Meridional Direction Features of Equatorial and Low-Latitude Plasma Depletions Observed by Swarm Satellites

The equatorial and low-latitude plasma depletions is an important space weather phenomenon, which often produce severe ionospheric scintillations. According to the well-known formation mechanism of the equatorial and low-latitude plasma depletions, i.e., the generalized Rayleigh-Taylor (RT) instability mechanism, these plasma depletions are expected to occur mainly at night and appear simultaneously at the geomagnetic conjugate locations in the northern and southern hemisphere. Although many studies on the equatorial and low-latitude plasma depletions have been performed in recent years, there are few reports on the variation features of the equatorial and low-latitude plasma depletions along the geomagnetic meridian direction. In this study, we have analyzed in situ electron density measurements obtained by the three Swarm satellites in the 55°E-155°E geographic longitude (GLON) sector during the equinox months of high solar activity years, i.e., February-April 2014, September-November 2014, February-April 2015, and September-November 2015. We found that the equatorial and low-latitude plasma depletions were observed mainly at night, which are consistent with the statements of the generalized RT instability mechanism. Moreover, we found that in most cases the appearance of the equatorial and low-latitude plasma depletions presented the north-south hemisphere symmetry. However, sometimes (with the occurrence rate of ~16-27%) they presented the north-south hemisphere asymmetry, namely, they were only detected at the low-latitude region in the northern hemisphere or southern hemisphere, which cannot be explained by the generalized RT instability mechanism.