On the E-W asymmetry of Forbush decreases (Ep ? 35 MeV)

Abstract

A ten-year survey of the effects of solar flare-generated shock-waves on the ambient cosmic rays (E ≥ 35 MeV) as well as on the solar particle population at low energies (E ≥ 300 keV) shows that the quasi-perpendicular side of the expanding shock front (sola flare site to the east of the Sun-spacecraft line) gives rise to prominent Forbusch decreases and large ESP events, whereas the presumably turbulent quasi-parallel part of the large-scale shock front-IMF configurations (source site to the west of the Sun-spacecraft line) is associated, on the average, with small (≲4%) or no Forbush decreases and weak or no ESP events. The role of grad-B drifts of the particles at quasi-perpendicular shocks is suggested as a common cause for both the ESP events and the Forbush decreases. The observations indicate that turbulence is not a dominant factor in the above shock effects on the energetic particle populations both at low and high energies.