6.1 Summary

We have discussed the origins and characteristics of coronal mass ejections and their associated phenomena. These have been discussed as occurring near the Sun in terms of their basic coronal properties and what we know about their source regions, and their manifestation in the heliosphere and near Earth. Kinematically there appears to be a continuum of accelerations and speeds of CMEs ranging from gradually accelerating CMEs that have smooth and balloon-shaped profiles, to rapidly accelerating CMEs that are less homogeneous and move through the outer corona at relatively constant speed. The rate of occurrence of CMEs correlates with solar activity, but the size scales of CMEs are much larger and their latitude distributions different than those of near-surface activity like flares or active regions. CMEs arise in large-scale closed coronal structures, the most evident being helmet streamers, which blow out and reform. Statistically, CMEs are most frequently associated with erupting filaments and X-ray long duration events, and not optical flares. However, new X-ray and EUV data are helping reshape our ideas on these associations. Large-scale coronal arcades are frequently observed, and may result from reconnection of closed field systems opened by CMEs , and indeed long current sheets are now observed connecting the arcade and the rising CME. The size scales and field strengths of these associated systems are a function of latitude. Thus, CMEs involve the destabilization of large-scale coronal structures which result in reconfiguration of the larger-scale, weaker fields at higher latitudes and of the smaller-scale, stronger fields at low latitudes. The magnetic structures involved with the source regions of CMEs can be complex and multipolar. The earliest X-ray signatures of the onset of a CME in the low corona appear to include outward-moving loops and the dimming or depletion of coronal material before and above the bright arcade. Against the disk the arcades are preceded by S-shaped structures associated with and aligned along the axis of filaments, which erupt forming cusp-shaped arcades. EUV waves are now frequently observed in association with CMEs.

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 24View Image). 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).


  Go to previous page Go up Go to next page