Cosmic ray decreases and interplanetary disturbances during early November 1969

Abstract

Using the cosmic-ray intensity data recorded with ground-based monitors at Mt. Washington, Deep River and Kula, and with the cosmic ray telescopes on Pioneer 8 and 9 spacecraft, the Forbush decrease events, 2–10 November 1969 were investigated, taking into account the associated interplanetary magnetic field and solar wind plasma data. A moving magnetic cloud, without any associated interplanetary shock wave, triggered the small Forbush decrease observed near Earth, with the onset time at 0700 UT on 2 November 1969 coinciding with the arrival time of the low density moving magnetic cloud. This result is consistent with shock unassociated moving magnetic cloud model (e.g., Zhang and Burlaga, 1988). We also found that an IMF shock wave caused the large Forbush decrease with onset time at 1837 UT on 8 November 1969 and with enhanced IMF intensity and SWP speed, a result which is consistent with Parkers blast wave model ( Parker, 1963). This IMF shock wave together with the accompanying moving magnetic cloud produced the large Forbush decrease with the onset time at 1800 UT on 9 November 1969, for which the arrival time of the moving magnetic cloud coincided with the onset time of the Forbush decrease, a result which is consistent with the shock associated moving magnetic cloud model (e.g., Zhang and Burlaga, 1988). The latter observations are also consistent with the model of Barnden, 1973which interprets the two-step Forbush decrease as caused by the post-shock turbulence followed by entry into the shock driver or ejecta. For the Forbush decrease event, 9–10 November 1969, there was no corotation delay in the Earth, Pioneer 8 and 9 data; nearly radial propagation of the IMF shock wave and the moving magnetic cloud, with expansion, is sufficient to explain the observations at Earth, Pioneer 8 and 9. The essential simultaneity of this large Forbush decrease at the Earth, at Pioneer 8, and at Pioneer 9, which established a heliocentric longitudinal spread of at least 109° from the flare/CME position on the Sun, is a notable result on the Forbush decrease process, considering the relatively small spread recently observed ( Cliver and Cane, 1996; Ifedili, 1997) for the angular extents of solar modulation of galactic cosmic rays during Forbush decreases.