Using the hard X-ray telescope (HXT) aboard Yohkoh, Masuda et al. (1994, 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 8 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, 1994). 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 37).
Later, Shibata et al. (1995) 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 5). Shibata et al. (1995) derived the following features of plasmoid ejection observed in impulsive flares:
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).
Living Rev. Solar Phys. 8, (2011), 6
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