The fraction of interplanetary coronal mass ejections that are magnetic clouds: Evidence for a solar cycle variation

Magnetic clouds'' (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) characterized by enhanced magnetic fields with an organized rotation in direction, and low plasma beta. Though intensely studied, MCs only constitute a fraction of all ICMEs detected in the solar wind. A comprehensive survey of ICMEs in the near-Earth solar wind during the ascending, maximum and early declining phases of solar cycle 23 in 1996-2003 shows that the MC fraction varied with the phase of the solar cycle, from similar to100% (though with low statistics) at solar minimum to similar to15% at solar maximum. A similar trend is evident in near-Earth observations during solar cycles 20-21, while Helios 1/2 observations at 0.3-1.0 AU show a weaker trend and larger MC fraction.