A. Kuznetsov
Institute of Solar-Terrestrial Physics Siberian Branch of
the Russian Academy of Sciences, Lermontov St. 126, Irkutsk 664033,
Russia
Zebra pattern is observed as a number of almost parallel bright and dark stripes in the dynamic spectrum of solar radio emission. In this work, the burst on April 21, 2002 is investigated. Zebra pattern was detected by the spectropolarimeter of the Huairou station (China) in 2.6-3.8 GHz frequency range. The number of simultaneously observed emission stripes was up to several tens. Zebra pattern had the superfine temporal structure: each stripe consisted of individual short pulses similar to millisecond spikes. Data analysis revealed a good one-to-one correspondence between the spikes located in different zebra stripes, with a certain temporal delay. The frequency separation of stripes and the occurrence rate of the spikes decrease with a decrease of the emission frequency. Zebra pattern had relatively high polarization degree (up to 80%); a temporal delay between the signals with left- and right-handed circular polarization was not found.
The observed fine spectral and temporal structure of emission can be interpreted as a reflection of quasi-periodic injection of electron beams (with energy about 20-40 keV) that generate emission at the levels of double plasma resonance. The observed zebra stripes corresponded to cyclotron harmonics with numbers about 30-50. The emission was generated, most probably, at the fundamental upper-hybrid frequency. The emission source was located in the post-flare arcade of magnetic loops. The results of observations allows us to reconstruct the height distribution of magnetic field and plasma density in that coronal structure. The quasi-periodic injection of electron beams can be caused by the process of dynamic magnetic reconnection in the current sheet.