2.1 Long-durational-event (LDE) flares

One of the biggest discoveries by the soft X-ray telescope (SXT) aboard Yohkoh is the cusp-shaped loop structure of long-durational-event (LDE) flares. Figure 4View Image shows a beautiful example of an LDE flare, which was observed on Feb. 21, 1992 at the west limb of the Sun (Tsuneta et al., 1992aJump To The Next Citation Point; Tsuneta, 1996Jump To The Next Citation Point; Forbes and Acton, 1996Jump To The Next Citation Point). This flare is accompanied by a large-scale coronal ejection (possibly coronal mass ejection). The ejection created a helmet streamer-like configuration, suggesting that a current sheet (Section 4.1) is formed. The apparent height of the cusp-shaped loop and the distance between the footpoints of the loop increased gradually at a few km s–1. This supports the idea that magnetic reconnection (Section 4.1) successively occurred above the loop.
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Figure 4: A soft X-ray image of an LDE flare with cusp shaped-loop structure, observed on Feb. 21, 1992 (Tsuneta et al., 1992aJump To The Next Citation Point; Tsuneta, 1996Jump To The Next Citation Point). Shown in reversed contrast.

Tsuneta et al. (1992aJump To The Next Citation Point) 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., 2006Jump To The Next Citation Point, 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., 2006Jump To The Next Citation Point).

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Figure 5: Soft X-ray images of plasmoid ejection associated with the flare on Oct. 5, 1992 (from Ohyama and Shibata, 1998Jump To The Next Citation Point). Shown in reversed contrast.

Yohkoh observations of the same flare as in Figure 4View Image (Hudson, 1994Jump To The Next Citation Point) 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 3View Imagea, Figure 43View Image) 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 5View Image).

The cusp-shaped loop is also observed in many other flares (e.g., Tsuneta, 1997Jump To The Next Citation Point), even in somewhat indistinct observations done during the pre-Yohkoh era (MacCombie and Rust, 1979; Hanaoka et al., 1986).


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