3.4 Coronal dimming to arcade formation

The most obvious coronal signatures of CMEs in the low corona are the arcades of bright loops that develop after the CME material has erupted (Kahler, 1977 – Skylab; McAllister et al., 1996Jump To The Next Citation Point; Hudson and Webb, 1997Jump To The Next Citation Point – Yohkoh; Hanaoka et al., 1994 – radio; Tripathi et al., 2004Jump To The Next Citation Point – EIT). Prior to the eruption, an S-shaped structure called a sigmoid can develop, typically observed in X-rays, sometimes in association with a filament activation. A sigmoid is indicative of a highly sheared, non-potential coronal magnetic field, and might be an important precursor to certain types of CMEs (e.g., Canfield et al., 1999). However, McKenzie and Canfield (2008) point out that X-ray sigmoids actually consist of separate J-shaped loops that support a bald-patch separatrix surface model for sigmoids. Eventually an eruptive flare can occur within or in the proximity of the sigmoid, resulting in the bright, long-duration arcade of loops. Sterling et al. (2000) call this process “sigmoid-to-arcade” evolution. These arcades suggest the eruption and subsequent reconnection of strong magnetic field lines associated with the CME system. Tripathi et al. (2004Jump To The Next Citation Point) found that nearly all (92%) EIT post-eruptive arcades (PEAs) from 1997 – 2002 were associated with LASCO CMEs (Figure 23View Image). Recent analyses of Hinode XRT and SDO AIA data reveal new information of the space and temperature evolution of arcades (e.g., Reeves et al., 2010; Reeves and Moats, 2010Jump To The Next Citation Point; Reeves and Golub, 2011), which help to further constrain the “standard” eruptive flare model (e.g., Section 3.2 and Figure 20Watch/download Movie).
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Figure 23: Erupting prominence, dimming regions and arcade associated with a fast CME on 12 September 2000. Top: SOHO EIT 195 Å running-difference images; bottom: CME leading edge and erupting prominence (EP) seen in SOHO LASCO C2 images. Image reproduced with permission from Tripathi et al. (2004Jump To The Next Citation Point), copyright by ESO.

Coronal dimming is the reduction in intensity on the solar disk across a large area, observed in X-ray, EUV and more recently in Hα, 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., 2003Jump To The Next Citation Point). 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 (2008Jump To The Next Citation Point) 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 24View Image). Webb (2002Jump To The Next Citation Point) 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.

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Figure 24: A filament eruption and post-eruption arcade near Sun center on 17 February 2000 (top). It was associated with a symmetrical LASCO halo CME (bottom). Image reproduced with permission from Tripathi et al. (2004Jump To The Next Citation Point), copyright by ESO.

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