Thermospheric densities during an intense magnetic storm, from the LOGACS experiment

Abstract

One of the most intense magnetic storms of recent years occurred on 25–26 May 1967 and coincided with the flight of the LOGACS (LOw-G Accelerometer Calibration System) experiment aboard an Agena vehicle in a near-polar orbit. The accelerometer gave density values with an estimated accuracy better than 10 per cent and a time resolution of 1 s, although at much longer and irregular intervals. The dayside response (at 1030 LT) is clearest as the geomagnetic index a p starts to decline, and shows peaks on the magnetic equator, near ±45° geomagnetic latitude, and near the polar caps. There are pronounced troughs near ±60° geomagnetic latitude; in the northern trough (at 150 km height) the density falls to 70 per cent of the undisturbed value. In succeeding passes the structure collapses irregularly. The equatorial peak nearly disappears within 3 h. At the peaks the density is near its maximum as a p starts to decline, indicating a very rapid response; but the density in the troughs increases for some hours as the structure collapses, before the atmosphere returns to normal. We suggest that the structure shows where the energy is injected, at least in the morning sector, in an intense storm, namely: (i) near the magnetic equator, (ii) near the particle precipitation zones, which are displaced equatorwards in the storm, (iii) over the polar caps. Most of the energy input is probably due to particle precipitation.