Tsuneta et al. (1992a) derived the following features of this LDE flare. These are now recognized as the common properties of LDE flares: The temperature distribution is somewhat chaotic in the early phase when the flare started, while in the late phase the temperature is systematically higher near the edge of the cusp-shaped loop (Veronig et al., 2006, and references therein). This can be explained by the radiative cooling efficiently working at inner central part of the loop (Forbes and Malherbe, 1991; Vršnak et al., 2006).
Yohkoh observations of the same flare as in Figure 4 (Hudson, 1994) showed that a small island-like feature (plasmoid) with the size of a few 104 km in soft X-ray is ejected at about a few 100 km s–1 during the precursor phase (see Figure 3a, Figure 43) of the flare. A soft X-ray movie of this flare suggests that the ejection of the plasmoid triggers the flare. It also shows that a filamentary structure existed before the onset of the flare and after it was ejected, the flare occurred. The filamentary structure seems to be located inside a current sheet which is formed during the preflare phase of this flare. It is expected that the structure formed inside a current sheet tends to inhibit strong inflows from entering a current sheet, and after this structure is ejected from the current sheet, strong inflows can drag magnetic flux into the current sheet and the rapid energy release starts (Section 4.1.6). A similar ejection phenomenon is also found in another flare (see Figure 5).
The cusp-shaped loop is also observed in many other flares (e.g., Tsuneta, 1997), even in somewhat indistinct observations done during the pre-Yohkoh era (MacCombie and Rust, 1979; Hanaoka et al., 1986).
Living Rev. Solar Phys. 8, (2011), 6
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