The strong Pc5 geomagnetic pulsations in the initial phase of the great magnetic storm of March 24, 1991

Pc5 pulsations which were excited during the great magnetic storm of March 24, 1991 are analysed using the data from three French observatories located in the southern Indian Ocean at the subauroral and middle latitudes. The unusual strong Pc5 pulsations observed display features depending on the interval of occurrence: a quasi-noisy regime in the frequency range of 2.5–3.2 mHz characterizes the 04–10 UT interval, whereas the oscillations are nearly monochromatic with spectral maximum around 1.8–1.9 mHz in the 10–14 UT interval. The sharp change occurring at 10.17 UT could be associated to a sharp change in the magnetosphere structure or dynamics.