Cosmic-ray intensity variations during the Forbush decreases, 2–15 May 1969

Abstract

Using the cosmic-ray intensity data recorded with ground-based monitors at Mt. Washington, Deep River and Kula, and the cosmic ray telescopes on board the Pioneer 8 and 9 spacecraft, the large Forbush decrease events, 2–15 May 1979, were investigated, taking into account the associated interplanetary magnetic field and solar wind plasma data. The Forbush decrease, 2–3 May 1969 appears to be caused by the interplanetary shock wave that was associated with high-speed solar wind plasma, and whose arrival at Earth was marked by the SSC that occurred at about noon on 2 May 1969. We also found that the interplanetary magnetic field shock wave, which originated from the solar flare that occurred at 40° East of the central meridian at 0500 UT on 12 May 1969 at McMath Plage 10088, was responsible for the major event, the Forbush decrease, 14–15 May 1969. The shock wave propagated nearly radially, with expansion, to Earth and Pioneer 8, but the post-shock turbulent region rotated and expanded to the Pioneer 9 telescope. Magnetic clouds were not associated with the shock wave. Our results are consistent with Parker's blast wave model (1963), but are inconsistent with Gold's magnetic tongue model (1960) as well as with models that require the presence of moving magnetic clouds (e.g. Zhang and Burlaga, 1988).