Longitudinal and seasonal variations of the equatorial ionospheric ion density and eastward drift velocity in the dusk sector

Abstract

The longitudinal and seasonal variations of the plasma density and drift velocity in the evening equatorial ionosphere show complex characteristics. In this paper, we analyze the ionospheric ion density and velocity measured by the Defense Meteorological Satellites Program (DMSP) F13 and F17 satellites at ∼1800 LT. Our analysis focuses on whether the longitudinal structure of the equatorial ionospheric density and velocity has multiple wave number components, how the longitudinal structure of the equatorial ion eastward drift varies with season and year, and what causes the longitudinal variations of the ion density and velocity. We find that the longitudinal structure of the ion density and velocity show a wave‐4 pattern during the equinox months (March, April, September, and October), a wave‐3 pattern in northern summer (May–August), and a wave‐2 pattern in northern winter (November–February). The longitudinal structure of the ion density and velocity becomes significantly different from the wave‐4 pattern in the longitude regions with large magnetic declination in the summer and winter. The variation of the equatorial ionospheric ion density shows an in‐phase correspondence with the ion eastward velocity and an anti‐phase correspondence with the ion upward velocity. We suggest that the enhanced upward drift causes the density decrease through the fountain effect and that the enhanced eastward drift causes the density increase through horizontal transport. The change of the ion eastward velocity with longitude can be as high as 180 m s−1 over a longitudinal range of 120° and is related to the solar activity. The results of this study reveal a number of new characteristics of the equatorial plasma horizontal drift in the east–west direction.