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Astron. Astrophys. 364, 859-872 (2000)

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3. Flare properties and evolution

The flare was a GOES M1.7 class event which occurred in NOAA Active Region 8592 (N20E40) on the 22[FORMULA] June 1999 as part of the first Max Millennium observing campaign. The GOES start time was approximately 18:15UT with a maximum at 18:29UT in the 1 to 8 Å channel.

In the pre-flare stages Big Bear Solar Observatory (BBSO) H[FORMULA] images show the ejection of a filament beginning at 18:10UT. After 18:10UT the filament can be seen in emission in all TRACE filters, indicating that the structure is multi-thermal. A Coronal Mass Ejection (CME) was also observed by the Large Angle Spectroscopic Coronograph (LASCO) ejected from the North-East quadrant. The calculated onset time of the CME, from its velocity profile, was 18:00UT to within an hour. During the filament eruption SXT was observing a different active region in which a GOES C2.6 class event was decaying and within which a small brightening occurred. An SXT and GOES temperature analysis of these events revealed that we cannot reliably discount CaXIX emission originating from these events. Therefore the BCS data before 18:20UT, the start of the studied M-class event, has two possible sources and hence cannot be utilized fully. SXT began observing AR8592 at 18:20:30UT when flare mode was triggered.

The flare displays two footpoint regions that outline the bases of the flare arcade and are visible in HXR and TRACE images (Fig. 2). The footpoints are located on either side of a simple photospheric magnetic neutral line (Fig. 2a). These footpoints are observed (in L[FORMULA]) to move apart, away from the neutral line, as the flare progresses. Both footpoint areas are elongated in shape with the longer axis parallel to the neutral line (Fig. 2b). The suth footpoint area exhibits stronger emission at both L[FORMULA] and HXR wavelengths, indicating greater energy deposition. The magnetic field strength at the south footpoint is weaker than the north footpoint, in agreement with the results of Sakao (1994). He found that greater electron deposition occurs at the magnetically weaker (south) footpoint due to reduced mirror forces experienced by the electrons propagating along the magnetic field.

[FIGURE] Fig. 2a-f. Images of the studied flare: a MDI magnetogram, b TRACE Lyman[FORMULA], c TRACE FeXII /FeXXIV , d SXT Be119 e HXT LO channel and f HXT M1 channel. Spatial co-ordinates in this figure and throughout are arc-seconds from Sun centre.

HXT images of the impulsive phase show two footpoints, one on either side of the photospheric neutral line. These footpoints are visible in the LO(14-23keV; Fig. 2e), M1(23-33keV; Fig. 2f) and M2(33-53keV) channels. T HI(53-93keV) channel does not contain enough counts to formulate an image. Images in the low channel also show a HXR source located above the SXR loops, slightly offset to the south from the loop apex. This source is not present in the higher energy cha nnels so no diagnostics are possible. The lack of counts in the higher energy channels suggests this loop source is thermal. Unfortunately there are too few images to create reliable lightcurve comparisons with the footpoint sources. The double footpoint and loop top source are only visible from 18:21[FORMULA]18:23UT, the time at which the HXR flux in the LO, M1 and M2 channels was high. An arcade of flare loops connecting the two footpoints is visible first (18:24UT) at high temperatures (SXR and FeXXIV , Fig. 2d) and then later (18:35UT) at cooler temperatures (FeIX and FeXII, Fig. 2c). These cool flare loops continue rising, visible in TRACE FeXII and FeIX lines, until at least 21:00UT. The orientation of the flare loop arcade and footpoints is such that we observe the top of the loop system against the dark background of the disk. Hence the signal from the top of the loop arcade is not contaminated by emission from the footpoints.

The combined multi-wavelength images of this flare suggest it was of the type described by Carmichael (1964), Sturrock (1966), Hirayama (1974) and Kopp & Pneuman (1976) (hereafter CSHKP) and subsequently by other authors. In this model reconnection begins low in the corona after the ejection of mass, and proceeds upwards creating a loop arcade with a temperature differential in height and footpoints that move apart with time.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001