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 β. 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 ∼100% (though with low statistics) at solar minimum to ∼15% 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.