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Astron. Astrophys. 364, 793-798 (2000)

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1. Introduction

Solar type III bursts are probably better studied and elucidated than other forms of radio emission in all of astrophysics. They are attributed to electron beams propagating out through the corona along magnetic field lines at a speed of about c/3 (Wild 1950); on its passage the beam sets up Langmuir waves (plasma oscillations) which then are converted into electromagnetic waves near the local plasma frequency. Other theory of type III bursts, however, has also been proposed very recently (Wu et al. 2000). On the dynamic spectrum, ordinary type III bursts observationally appear as an emission rapidly drifting toward lower frequencies (higher altitudes). In some cases, bursts start just like ordinary type III bursts, but their frequency drift rates gradually change from negative through zero to positive; if the emission stops here, the bursts are called inverted-U bursts (simple U bursts) because of their appearance in the dynamic spectrum (sometimes they are called J-burst due to a shorter descending branch). Certain observations made before revealed a third ascending branch following the type U-event, which is called "N-burst" (Caroubalos et al. 1987; Hillaris et al. 1988). It is possible, however, that a fourth descending branch following the type N-event, thus suggesting the capital letter M in the dynamic spectrum. We call the bursts "M-burst" . A microwave M burst, which is a fine structure of type IV-DCIM bursts detected on May 3 1999 by Beijing Asronomical Observatory (BAO) spectrometer over the band 2.6-3.8 GHz is studied in this paper.

Observational and theoretical aspects of type IV bursts have been less studied than those of type III bursts, but type IV bursts are better known around meter wavelength than above 1 GHz. Boischot (1957) was the first to identify that type IV burst was meter-wavelength continuum emitted by a solar flare or by an eruptive prominence. Weiss (1963) used the name "moving type IV" (IVM) to distinguish it from the stationary component of type IV bursts. The type IV burst is also called "decimetric pulsation" (DCIM) in the decimetric and microwave bands, because DCIM is characterized by long duration (tens of minutes), broad bandwidth (more than 2.0 GHz as present event), no obvious frequency drift and association with or following a solar flare. Wiehl et al. (1985) reported a comprehensive study on a number of DCIM bursts and classified them into 4 main subtypes: short and long lasting bursts, narrowband and wideband bursts.

We will put the emphasis on the fine structure, of a kind that has never been recorded before in such high frequency range, around 3.0 GHz. In Sect. 2, we will describe the instrument and give an overview of the event on May 3 1999. In Sect. 3, we will analyse the properties of the fine structure and the event. In Sect. 4, we will discuss a plausible model of this fine structure and draw a conclusion in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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