Fraction of Interplanetary Coronal Mass Ejections That Are Magnetic Clouds: Evidence for a Solar Cycle Variation

摘要

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 the ICMEs that are 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 varies with the phase of the solar cycle, from approx. 100% (though with low statistics) at solar minimum to approx. 15% at solar maximum. A similar trend is evident in near-Earth observations during solar cycles 20-21, while Helios 1/2 spacecraft observations at 0.3-1.0 AU show a weaker trend and larger MC fraction.