fl151.masuda03 Posted: 25-Oct-93 Updated: 24-Jul-94, 03-Apr-95, 26-Nov-95, 20-Jul-96 Events specified: 11 flares (see text)
Authors: S. Masuda, T. Kosugi and T. Sakao National Astronomical Observatory of Japan
S. Tsuneta Institute of Astronomy, University of Tokyo
H. S. Hudson Institute for Astronomy, Unversity of Hawaii
Abstract:
The hard X-ray time profiles of solar flares often seem to be a superposition of spikes, clearly seen at higher energy bands, upon a "super-hot" (T > 30 MK) thermal background emission which varies more gradually. The latter component usually develops after impulsive spikes, so one might suppose that the super-hot plasma is an aftermath due to collisional heating by accelerated electrons. This is not necessarily true, however. In the case of the 24 October, 1991 flare, HXT imaging observations clearly reveal that, at least at one prominent spike peak, the spike component originates from a remote site, some 40 arcsec apart from the location of the super-hot "background" component. In order to pursue this problem further, we investigate several other flares which occurred near the solar limb. We concentrate our attention to the vertical separation between thermal and non-thermal sources as well as their evolutionary differences. The present analysis contains the following list of events, which were selected by two criteria: 1) the longitude exceeds 80 degree and 2) hard X-ray count in M2-band exceeds 10 counts/sec/subcollimator.
91/12/02 045527 M3.6 SF N16E87 6952 91/12/15 024411 M1.2 92/01/13 172937 M2.0 92/02/06 032511 M7.6 SN N05W82 7030 92/02/17 154209 M1.9 SF S16W81 7050 92/04/01 101407 M2.3 92/10/04 222107 M2.4 SN S05W90 7293 92/11/02 X9.0 92/11/05 061959 M2.0 92/11/23 202602 M4.4 SF S08W89 7342 93/02/17 103630 M5.8 SF S07W87 7420
Remarks;
This study is a main part of my PhD thesis. The superviser of this study is Takeo Kosugi (NAOJ). I will submit the thesis by the end of the year 1993.
Update 20-Jul-96
I am preparing to submit a paper, entitled "Characteristics of Hard X-ray Sources in Solar Flares", to ApJ. In this paper, the observational results of three types of hard X-ray sources, so-called ``(double) footpoint source(s)'', ``loop-top impulsive source'', and ``loop-top gradual source'' are summarized. The abstract is as follows.
Hard X-ray images of ten flares sampled in an unbiased manner, which occurred near the solar limb and were observed with the Hard X-ray Telescope (HXT) on board Yohkoh during the period from October, 1991 to September, 1993, are analyzed in detail to investigate the vertical structure of hard X-ray sources. From temporal, spectral, and structural behaviors derived from a series of images, it is found that hard X-ray sources are classified into three types: ``(double) footpoint source(s)'', ``loop-top impulsive source'', and ``loop-top gradual source''. Characteristics of these components, as well as their interpretation and implication, are discussed.
As for the loop-top impulsive sources, instead of the MEM, the Gaussian- source fitting method is adopted to investigate the temporal behaviors of the source above the loop and the footpoint sources with a better time resolution ( < 5 seconds ) in the HXT M2-band (33-53 keV). Though a preliminary result were reported at the Bath conference, this study is now in progress.
Update 26-Nov-95
A paper, whose title is "Hard X-ray Sources and the Primary Energy-Release Site in Solar Flares", appeared on Publ. Astron. Soc. Japan, 47, 677-689, (1995). In this paper, we discussed the characteristics of loop-top impulsive source in three flares (the 13-Jan-92, 4-Oct-92, 17-Feb-93 events). Observational results for the loop-top impulsive source are summarized. And we are preparing another paper in which we discuss another type of loop-top source, so-called loop-top gradual source. In this study, we analyzed ten events and found a loop-top gradual source in all of the events. Now we consider the relation between the impulsive loop-top source and the gradual loop-top source. This is related to the origin of the loop-top gradual source.
Update 03-Apr-95
As we described in the last report, we found two types of 'loop-top' hard X-ray sources, loop-top impulsive source and loop-top gradual source, in addition to (double) footpoint sources.
