5.2 Interplanetary counterparts of CMEs: ICMEs
The fast CMEs often drive large-scale density waves out into space which eventually steepen to form
collisionless shock waves, similar to the bow shock in front of the Earth’s magnetosphere. The shock front is
the outer boundary of a plasma sheath (see, e.g. Tsurutani et al., 1988
) that results from compression,
deflection, and heating of the ambient solar wind by the ensuing ejecta. The sheath may contain substantial
distortions of the interplanetary magnetic field due to field line draping (McComas et al., 1988) around the
ejecta cloud pressing from behind. It has become common to summarize all these CME driven effects under
the term ICME (for interplanetary counterparts of CMEs, see terminology discussion by Burlaga (2001)
and Russell (2001)).
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.
. 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.
