3.1 Coronal streamers and blowouts

CMEs in general are associated with previously closed magnetic field regions in the corona, the opening of which is a consequence of the eruption. Many CMEs viewed at the solar limb also appear to arise from large-scale, pre-existing coronal streamers which often overlie active regions (e.g., Hundhausen, 1993Jump To The Next Citation Point). Many energetic CMEs actually involve the disruption (“blowout”) of such a structure, which can increase in brightness and size for days before erupting as a CME (Howard et al., 1985; Illing and Hundhausen, 1986Jump To The Next Citation Point; Hundhausen, 1993Jump To The Next Citation Point). Possible causes of such disruptions include the emergence through the surface of new magnetic flux, the dynamical evolution of arcades, or the shearing of magnetic field lines. Other variants of streamer changes associated with different types of filament activity have been noted by Gopalswamy et al. (2004a).

A streamer is a bright (dense) structure containing closed and open fields, which help guide denser, outward-flowing solar wind material. They are observed by coronagraphs (and during solar eclipses) above the solar limb and are often found above active regions. Blowout CMEs viewed when the surface eruption is at the solar limb mostly display the classic three-part structure (Burkepile et al., 2004). In these cases prominence material can actually be followed from at or near the solar surface (as viewed in the Hα line) into the coronagraph field of view (Figures 2View Image, 19View Image, and 20Watch/download Movie), where it forms the bright core of the CME. CMEs exhibit radial velocity dispersion, with the leading edge being fastest, followed by the speed decreasing through the prominence material (Webb and Jackson, 1981; Simnett, 2000Jump To The Next Citation Point). The kinematic profiles of erupting prominences and their associated CMEs are usually similar in that both will exhibit acceleration, deceleration or constant speed with height. The SMM coronagraph had an Hα filter, which was used for studies of a few CMEs containing large prominences. Illing and Athay (1986) compared the Hα and white light images from eight prominence/CMEs finding that some CME prominence masses exceed 1012 kg: a large fraction of the total CME mass. They also concluded that the prominence material usually becomes nearly fully ionized as it moves outward through the low corona. UVCS results are limited in this regard, because its best diagnostics are for plasma typically in the 105 K range. The brightest UVCS emission seen during CMEs is likely in the core or prominence material. Proton temperatures and ionization states suggest plasma of 104.5 – 5.5 K, so the material has probably been heated from the original prominence temperatures and it must be heated continually as it moves out to counteract cooling and radiative losses (Kohl et al., 2006Jump To The Next Citation Point; J. Raymond, 2011, priv. comm.). In one event, Ciaravella et al. (2003bJump To The Next Citation Point) noted that prominence material likely was heated to above 106 K. The cleanest evidence for heated prominence plasma is the EIS result for the 9 April 2008 event by Landi et al. (2010). Also, many EIT and TRACE observations of erupting prominences near the surface show them changing from absorption to emission, indicative of heating.

View Image

Figure 19: LASCO C2 image from 4 January 2002 image of a Coronal Mass Ejection (CME) showing detail in the ejected material. The solar limb Sun is represented by the white circle. Available from SOHO online image gallery: External Linkhttp://sohowww.nascom.nasa.gov/gallery/bestofsoho.html.

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Figure 20: mpeg-Movie (455 KB) LASCO C3 images of lightbulb shaped CME on 27 February 2000. Classic three-part structure with outer shell, void and inner bright structure, in this case an erupting prominence. From SOHO online movie gallery: External Linkhttp://sohowww.nascom.nasa.gov/gallery/Movies/flares.html.

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