-X) v2.1 simulations. Both observations and WACCM-X show that day-to-day variability is large at dawn and dusk and it also changes with season and longitude. To better study the variability, we carried out numerical experiments with a new standalone electro-dynamo model. We found that wind variability below similar to 130 km is the main contributor, and winds above similar to 130 km plays a secondary role. The ionospheric conductivity mediates the drift variability response to the wind variability and thus affects its strength and seasonal and longitudinal variations. Further, we examine the variability of dawn drifts generated by different tidal components in June and find that DW1, SW2, D0, and SW1 are the most important ones. This work emphasizes the importance of lower atmospheric variability in studying and predicting the day-to-day variations of ionosphere and space environment.