Effects of storm-time electrodynamic phenomena on the nighttime F-layer height at the magnetic equator: Analysis of ionograms data

Abstract

In this paper, we report on a comparative study of the observed changes in the nighttime-profile of the F-layer height during the main and recovery phases of the 10–19 March 1998 storm period and the quiet night of 8–9 March preceding that period. The disturbed nights i.e. 10–11 March and 16–17 march were selected from the time variation of the Dst magnetic index in the main and recovery phases, respectively. The F-layer height data were collected from the quarter-hourly ionograms of an IPS 42-type vertical sounder at Korhogo (Ivory Coast; 9.2° N; 5° W; 2.4° S dip. lat.) in the West African sector. A common feature is that both time profiles show a single pulse height, peaking at about 20:15 LT. However, while the time profile in the quiet night is quite similar to the well known ExB pulse, with altitude and thickness about 330 km and 100 km respectively, the time profiles of the nights in the main and recovery phases show amplified altitudes up to 450 km. In view to understand these tendencies, the time variations of the polar magnetic disturbances DP of the 10–11 March and 16–17 March nights were represented as a function of the latitude varying from polar to equatorial regions conjointly to that of the polar magnetic activity indices AU and AL. A clear magnetic disturbance was detected on the polar magnetic activity indices AU and AL, in the night 10–11 March of the storm main phase, and was found to move equator-ward on the time profile of the polar magnetic disturbances DP, qualitatively explaining the ExB pulse reinforcement in this night. In contrast, the time profiles of the AU and AL indices and DP disturbances remained featureless in the 16–17 March night of the storm recovery phase. We discussed these trends in the light of the magnetospheric and ionospheric-disturbance dynamo theories.