Coronal dimming is the reduction in intensity on the solar disk across a large area, observed in X-ray, EUV and more recently in H, and coincident in timing with the launch of a CME above. Measurements imply that the reduction in intensity is due to the evacuation of mass from the low corona (Hudson and Webb, 1997) and not a temperature change (e.g., Harrison and Lyons, 2000; Harrison et al., 2003). The dimming regions can be much more extensive than any associated flaring activity and can map out the apparent base of the associated CME (Thompson et al., 2000; Harrison et al., 2003). They have also been shown to extend deep into the corona and possibly the chromosphere and photosphere (McIntosh et al., 2007), thereby indicating that the initial terminology of “transient coronal hole” is probably more physically appropriate. Coronal dimmings are good indicators of the area on the Sun corresponding to the CME and of the behavior of the local magnetic fields following the CME launch. It is likely that at least part of the mass observed leaving the low coronal dimming region becomes part of the CME (e.g., Webb et al., 2000a), but what part and how much are uncertain. In a recent survey of six STEREO events observed as dimmings by EUVI and as CMEs by COR2, Aschwanden et al. (2009) found a nearly 1:1 correspondence between the EUV and white light masses. The self-similar evolution of the mass from the low to outer corona was also successfully modeled. For their sample of EIT dimming events, Reinard and Biesecker (2008) found mean lifetimes of 8 hours, with most disappearing within a day. Other results suggest that there may be two types of dimming, “core” dimmings directly associated with the source active region and flare, and “secondary” dimmings farther away that may be associated with loop motions or evacuation (e.g., Attrill et al., 2010).
Surveys of solar activity associated with frontside halo CMEs have been made primarily with low coronal images from the SOHO EIT and Yohkoh Soft X-ray telescope (SXT) instruments, although surveys with STEREO and Hinode are emerging. The activity associated with halo CMEs includes the formation of dimming regions, long-lived loop arcades, flaring active regions, large-scale coronal waves and filament eruptions (Figure 24). Webb (2002) found that 2/3 of halo CMEs were associated with either or both filament eruptions and dimmings, and Reinard and Biesecker (2008) found that about half of all frontside halo CMEs have dimmings. Coronal dimming has not been observed as frequently as other associated eruptive phenomena but the most recent, very sensitive results (e.g., Schrijver and Title, 2011) from SDO imply that dimming is more common than measurements from previous instruments have implied.
Living Rev. Solar Phys. 9, (2012), 3
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