Study of the radial dependence of Forbush decreases at 0.28–1 au using data from theHelios1 and 2 spacecraft

Abstract

ABSTRACTWe identify and investigate cosmic ray Forbush decreases (FDs) observed in the E > 50 MeV data from the Helios 1 and 2 spacecraft, spanning from 1974–1985 and covering heliocentric distances in the range 0.28–1 au. A Helios FD catalogue is compiled, including the characteristics of the cosmic ray variations, as well as those of the solar wind (SW) and the interplanetary magnetic field (IMF) of the associated interplanetary disturbances. An extended statistical study considers the radial dependence of the FD magnitude, the SW velocity, and the IMF intensity in these disturbances. It is found that the Helios FD sizes at different distances from the Sun are determined by the parameters of the interplanetary disturbances. In particular, the FD magnitudes observed at Helios, as well as near Earth, correlate well with VB, which is the product of the maximum SW velocity and the IMF intensity when normalized by the average values of these parameters at the radial distance of the observations. However, we found that, on average, the Helios FD sizes are statistically independent of the radial distance in the range of 0.28–1 au.