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Astron. Astrophys. 355, 769-780 (2000)

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2. Observations

The analysed PFL system was a remnant of a small flare (GOES class C2.9) which occurred on 16th December 1996 on the south-west limb of the Sun. GOES X-ray fluxes showed that the flare started at 12:20 UT, its maximum occurred at 12:29 UT and the GOES event finished at approximately 13:00 UT. There are no direct observations of the flare itself because during the flare Yohkoh was in the shadow of the Earth and the slit of CDS was high above the flare region. The data concerning the PFL system was taken only before and after the GOES event and its time distribution is presented on the background of GOES-9 X-ray fluxes in the Fig. 1.

[FIGURE] Fig. 1. GOES-9 X-ray fluxes and times of CDS and SXT observations. The hatched regions correspond to the times when the Yohkoh satellite was in the shadow of the Earth, the region between the two dashed lines corresponds to the time when the whole CDS raster was built and the region between the two dot-dash lines to the times when the loop system itself was scanned.

2.1. CDS

The Coronal Diagnostic Spectrometer - CDS on board SOHO is fully described in Harrison et al. (1995) and Harrison & Fludra (1995). The analysed raster was taken using the Normal Incidence Spectrometer (NIS) with the [FORMULA] arcsec slit oriented in the N-S direction.

The size of the raster analysed in this work is [FORMULA] arcsec, the spatial dimensions of one raster element are 2.032 arcsec in the E-W direction and 1.68 arcsec in the N-S direction. The exposure time of one spectrum was 45 s and the scanning started at 11:58:27 UT and finished at 13:47:05 UT (see also Fig. 1). The whole raster consists of 120 exposures and the total raster duration was 1h 48min 38s. This gives the CDS scanning speed [FORMULA] s per one N-S stripe. It is clear that when highly dynamic phenomena are observed, as for example PFL, a long scanning time can influence the shape of the observed structures and one has to be very careful when interpreting such observations. The observation was carried out in 16 spectral windows 20 pixels wide, in the spectral direction. Each window corresponds to spectral width approximately 1.33 Å for NIS I and 2.23 Å for NIS II. The spectral lines are usually in the middle of these windows. A list of the 14 spectral lines available and their approximate formation temperatures is given in Table 1. In addition to the 14 lines referred to in Table 1, there are two more spectral windows which are adjacent to the important density sensitive line pair of Fe XIV at 334.2 Å and 353.8 Å. These windows were used for determination of the scattered light level.


Table 1. List of available lines and their formation temperatures. Asterisks indicate the density and crosses the temperature sensitive line pairs used in this analysis.

2.2. SXT

The Soft X-ray Telescope - SXT on board Yohkoh is described in detail in Tsuneta et al. (1991). The SXT is able to obtain time sequences of images in one of five filters with the time resolution better than one second. The pixel size of the CCD detector is 2.45 arcsec.

As we mentioned above, the Yohkoh satellite was in the shadow of the Earth during the flare (see Fig. 1), so that the only SXT images available were taken before and after the flare. In this analysis we used only the SXT data obtained after the flare. The first usable, not overexposed SXT image of the sequence of interest was taken at 13:07:28 UT and the last one a long time after the examined PFL system had disappeared. The time gap between two consecutive images was 64 s. The observational sequence was carried out in two filters. One image taken in Al1 filter was followed by two images taken in AlMg filter. From the intensity ratio in these two filters the time evolution of the temperature and emission measure of the observed hot PFL plasma was determined.

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© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000