Ja. Vanallen, RECOVERY OF INTERPLANETARY COSMIC-RAY INTENSITY FOLLOWING THE GREAT FORBUSH DECREASE OF MID-1991, Geophysical research letters, 20(24), 1993, pp. 2797-2800
There was a succession of major solar flares in late-May to mid-June 1
991. Their effects on cosmic ray intensity were observed by six spacec
raft at various points in the heliosphere and by terrestrial neutron m
onitors. Study of these observations is summarized as follows: (a) An
abrupt and extraordinarily large Forbush decrease in intensity occurre
d at the Earth on DOY 164/1991 (13 June) and ones of similar magnitude
(almost-equal-to 20 percent) occurred in an orderly sequence of timin
g thereafter over ranges of heliocentric ecliptic longitude of essenti
ally 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 16
2/1991 (11 June) was the dominant cause of the blast wave. (d) Two dif
ferent representations of the subsequent recoveries of intensity yield
substantially different numerical values of ''recovery time'' but eit
her set of values shows an increase by a factor of about three between
1.0 and 53 AU.