A paper, in which we discuss the loop-top impulsive source, was submitted to Publ. Astron. Soc. Japan in February 1995. The loop-top gradual source will be summarized in another paper.
Abstract of the paper submitted to PASJ
Accurately coaligned hard and soft X-ray images, taken simultaneously with the Hard X-ray Telescope (HXT) and the Soft X-ray Telescope (SXT) aboard Yohkoh, of the impulsive solar flares on 13 January, 1992 (17:29 UT), 4 October, 1992 (22:21 UT), and 17 February, 1993 (10:35 UT) occurring near the limb, clearly reveal that, in addition to double-footpoint sources, a hard X-ray source exists well above the corresponding soft X-ray loop structure around the peak time of the impulsive phase. This hard X-ray source shows an intensity variation similar to the double-footpoint sources and a spectrum relatively hard in comparison with the loop-top gradual source appearing later in the flare. We believe that this is the first clear evidence that the magnetic reconnection, which is responsible for the primary flare energy release, is under progress above the soft X-ray flaring loop. Maybe this ``loop-top'' hard X-ray source represents the reconnection site itself or the site where the downward plasma stream, ejected from the reconnection point far above the hard X-ray source, collides with the underlying closed magnetic loop. Characteristics of this hard X-ray source are quantitatively discussed in the schemes of thermal (T $\gsim 10^8$ K) and nonthermal interpretations of the hard X-ray emission.
Update 24-Jul-94
Analysis
Hard X-ray images of unbiasedly sampled ten flares (see event list), which near the solar limb and were observed with the Hard X-ray Telescope (HXT) on board Yohkoh during the period from October, 1991 to September, 1993, are analyzed in detail to investigate the vertical structure of hard X-ray sources.
Event List
91/12/02 045527 M3.6 SF N16E87 6952 91/12/15 024411 M1.2 6972 92/01/13 172937 M2.0 6994 92/02/06 032511 M7.6 SN N05W82 7030 92/02/17 154209 M1.9 SF S16W81 7050 92/04/01 101407 M2.3 7123 92/10/04 222107 M2.4 SN S05W90 7293 92/11/05 061959 M2.0 7323 92/11/23 202602 M4.4 SF S08W89 7342 93/02/17 103630 M5.8 SF S07W87 7420
Results
From temporal, spectral, and structural behaviors derived from series of images, it is found that hard X-ray sources are classified into three types: ``(double) footpoint source(s)'', ``loop-top impulsive source'', and ``loop-top gradual source''. The characteristics of the individual types of source are as follows:
(Double) Footpoint Source(s): Sources of this type usually dominate over the hard X-ray emission in the impulsive phase, especially so in higher energy X-rays above 30 keV. They tend to appear in pairs at the two ends (or footpoints) of a soft X-ray flaring loop; the double-source footpoint structure is seen in seven events out of the ten. The impulsive time variation and the hard spectrum are explained as the emission originating from non-thermal electrons accelerated around the top of the loop and precipitating down along the loop into the chromosphere.
Loop-top Impulsive Source: Sources of this type appear as a single source in six events out of the ten. The source is relatively compact and is located well above the apex of the corresponding soft X-ray flaring loop (three events out of the six), or at or near the apex (the remaining three events). The temporal and spectral characteristics of the source are similar to those of the footpoint source, so that this source is intimately related to the footpoint source. Maybe the loop-top source represents the primary energy release site of solar flares.
Loop-top Gradual Source: Sources of this type appear at the apex portion of
the corresponding soft X-ray flaring loop as a single source in all of the ten flares analyzed. This loop-top source is slightly diffuse and sometimes seems to trace the loop seen in soft X-rays. Although the source begins to brighten in the impulsive phase, it is most clearly seen in the X-ray energy range < 30keV and in the gradual phase. This source represents the creation of high- temperature plasma.
Current status
(1) This topic is a part of my PhD thesis which was accepted in February 1994. (2) Full paper in preparation. (3) Abstract to appear in the Proc. of the Kofu Symposium.