MIDLATITUDE IONOSPHERIC DISTURBANCES PRODUCED BY MAJOR MAGNETIC STORMS

Large-scale ionospheric disturbances were observed in France by HF ionospheric sounding measurements during major magnetic storms, specially during the March 13-14, 1989 storm. The experiment used a network of four stations (one continuous wave transmitting station and three receiving stations) to determine the horizontal component of the phase velocity of the travelling disturbances by triangulation. The vertical component was determined by using several frequencies on one of the links and ionograms indicated overall ionospheric changes every 30 min. During the first diurnal phase of the March 13, 1989 magnetic storm, four large oscillations of the F2-layer critical frequency f0F2 were observed. During the first and second f0F2 decreases, a dense F1 region, revealing strong compositional changes in the lower thermosphere, modulated the oscillations and contributed to a F2-region virtual height increase to above 800 km. The Doppler measurements at 150-180 km detected only magnetic pulsations and southward travelling acoustic waves. The large-scale disturbances reached the altitude of the Doppler measurements 30 min after the fourth f0F2 peak, with a rapid upward and downward plasma drift of 250 m s-1. This disturbance moved to the southwest at 860 m s-1. During the March 13-14 nocturnal phase an auroral type Es appeared, formed by irregularities extending over 800 km. The ionosphere almost disappeared at 0330 UT and a low-density spread F was observed until the sunrise; the Doppler links were interrupted all the night. During the March 14 morning phase, southward travelling acoustic waves were observed and the F region seemed to be limited to the F1 region. This is probably an effect of the extreme G condition, arising from a decrease of the F2-layer maximum below the F1-layer maximum. The principal features of this storm are compared with those of two other storms on April 12, 1990 and August 14-15, 1989.