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Astron. Astrophys. 356, 1067-1075 (2000)

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2. Instrumentation

The H[FORMULA] observations were carried out with a coelostat at the `Specola Solare Ticinese' in Locarno-Monti, Switzerland. A Zeiss birefringent filter (0.05 nm bandpass) and a fast digital CCD-camera were installed at the focus of the telescope with a focal length of 1.95 m and a free aperture of 0.15 m (Eder 1990). The instrument is an improved version of the one used by Wülser & Marti (1989). Our setup had a field of view (FOV) of 160" [FORMULA] 160" with a pixel size of 0.92" [FORMULA] 0.88" and a temporal resolution of 0.2 s. The images were flat-field calibrated and seeing induced shifts were corrected with the help of sunspot positions in the images. The temporal variation of the transparency of the Earth's atmosphere was taken into account by normalizing the image intensity relative to that of a quiet Sun region in the same image.

The microwave observations of the event were made by the patrol radio telescopes in Bern at 3.1, 5.2, 8.4, 11.8, 19.6, 35.0 and 50.0 GHz in right and left circular polarization with a temporal resolution of 0.1 s (e.g. Bruggmann & Magun 1990). The data were calibrated with published absolute quiet-Sun fluxes from Sagamore Hill (see Melnikov & Magun 1998). A sky calibration just ended at 08:01:00 UTC for 3 and 5 GHz and at 08:01:30 UTC for the higher frequencies. Therefore the first few seconds at the onset of the flare are missed at higher frequencies.

The hard X-ray/gamma-ray (HXR/GR) observations were carried out by the PHEBUS experiment on board the GRANAT satellite. PHEBUS has been described in Barat et al. (1988); Barat (1993) and Talon et al. (1993). During the 1991 March 13 flare, HXR/GR data were acquired from 08:01:38 to 08:14:24 UTC. The observations used here consist of spectral measurements performed in the 0.073-89 MeV energy range. Until 08:03:34 UTC spectra have been recorded over 116 energy channels with a variable accumulation time ranging from 0.37 to 0.88 s. After 08:03:34 UTC, 40 channel spectra have been recorded each second below [FORMULA] 8 MeV and every 4 seconds above. In addition we have used records of the [FORMULA] 0.073 MeV integral count rate obtained from 08:01:26 to 08:03:06 UTC with a time resolution varying from 8.5 ms to 48 ms, which were integrated to [FORMULA] ms, to improve the signal-to-noise ratio.

Finally, we have used a magnetogram recorded at Kitt Peak 16 hours before the flare.

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000