CMEs carry into the heliosphere large quantities of coronal magnetic fields and plasma which are detected by remote sensing and in-situ spacecraft observations. Measured at a single spacecraft, this material has distinctive plasma and magnetic field signatures with a large amount of variation. One of the most important signature classes is of magnetic clouds which are thought to be the flux ropes embedded in CMEs and have been identified, for one case, as originating from the cavity component of the three-part coronagraph CME. These carry strong, directional fields that can be very geoeffective. As carriers of such magnetic structures, CMEs may provide an important way for the Sun to shed the magnetic flux and helicity that is built up over the solar magnetic cycle.
In terms of space weather, CMEs are now identified as a crucial link between activity at the Sun and its propagation through the heliosphere to the Earth and other locations. The interplanetary manifestations of CMEs can result in extensive transient disturbances that, when directed Earthward, can cause major geomagnetic storms at Earth. In addition, significant particle and radiation hazards at Earth can arise from CMEs launched from western solar longitudes and, thus, are not necessarily Earth-directed (see Figure 24). The association of erupting filaments and magnetic clouds with CMEs has led to the view that flux ropes form the cavity of a CME and help drive it outward. Halo CMEs are important in terms of forecasting space weather and enhancing our understanding of CMEs and flux ropes. It appears that shocks and magnetic clouds are also likely to be detected at Earth following such solar events. Moderate storms not associated with CMEs are usually caused by Earth passage through the heliospheric current sheet (HCS) and related corotating interaction regions (CIRs). More details about space weather appear in two respective Living Reviews (Schwenn, 2006; Pulkkinen, 2007).
Living Rev. Solar Phys. 9, (2012), 3
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