2.4 Transient brightenings and X-ray jets

Yohkoh/SXT has revealed that the corona is full of transient brightenings (Shimizu et al., 1992) and X-ray jets (Shibata et al., 1992aJump To The Next Citation Point). Shimizu et al. (1994) investigates the structure of transient brightenings in active regions, and they found that these transient brightenings usually have single or multiple loop structure with the length of 0.5 – 4 × 104 km. The total thermal energy estimated in a single transient brightening is 1025 – 1029 erg and the time scale is 1 – 10 min. They further showed that the transient brightenings correlate with GOES C-class or sub-C class flares, suggesting that these brightenings are the spatially resolved soft X-ray counterpart of hard X-ray microflares (Lin et al., 1984Jump To The Next Citation Point). Later, Watanabe (1994) shows that the maximum temperature of sub-C class flares is the order of 107 K. The multiple loop structure of transient brightenings might be the evidence of magnetic reconnection via loop-loop interactions (Gold and Hoyle, 1960Jump To The Next Citation Point; Tajima et al., 1987Jump To The Next Citation Point; Sakai and Koide, 1992). Shimizu (1995Jump To The Next Citation Point) shows a simple scaling law between the number N and total thermal energy W of transient brightenings as follows:
dN ∕dW ∝ W −1.5∼ −1.6, (4 )
where W ranges from 1027 to 1029 erg. Since this result is essentially the same as large flares and hard X-ray microflares (Hudson, 1991Jump To The Next Citation Point), it is likely that the same physical mechanism works for transient brightenings and flares.

X-ray jets are defined as soft X-ray brightenings with the shape of a collimated plasma outflow (Shibata et al., 1992aJump To The Next Citation Point; see Figure 9View Image). They are accompanied by small flares and X-ray bright points. The occurrence rate of X-ray jets is more than 20 per month between November 1991 and May 1992. Shimojo et al. (1996) shows that their average length and apparent velocity are 1.7 × 105 km and 200 km s–1, respectively. Shibata et al. (1992aJump To The Next Citation Point) pointed out that an X-ray jet is often observed in an emerging flux region where emerging field might interact with preexisting field. Until now two types of X-ray jets have been identified (Figure 10View Image):

(1) Two sided-loop jet: When emerging field interacts with the preexisting field that extends horizontally, jets are produced in the horizontal direction toward both sides of an emerging flux region.

(2) Anemone-type jet: When a newly emerging flux region appears in a unipolar region such as coronal holes, vertical jets are generated via the interaction of emerging field and the preexisting field that extends vertically. This forms an anemone-like loop structure in three-dimensional space (see the bottom panel of Figure 31View Image).

View Image

Figure 9: (Left) X-ray jet observed in soft X-rays (from Shibata et al., 1992a). Shown in reversed contrast. (Right) Longitudinal magnetic field distribution in the same field of view as in the left panel. The contour shows the soft X-ray intensity, revealing that the footpoint of the X-ray jet correspond to the mixed polarity region (from Shibata, 1999Jump To The Next Citation Point).
View Image

Figure 10: Two types of X-ray jets. (a) Two sided-loop jet. (b) Anemone-type jet. (c) Typical configuration for the two sided-loop jet at left panel and anemone-type jet at right panel (modified from Yokoyama and Shibata, 1996Jump To The Next Citation Point).

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