Daytime Ionospheric Large‐Scale Plasma Density Depletion Structures Detected at Low Latitudes Under Relatively Quiet Geomagnetic Conditions. Issue 2 (3rd February 2022) - Sun, Wenjie
Abstract: The ionospheric electron density is expected to enhance after sunrise during geomagnetic quiet days. In this paper, we report an anomalous large‐scale ionospheric TEC depletion structure occurring in morning hours over low latitudes in China under relatively quiet geomagnetic conditions. The depletion structure, which occurred over a wide longitude range of >35°, developed toward higher latitudes at the speed of ∼250 m/s. A statistical analysis using the data in 2018–2020 showed that such large‐scale daytime TEC depletion mainly occurred in summer (mostly with Kp < 3), with large intensities at ∼16–30°N. The geomagnetic Δ H measurements at the stations near and away from the magnetic dip equator showed positive values, which rules out the possible contribution from a westward electric field. Observations from multiple ionosondes revealed a net downward plasma transport during the TEC depletion. An enhanced poleward neutral wind during the early morning hours, which may induce downward plasma transport, was surmised to play a leading role in driving the large‐scale morning TEC depletion. Plain Language Summary: Under geomagnetic storms, the daytime ionosphere may undergo significant plasma density depletion over wide longitude and latitude regions. Possible driving mechanisms of large‐scale plasma density depletion include the suppression of the fountain effect by westward equatorial electrojets, the upward plasma diffusion caused by increasing electron temperature, and the decreased O/N2 ratio transported by perturbed neutral circulation. In this paper, we report cases of anomalous large‐scale plasma density depletion structures in morning hours of geomagnetic quiet days over low latitudes in China. The large‐scale plasma density depletion structure, which produced TEC reduction of ∼4 TECu, covered >3, 500 km in the east‐west direction and developed poleward. Based on the observations from multiple ionosondes and magnetometers at equatorial and low latitudes, it is unlikely that the possible driving sources are from geomagnetic activities. We surmise that there could be an enhanced poleward neutral wind over a wide longitude region which induced downward plasma transport and produced the depletion structure. Key Points: A daytime large‐scale TEC depletion structure covering ∼35° in longitude under relatively quiet geomagnetic conditions was reported The daytime large‐scale depletion structure with poleward development mainly occurred in summer with large intensities at 16–30°N It is surmised that enhanced poleward neutral wind could be a possible driver for the daytime large‐scale depletion structure