The effect of zonal wind reversal around sunset on ionospheric interhemispheric asymmetry at March equinox of a solar maximum year 2000

Abstract

AbstractInterhemispheric asymmetry of the topside ionosphere around March equinox of a solar maximum year 2000 was investigated using ROCSAT‐1 total ion density (Ni) measurements. The asymmetry during the local time sector of late afternoon to midnight was analyzed for two longitude sectors with larger geomagnetic declinations (positive at 120°W–180°W and negative at 0–60°W). The asymmetry of equatorial ionization anomaly (EIA) crests is consistent with (but much weaker than) that of Ni at middle dip latitudes in each longitude sector, and the asymmetry is contrary between two longitude sectors. The zonal wind was confirmed to be the primary reason for the asymmetry; it induces opposite vertical plasma transports between two hemispheres under the effect of geomagnetic declination. For the asymmetry of EIA crests, the effect of vertical plasma transport dominates over that of transequator plasma transport. Zonal wind reversal around sunset results in Ni at middle dip latitudes increases in one hemisphere while quickly decreases in the other one, which induces a quick reversal of asymmetry at about 18:00 LT. Nighttime ion composition at 600 km was found to have similar asymmetries with those of Ni, indicating that wind‐induced vertical plasma transport is important for topside ion composition. Asymmetry intensity is somewhat different between two longitude sectors, and the difference depends on dip latitudes. That, on the one hand, is related to the longitudinal variation of the fountain effect and, on the other hand, implies that neutral winds are possibly longitudinally dependent.