Abstract
Abstract. The apparent contradiction between small-scale source regions of, and large-scale coronal response to, coronal mass ejections (CMEs) has been a long-standing puzzle. For some, CMEs are considered to be inherently large-scale events – eruptions in which a number of flux systems participate in an unspecified manner, while others consider magnetic reconnection in special global topologies to be responsible for the large-scale response of the lower corona to CME events. Some of these ideas may indeed be correct in specific cases. However, what is the key element which makes CMEs large-scale? Observations show that the extent of the coronal disturbance matches the angular width of the CME – an important clue, which does not feature strongly in any of the above suggestions. We review observational evidence for the large-scale nature of CME source regions and find them lacking. Then we compare different ideas regarding how CMEs evolve to become large-scale. The large-scale magnetic topology plays an important role in this process. There is amounting evidence, however, that the key process is magnetic reconnection between the CME and other magnetic structures. We outline a CME evolution model, which is able to account for all the key observational signatures of large-scale CMEs and presents a clear picture how large portions of the Sun become constituents of the CME. In this model reconnection is driven by the expansion of the CME core resulting from an over-pressure relative to the pressure in the CME's surroundings. This implies that the extent of the lower coronal signatures match the final angular width of the CME.
Highlights
The apparent contradiction between small-scale source regions of, and large-scale coronal response to, coronal mass ejections (CMEs; cf. Fig. 1) has been highlighted by Klimchuk (2001): “Coronagraph observations suggest that the horizontal scale of the opened field can be many times greater than that of the reconnection arcade and this may be difficult to reconcile with the geometry of the [existing] model[s].”High-quality data coupled with some new ideas have brought us closer to solving this puzzle
CMEs are considered to be inherently large-scale events – eruptions in which a number of flux systems participate in an unspecified manner, while others consider magnetic reconnection in special global topologies to be responsible for the large-scale response of the lower corona to CME events
4.5 Reconnection with coronal holes. Coronal holes are another type of magnetic structure that, upon interaction and reconnection with a CME can dramatically alter the global topology of the CME
Summary
The apparent contradiction between small-scale source regions of, and large-scale coronal response to, coronal mass ejections (CMEs; cf. Fig. 1) has been highlighted by Klimchuk (2001): “Coronagraph observations suggest that the horizontal scale of the opened field can be many times greater than that of the reconnection arcade and this may be difficult to reconcile with the geometry of the [existing] model[s].”. CMEs may become large-scale even in the low corona due to interaction between the expanding magnetic structure of the CME and other low-coronal magnetic structures (e.g. Maia et al, 1999; Pohjolainen et al, 2001; Bemporad et al, 2005; Attrill et al, 2007a; Mandrini et al, 2007; Moore and Sterling, 2007). In this case, CMEs become large-scale by nurture – they are not large-scale by nature.
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