In defense of the term ICME


  • C. T. Russell

    1. Institute of Geophysics and Planetary Physics and Department of Earth and Space Sciences, University of California-Los Angeles, Calif., USA
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As Burlaga [2001] correctly points out, the term “cloud” has been used for many years to describe solar wind disturbances. “Plasma cloud,” “magnetized plasma cloud,” and now “magnetic cloud” have all seen use. While the first two terms were rather inclusive in their definition, the last one is rather restrictive [Burlaga et al., 1981]. A magnetic cloud has to have a particular duration, a smooth rotation, strong magnetic field, and a low proton temperature. Thus, we are forced into adopting a separate name for many structures that do not fit this restrictive scheme. The term “ejecta” could be used, but there are two types of ejecta: flare ejecta and coronal mass ejecta. The latter term has been used extensively for the disturbance seen at 1 AU that arises in response to a coronal mass ejection (CME) detected at the Sun. This usage has led to some difficulties, because one line of research in this field is to compare the properties of CMEs in the corona with the disturbance seen later in the interplanetary medium. Both cannot be simply called CMEs, or confusion reigns. Thus arose the practice of using the term ICME for the interplanetary counterpart of a CME; e.g. [Lindsay et al., 1999; Mulligan et al., 1999a, b]. This term includes a wide variety of structures; in particular, structures missing one feature of a magnetic cloud, but clearly being associated with a CME on the Sun. While not every ICME detected has been identified with a specific CME, enough have that we can be confident of the association [e.g. Lindsay et al., 1999].