Abstract
The unusually large cosmic ray disturbance commencing on August 4 had three unusual features. First, a ground level event (GLE) was seen in neutron monitors at Pc ≲ 1.5 GV beginning at about 1400 UT, almost 8 hours after the large solar flare at 0630 UT. This GLE can be attributed to relativistic solar particles that experienced coronal spreading onto flux tubes coming out toward earth and had a rigidity spectrum for 0.7 ≤ P ≤ 2.0 GV given by 7 × 104P(−8.5±1.0) (m²s GV sr)−1. Second, a large precursory increase was associated with the rapid Forbush decrease (Fd) at 2100–2200 UT. This precursory increase resulted from a free space anisotropy of ∼5% in magnitude, about 2 hours wide and in a direction 0°–30°W of the E-S line. Third, the Fd had a short time scale, with an overshoot in the initial recovery phase. These rapid cosmic ray intensity changes are related to the interplanetary field changes and a model proposed for this rapid Fd.
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