Geophysical effects of the interplanetary magnetic cloud on October 18-19,1995, as deduced from observations at Irkutsk

During an interaction of the Earth's magnetosphere with the interplanetary magnetic cloud on October 18-19, 1995, a great magnetic storm took place. E xtremely intense disturbances of the geomagnetic field and ionosphere were recorded at the midlatitude observatory at Irkutsk (Phi' approximate to 45 degrees, Lambda' approximate to 177 degrees, L approximate to 2) in the cou rse of the storm. The most important storm features in the ionosphere and m agnetic field are: a significant decrease in the geomagnetic field Z compon ent during the storm main phase, unusually large amplitudes of geomagnetic pulsations in the Pi1 frequency band; extremely low values of critical freq uencies of the ionospheric F2-layer; an appearance of intense E-s-layers si milar to auroral sporadic layers at the end of the recovery phase. These ma gnetic storm manifestations are typical for auroral and subauroral latitude s but are extremely rare in middle latitudes. We analyze the stormtime midl atitude phenomena and attempt to explore the magnetospheric storm processes using the data of ground observations of geomagnetic pulsations. It is con cluded that the dominant mechanism responsible for the development of the O ctober 18-19, 1995 storm is the quasi-stationary transport of plasma sheet particles up to L approximate to 2 shells rather than multiple substorm inj ections of plasma clouds into the inner magnetosphere. (C) 2000 Elsevier Sc ience Ltd. All lights reserved.