CMEs carry into the heliosphere large amounts of coronal magnetic fields and plasma, which can be detected by remote sensing and in-situ spacecraft observations. Here they are known as interplanetary CMEs or ICMEs (Zhao and Webb, 2003; Dryer, 1994). The term ICME or “interplanetary CME” was originally devised as a means to separate the phenomena observed far from the Sun (e.g., by in-situ spacecraft) and those near the Sun (e.g., by coronagraphs). However, in the SMEI and STEREO era, where CMEs can now be tracked continuously from the Sun to 1 AU and beyond, the term has become largely redundant. Consequently, in a recent workshop on remote sensing of the heliosphere in Wales (June 2011) it was decided to no longer use the term ICME, and in this review we drop the term.
The passage of CME material past a single spacecraft is marked by distinctive signatures, but with a great degree of variation from event to event (e.g., Gosling, 1993). These signatures include transient interplanetary shocks, depressed proton temperatures, cosmic ray depressions, flows with enhanced helium abundances, unusual compositions of ions and elements, and magnetic field structures consistent with looplike topologies. Many of these signatures were first identified in the plasma which followed an IP shock by several hours and was considered to be the piston (CME) driving the shock. Some signatures can also be observed elsewhere in the solar wind where they may identify relatively slower CMEs not driving shocks.
Often observed in in-situ data are highly structured magnetic field configurations corresponding to the arrival of a CME. The field assumes the structure of a spiral (or helix), and is accompanied by other signatures including strong magnetic field with low field variance, low plasma beta, and low temperature. Such structures were called magnetic clouds by Burlaga et al. (1981) citing early theoretical work dating back to the 1950s (Morrison, 1954). Figure 30 shows a schematic of such a cloud impinging on the Earth in May 1997. Such a structure is often modeled as a flux rope, which is a series of helical field lines like the coils of a spring with pitch angles increasing toward the outer edge. Since, as we have seen, many if not all CMEs are now considered to contain flux ropes, it is logical to expect magnetic clouds to form the core of CMEs. In a recent report, one magnetic cloud observed in-situ was tracked continuously back to its coronagraph origins and it was found to be the cavity component of the three-part CME structure (Howard and DeForest, 2012a). This reinforced the largely-accepted view that the cavity component was the CME flux rope (e.g., Forsyth et al., 2006). Theoretical work involving the development of cavities includes Fuller et al. (2008).
Models have been developed for the force free (e.g., Lepping et al., 1990; Lynch et al., 2005) and non-force free (Hu and Sonnerup, 2001) states of magnetic clouds observed in-situ, the latter also known as the Grad–Shafranov technique. Around 30% (Gosling et al., 1991) to 50% (Cane et al., 1997) of CMEs observed in-situ show a clear signature of a magnetic cloud. It remains unknown whether the remainder does not show the signature because the imbedded flux rope is less structured, is absent, or whether the spacecraft did not pass through the flux rope component (i.e., skirted its flank).
Some magnetic clouds have been associated with solar filament disappearances. Since filament plasma is embedded in helical, horizontal magnetic fields, the close association of CMEs with filament eruptions and shearing fields near the surface also supports the view that flux ropes form the core of CMEs. One idea is that the interior fields of a rising, sheared CME reconnect, resulting in an ejected flux rope and new, closed coronal loops at the Sun. In several studies magnetic clouds have been found to have the same orientation and polarity as associated erupting filaments at the Sun. Furthermore, larger filaments always have twist in the same sense in a given hemisphere, even though the hemispherical polarity reverses every solar cycle. Filament eruptions and CMEs may be important ways that the Sun sheds magnetic helicity, as well as flux built up over the solar magnetic cycle.
Since the in-situ signatures of CMEs are well described in several recent reviews (Schwenn, 2006; Zurbuchen and Richardson, 2006; Richardson and Cane, 2010), we will not discuss them further here. We will, however, discuss the remote sensing of CMEs, especially as achieved recently by the new class of white light imagers: heliospheric imagers.
Living Rev. Solar Phys. 9, (2012), 3
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