The Decay of Interplanetary Coronal Mass Ejections and Forbush Decrease Recovery Times

We investigate the relation between Forbush cosmic ray decrease recovery time, and coronal mass ejection angular size and transit time between the Sun and Earth. We identify 10 Forbush decreases from ground based neutron count rates between 1997 and 2000, with the addition of the Halloween 2003 event, that can be associated with single coronal mass ejections (CMEs) in the SOHO LASCO CME Catalog and specific interplanetary coronal mass ejections (ICMEs) crossing the vicinity of Earth. Contrary to what is predicted by simple cosmic ray diffusive barrier models, we find that Forbush recovery times are anti-correlated with the transit time. The anti-correlation suggests that the decay rate of ICMEs is anti-correlated with their travel speed. Forbush recovery times range from five times the transit time for the fastest disturbance to a fifth the Sun-Earth transit time for the slowest. To account for the large range of measured recovery times we infer that the slowest disturbances must decay rapidly with radius whereas the fastest ones must remain strong. The longest recovery times suggest that the fastest disturbances in our sample decayed less rapidly with radius than the ambient solar wind magnetic field strength.