The ejecta themselves (called “piston gas” or “driver gas” in earlier papers) have properties that differ radically from those of the ambient solar wind. At first, the ejecta are often separated from the sheath plasma by a tangential discontinuity. Their very different origin is discernible from their different elemental composition (Hirshberg et al., 1971), ionization state (Bame et al., 1979; Schwenn et al., 1980; Henke et al., 1998; Rodriguez et al., 2004), temperature depressions (Gosling et al., 1973; Montgomery et al., 1974; Richardson and Cane, 1995), cosmic ray intensity decreases (“Forbush decreases”, see, e.g. Cane et al., 1994), the appearance of bi-directional distributions of energetic protons and cosmic rays (Palmer et al., 1978) and supra-thermal electrons (Gosling et al., 1987). In many ejecta, major overabundances of Helium are observed, up to 30%, as was first noted by Hirshberg et al. (1971). This indicates that ejecta material originates from low layers in the solar atmosphere, where gravitational stratification allows substantial enrichment of heavy ions.
For about one third of all shocks driven by ICMEs, the succeeding plasma exhibits to an in situ observer the topology of magnetic clouds (Burlaga et al., 1981), see reviews by, e.g., Gosling (1990); Burlaga (1991); Osherovich and Burlaga (1997). Smooth rotation of the field vector in a plane vertical to the propagation direction, mostly combined with very low plasma beta, i.e., low plasma densities and strong IMF with low variance give evidence of a flux rope topology (Marubashi, 1986; Bothmer and Schwenn, 1998) of these magnetic clouds. This is consistent with the concept of magnetic reconnection processes (we might better call them “disconnection” processes) in coronal loop systems in the course of prominence eruptions at the Sun (Priest, 1988). It is true though that the boundaries of magnetic clouds are often difficult to identify (Goldstein et al., 1998; Wei et al., 2003).
Most of these ICME signatures can be seen in the event shown in Figure 34. Usually, only a subset of the criteria for identifying ejecta is encountered in individual events, and to this day a trained expert’s eye is needed to tell what is ejecta and what not. The situation is additionally complicated by the class of very slow CMEs found to take off more like balloons rather than as fast projectiles (Srivastava et al., 2000). After many hours of slow rise, they finally float along in the ambient slow solar wind. Naturally, they do not drive a shock wave. Only in rare cases, a few of their ejecta signatures (e.g., composition anomalies, magnetic cloud topology) remain and disclose their origin.
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