Meridional Direction Features of Low-Latitude Plasma Depletions in the 90°-1500E Sector Observed by the Swarm Satellites

The presence of plasma depletions in the low-latitude ionosphere is a well-known phenomenon. Generalized Rayleigh-Taylor (RT) instability is generally considered to be responsible for triggering the low-latitude plasma depletions. Therefore, these plasma depletions are predicted to appear mostly at night and in the magnetic conjugate positions of the north and south hemispheres. In recent years, there have been intensive studies on low-latitude plasma depletions, but only a minority of research has been done on the variation characteristics of plasma depletions along geomagnetic field lines at low latitudes. The in-situ Ne (electron density) observations provided by the Swarm constellation within 90°-150°E GLON have been analyzed in this study, which were collected in the equinoctial months (i.e., February to April and September to November) of the years 2014-2016. As predicted by the generalized RT instability mechanism, we found that the plasma depletions occurred primarily at night. In addition, our results indicate that low-latitude plasma depletions took place in the north and south hemispheres symmetrically on most occasions. However, there are also a few cases that can't be expounded by the generalized RT instability mechanism, namely, plasma depletions exhibited in the north and south hemispheres asymmetrically. In other words, the plasma depletions at low latitudes were only observed in the north or south hemisphere. Furthermore, the occurrence rate of the north-south asymmetric depletions events increased significantly (with the occurrence rate of ~26-48%) in 2016 (i.e., the low solar activity year) as compared with those in the years of 2014-2015 (i.e., the high solar activity years).