Deformation of the night side plasmasphere and ionosphere during the August 1972 geomagnetic storm

Isis 2 satellite measurements of electron and ion temperatures and densities and of airglow emissions have resolved certain features of the response of the night side ionosphere to the major magnetic storm of August 4-6, 1972, an event in which Kp reached 9o at one point. The near-polar orbit of Isis 2 is circular at about 1400-km altitude and at the time of the storm was oriented in the 0500-1700 LT plane. During the initial phases the plasmapause moved equatorward from L=4.5 to about 2.7, followed by a further compression to L=1.9-2.0 at the peak of the storm and a slow recovery during the ensuing days. The plasmapause was identified by an abrupt decline in H⁺ accompanied by a sharp peak in both T_e and T_i, T_e approaching 6000°K and T_i exceeding 3000°K in some passes. A pronounced trough developed within the plasmasphere during the days of greatest disturbance. Equatorial temperatures were also enhanced from normal values, T_e exhibited great spatial variation there on some passes, and this variation was associated with the development of troughs of N_e at the dip equator. On the most disturbed day (August 5), when the plasmapause had fallen to L=2.3, a deep trough developed in the region L=1.3-1.8, N_e falling below 1 × 10² cm^-3, more than 2 orders of magnitude below the normal protonosphere concentration at this altitude. Top side sounder observations of radio breakthrough from the ground indicated that the trough extended through the F region as well. A red arc observed on this day at L=1.23 apparently was associated with an equatorial trough in N_e and enhancement in T_e. Whether the arc and the troughs were caused by magnetospheric processes or by enhanced recombination within the F region is not clear.