Galactic cosmic ray modulation and interplanetary medium perturbations due to a long‐living active region during October 1989

Abstract

During October 1989, three very energetic flares were ejected by the same active region at longitudes 9° E, 32° W, and 57° W, respectively. The shape of the galactic cosmic ray variations suggests the presence of large magnetic cloud structures (Nagashima et al., 1990) following the shock‐associated perturbations. In spite of long data gaps the interplanetary observations at IMP 8 (near the Earth) and ICE (∼1 AU, ∼65° W) confirm this possibility for the event related to the 9° E flare; the principal axes analysis shows that the interplanetary magnetic field variations at both spacecraft locations are mainly confined on a meridian plane. This result suggests that the western longitudinal extension of this cloud is indeed very large (≥75°). The nonnegligible depression in the cosmic ray intensity observed inside the possible cloud related to the 57° W flare indicates that also the eastern extension could be very wide. The analysis of neutron monitor data shows clearly the cosmic ray trapping effect of magnetic clouds; this mechanism seems to be responsible for the enhanced diurnal effect often observed during the recovery phase of Forbush decreases. We give an interpretation for the anisotropic cosmic ray peak occurring in the third event, and, related to that, we suggest that the Forbush decrease modulated region at the Earth's orbit could be somewhat wider than the magnetic cloud, as already anticipated by Nagashima et al. (1990). By this analysis, based mainly on cosmic ray data, we show that it is possible to do reasonable inferences on the large‐scale structure of flare‐related interplanetary perturbations when interplanetary medium data are not completely present.

Cosmic ray decreases and solar wind disturbances during late October 1989

“…The cosmic ray record at Earth is consistent with this picture such that only the first two shock-associated cosmic ray decreases had the signature expected for intercepting ejecta material. This same time period was also examined by Bavassano et al (1994). However, we do not agree with their conclusion that "magnetic clouds" extending at least 75 ø from the source longitude were present.…”

Space weather and dangerous phenomena on the Earth: principles of great geomagnetic storms forcasting by online cosmic ray data

“…This effect was analyzed recently, both theoretically (Dorman et al, 1995) and experimentally on the basis of the network of CR stations (Belov et al, 1995a,b). The predecrease effect can be due to a magnetic connection of the Earth with regions (moving from the Sun) with reduced CR density; this lower density can be observed at the Earth along the actual direction of IMF lines (Nagashima et al, 1990;Bavassano et al, 1994); 3. CR fluctuations: Many authors found some peculiarities in the behavior of CR fluctuations before FD: changes in frequency spectrum; appearance of peaks in spectrum at some frequencies; variations in some special parameter introduced for characterizing the variability of fluctuations.…”

Long-duration high-energy proton events observed by GOES in October 1989

“…According to Cane and Richardson, there was another shock passing the Earth as early as 09:16 UT, but that was suggested to be related to one of the Ha or X-ray¯ares at the Sun which preceded the large 19 October¯are. However, Bavassano et al (1994) relate the ®rst shock to the 19 October¯are. Direct evidence of ejecta from the 22 October event was not observed, because IMP 8 was in the magnetosphere, but their passage was considered to be plausible by Cane and Richardson (1995).…”

“…On 26 October, the shock produced by the last CME was seen as an SC, too, but Cane and Richardson concluded that the ejecta from this event did not pass the Earth. Bavassano et al (1994) state that the trapping eect of these ejecta is`marginally``seen'' by cosmic rays'.…”