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., 1988Jump To The Next Citation Point) 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., 1971Jump To The Next Citation Point), ionization state (Bame et al., 1979Schwenn et al., 1980Henke et al., 1998Rodriguez et al., 2004), temperature depressions (Gosling et al., 1973Montgomery et al., 1974Richardson 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, 1986Bothmer and Schwenn, 1998Jump To The Next Citation Point) 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., 1998Wei et al., 2003).

Most of these ICME signatures can be seen in the event shown in Figure 34View Image. 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.

View Image

Figure 34: This shock event was observed by the Helios 1 solar probe during 3 days in 1981, at a distance from the Sun of 0.53 AU and at 92 west of the Earth–Sun line, i.e., right above the Sun’s west limb as seen from the Earth. The panels show the solar wind parameters (from bottom to top): proton density, temperature, flow speed, magnetic field magnitude and its azimuth and elevation angles. The upper panel shows the azimuth flow direction of suprathermal electrons (at 221 eV), the direction away from the Sun being at 180. The jumps in all parameters and the sudden widening of the electron angular distribution at 19:20 UT on day 171 denotes the arrival of a fast shock wave. The time between 02:00 and 19:00 UT (shaded area) on day 172 denotes the passage of a magnetic cloud, with its characteristic change of the field direction in the sense of a magnetic flux rope, and the simultaneous appearance of oppositely flowing (bi-directional) suprathermal electron streams. Note also the mono-directional electron flow before and after the event series. This one of the cases for which Sheeley Jr et al. (1985), using the SOLWIND coronagraph, could observe a uniquely correlated CME. From Schwenn et al. (2005Jump To The Next Citation Point).

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