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., 1992a). 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., 1984). 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,
1960; Tajima et al., 1987; Sakai and Koide, 1992). Shimizu (1995) shows a simple scaling law
between the number and total thermal energy of transient brightenings as follows:
where ranges from 1027 to 1029 erg. Since this result is essentially the same as large flares and hard
X-ray microflares (Hudson, 1991), 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., 1992a; see Figure 9). 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. (1992a) 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
(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
(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 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, 1999).
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, 1996).