Daytime Ionospheric Large-Scale Plasma Density Depletion Structures Detected at Low Latitudes Under Relatively Quiet Geomagnetic Conditions

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 degrees, developed toward higher latitudes at the speed of similar to 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 similar to 16-30 degrees N. The geomagnetic Delta 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.