Of the 22 total events, 6 appear to be morphologically similar to the canonical
Masuda flare, namely, a looptop source connected by a single flaring loop to
two footpoints. PIXON images of these flares were obtained around the peak time,
and other times whenever the count rate was sufficiently large to allow imaging in the
25-50 keV band. We use these images as the input for imaging spectroscopy (see the
accompanying paper by Jiang et al; poster #18.03).
We describe
Flare 2092002
which is a GOES M1.8 flare
observed by RHESSI up to 800-7000 keV.
This event started at 09:22:08UT on September 20, 2002, continued rising to a
sub-peak at 09:25:59 in energy channel between 25 keV and 300 keV,
and peaked at 09:26:42UT
nearly simultaneously in the energy channels from 3 keV through 300 keV as can
be seen from the lightcurves in Fig. 1. The HXR images were reconstructed by
the PIXON algorithm using front segments of detectors 3 through 6, and detectors
8 and 9, with a minimal spatial resolution of
. Fig. 2 shows
the PIXON images in separate energy bands for a time interval [09:26:36, 09:27:20],
covering the main peak. It is clearly shown that in the 11.4-13.4 keV image a
diffuse source dominates, suggesting a hot looptop source of
keV. Two footpoint sources appear in about 21.7-25.5 keV
and become more and more dominant as the energy goes higher. The boxes in Fig.
1b define the three sources. The corresponding fluxes were calculated from these
image with background subtracted (see the Miscellany section). Fig. 3 shows
the lightcurves at a cadence of 2 seconds with a
seconds
integration time in the 12-25 keV and 25-50 keV band.
The ratio of fluxes of the two FPs to the LT source averaged over
the rising, peak, and decaying phase, respectively, is shown in red.
Again, we note that the looptop source dominates in 12-25 keV and
its importance gradually increases with time; in contrast, the footpoint sources
are much brighter in the 25-50 keV channel but it decays more rapidly
after the peak.
Flares 2081203 and 2112532 have qualitatively similar characteristics
(see Fig. 4 through 9).
Many flares appear to haVe a more complex morphology similar to the flares studied by Petrosian et al. (2002) during the rising phase of this sunspot cycle. An example of such a case is Flare 2080327 (see Fig. 10 through 12).