Recovery of interplanetary cosmic ray intensity following the Great Forbush Decrease of mid‐1991

Abstract

There was a succession of major solar flares in late‐May to mid‐June 1991. Their effects on cosmic ray intensity were observed by six spacecraft at various points in the heliosphere and by terrestrial neutron monitors. Study of these observations is summarized as follows: (a) An abrupt and extraordinarily large Forbush decrease in intensity occurred at the Earth on DOY 164/1991 (13 June) and ones of similar magnitude (≈ 20 percent) occurred in an orderly sequence of timing thereafter over ranges of heliocentric ecliptic longitude of essentially 360 degrees; of heliographic latitude +32 degrees to −5 degrees; and of radial distance 1.0 to 53 AU. (b) The apparent radial speed of propagation of the presumably causative blast wave was 865 (±75) km s−1 (c) It appears likely that the distinctive solar flare of DOY 162/1991 (11 June) was the dominant cause of the blast wave. (d) Two different representations of the subsequent recoveries of intensity yield substantially different numerical values of “recovery time” but either set of values shows an increase by a factor of about three between 1.0 and 53 AU.