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Astron. Astrophys. 364, 853-858 (2000)

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2. Observation

For a detailed study of fine structures in the high-frequency part of decimetric and microwave bursts, new spectral observations (intensity and polarization) with high resolution in time and in frequency over a wide frequency range are needed. For this purpose, a broad band radio spectrometer is being developed by the solar radio astronomical community of China (Fu et al. 1995). It is composed of five separate spectrometers covering the respective frequency ranges: 0.70-1.40 GHz, 1.00-2.00 GHz, 2.60-3.80 GHz, 4.00-5.20 GHz and 5.20-7.60 GHz. The 2.60-3.80 GHz component spectrometer started working in Oct. 1996 at BAO; it has 120 frequency channels of 10 MHz bandwidth each, a time resolution of 8 milliseconds, as well as allowing high sensitivity and high accuracy measurement of circular polarization (Ji et al. 2000).

An overview of solar radio bursts on Apr. 15 1998 observed by BAO over the band 2.60-3.80 GHz is given in Fig. 1, including the left- and right-handed circular dynamic spectrum and polarization; the data is integrated into a time resolution of 200 ms. There is a decimetric pulsation (DCIM) event between 0741 UT and 0749 UT, and about 9 minutes later, from 0758 UT to 0815 UT, a second DCIM was observed. On the rising phase of the second DCIM emission, from about 0747 UT to 0802 UT, various fine structures are detected, thanks to the high time and frequency resolution. Three of these structures are given in Fig. 2a-c (FS a, FS b, and FS c in chronological order respectively) with a right-handed circular component. FS b and FS c are very close to each other, however, there are other kinds of fine structures between FS a and FS b.

[FIGURE] Fig. 1. Dynamic spectrum of the DCIM on April 15 1998 observed by the BAO spectrometer in the 2.60-3.80 GHz band. From top to bottom: left- and right-handed circular components and polarization degree respectively. The data is integrated into the time resolution of 200 ms. The first triangle corresponds to the FS a in Fig. 2, and the second one indicates the FS b and c (the two are very close to each other).

[FIGURE] Fig. 2a. Three special fine structures in the rising phase of the second DCIM around the time 1998 0415 0800 UT. FS a, b and c (the data with a time resolution of 8 ms) are three EEL events, the other two pictures have a time resolution of 200 ms. The triangle marker in FS b points to the period where five strips are split during 84 ms.

[FIGURE] Fig. 2b and c.

These two DCIM were also recorded by the spectrometer over a range from 2000 MHz to 4395 MHz at the Ondrejov Observatory (ONDR). On the other hand, Solar Geophysical Data (SGD) reports a C8.8/SN flare detected by GOES and SVTO in active region(AR) 8203 located at N29W15; the flare occurred from 0737 UT to 0806 UT, reaching its maximum at 0746 UT. Therefor, AR 8203 and its flare are the likely source of the two DCIM events as the optical emssion begins slightly before the radio emission. In the observations of YOHKOH, a small arcade structure is detected in the soft X-ray (SXR) in AR 8203 between 0806 UT and 0816 UT. However, the hard X-ray event (HXR) recorded by BATSE on channel 0 (33.2-56.9KeV) lasts from 0740 UT to 0752 UT. In addition, there are also coronal mass ejection (CME) events by SOHO and type II bursts on Apr. 15-18 1998.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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