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1 Overview

Figure 1: Time evolution of the GOES X-ray fluxes. Here we focus on the October 28, 2003 X17 flare. In the bottom plot, the timings of the 5 successive CDS rasters are indicated. CDS observed the two precursor M-class flares F1 and F2 with the third raster.

2 The two pre-events, M-class flares

SOHO CDS observed with the 3rd raster during 09:37-11:02 UT. CDS observed two distinct M-class flares, with strong Fe XIX emission (saturated in the two cores). The first one (F1) was observed during 10:00-10:19 UT, while the second (F2) during 10:33-10:47 UT. The first flare was also observed by TRACE.

Figure 2: Sequence of TRACE 195 Å images, showing the two M-class flares. Between 10:24 and 10:49 UT TRACE did not observe. Note the contours of Fe XIX emission of the two flares observed by CDS.

Figure 3: Sequence of TRACE 1600 Å images, showing the two M-class flares. Between 10:24 and 10:49 UT TRACE did not observe.

Figure 4: TRACE 195,1600 Å with contours from SOHO/MDI magnetogram. The M-class flare occurred in the region of mixed polarity. Note also the filament eruption associated with this flare.

Figure 5: SOHO/MDI magnetogram.

Figure 6: CDS monochromatic images at transition region (O V), coronal (Mg X) and flare (Fe XIX) temperatures, during the long CDS scan which observed the two F1,F2 M-class flares. Overlaid are contours of TRACE 1600 Å. Notice the complex structure of the second flare (left) in the enhanced Fe XIX image.

3 The X17 flare

Figure 7: Sequence of TRACE 195 Å images during the X17 flare. The first three images are shown with a different intensity scale, compared to the following ones.

Post-flare arcade

Here we show results concerning the 5th CDS observation (12:28-12:52 UT). During this time, TRACE 195 and 1600 Å images show that the 2-ribbon structure and the arcade of loops is approximately stationary, with some apparent flows along the loops.

Figure 8: Top: TRACE 195 Å with contours of CIV. Bottom: TRACE 1600 Å

Figure 9: Top: SOHO/MDI magnetogram with contours of CIV (showing the ribbons). Bottom: TRACE 195 Å with contours of Fe XIX intensity from CDS. The contours show the hottest regions, where most TRACE emission is dominated by Fe XXIV.

Figure 10: CDS intensity maps with contours of CIV (showing the ribbons). The post-flare loops show emission at all temperatures

Figure 11: CDS velocity map in the transition region (O V) with contours of CIV (showing the ribbons). Strong downflows (v @ 50-100 km/s ) in the ribbons are observed.

Figure 12: CDS velocity maps in coronal lines with contours of C IV (showing the ribbons). Strong downflows (v @ 50-100 km/s ) in the ribbons are observed.

The X17 flare presented great complexity. We have used the CDS to understand the temperature structure of the various events.

Two precursor M-class events have been observed with CDS. The first one was also observed by TRACE, while the second, more important one, was not observed by TRACE. The second one had a complex structure of hot loops.

During the precursor events the filament becomes activated.

During the X17 flare the ribbons quickly separate and an arcade system of loops forms. These loops are multi-thermal, with the hottest part in the central region.

Strong downflows (v @ 50-100 km/s ), in particular at transition region temperatures are observed in the ribbons.

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On 14 Apr 2005, 15:46.