2.3 Impulsive flares

Those flares called impulsive flares show simple loop structure in soft X-ray, and they do not have a cusp-shaped structure, as found by Skylab before Yohkoh. Because of their apparent shape, impulsive flares are also called compact flares or confined flares (Pallavicini et al., 1977). Historically, it was considered that these flares were produced via energy release inside the loop observed in soft X-ray (Alfvén and Carlqvist, 1967Jump To The Next Citation Point; Spicer, 1977; Uchida and Shibata, 1988Jump To The Next Citation Point). Obviously, this is different from the energy release outside the loop, suggested by the CSHKP model for LDE flares, in which the energy release site is a current sheet formed above the soft X-ray (SXR) loop (see Figure 42View Image).
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Figure 8: Hard X-ray loop-top source (contours) of an impulsive flare observed on January 13, 1992 (from Masuda, 1994Jump To The Next Citation Point). Colors represent soft X-ray intensity.

Using the hard X-ray telescope (HXT) aboard Yohkoh, Masuda et al. (1994Jump To The Next Citation Point, 1996) discovered a hard X-ray loop-top source above a soft X-ray loop in several impulsive flares. This suggests that the energy source producing the hard X-ray loop-top source is not located inside the soft X-ray loop but above (outside) the loop. Figure 8View Image shows an example of hard X-ray loop-top and footpoint sources, observed at the limb of the Sun. The temporal behavior of the loop-top source is similar to the footpoint sources (Masuda, 1994Jump To The Next Citation Point). This suggests that at least part of impulsive flares are produced through the same process as LDE flares. A more detailed explanation is given in Section 4 (see also Figure 37View Image).

Later, Shibata et al. (1995Jump To The Next Citation Point) show an evidence that magnetic reconnection occurs outside a soft X-ray loop in several impulsive flares, where they also found plasmoid ejection (see Figure 5View Image). Shibata et al. (1995Jump To The Next Citation Point) derived the following features of plasmoid ejection observed in impulsive flares:

  1. The velocity is 50 – 400 km s–1.
  2. The size is 4 – 10 × 104 km.
  3. The soft X-ray intensity is 10–4 – 10–2 of the peak intensity of a soft X-ray loop.
  4. The ejection of a plasmoid starts almost simultaneously at the beginning of the impulsive phase during which hard X-ray intensity takes a peak value. This relation also holds true in the case of multiple ejection where multiple impulsive phases exist (e.g., Oct. 4, 1992 flare).
  5. A small soft X-ray bright point appears during the impulsive phase of a flare (Shibata et al., 1995Jump To The Next Citation Point), about a few 104 km away from a soft X-ray loop. It is suggested that this bright point corresponds to one of the footpoints of an erupting flux rope forming a plasmoid in three-dimensonal space.

Recently, Shimizu et al. (2008) made an analysis of fifteen impulsive flares to examine the correlation among the rise velocity of a soft X-ray loop and the ejection velocity of a plasmoid. The main conclusion is that there is a positive correlation between these two velocities (the ejection velocity is an order larger than the rise velocity), suggesting that the CSHKP model can be applied even for these impulsive flares. This further suggests that the plasmoid-induced-reconnection may play a key role in flares (see Section 4.1.6).

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