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New Features in Type IV Solar Radio Emission: Combined Effects of Plasma Wave Resonances and MHD Waves

An intense and complex type IV solar radio burst over the time period 1992 0217 08-12 was recorded simultane- ously by 3 spectrographs, ARTEMIS (100-500 MHz), OSRA (200-400 MHz) and IZMIRAN (180-270 MHz), and by the Nancay radioheliograph. For � 2 hours, the event exhibited strong pulsations on various time scales and "zebra patterns" with new features: sudden frequency shifts of the whole pat- tern, splitting of individual zebra stripes into two stripes, struc- turation of the upper-frequency split stripes into emission dots in phase with � 0.2 s pulsations. Another new and spectacular feature was a � 10 MHz bandwidth emission at the high fre- quency cut-off of the whole event, oscillating between 350 and 450 MHz in phase with� 3 min pulsations, and itself structured by the � 0.2 s pulsations. Another property observed for the first time was that the circular polarization of zebra patterns changed sign during the event, possibly due to magnetic field reversal at some point of a long-lasting magnetic reconnection process in the upper corona. According to a classical picture, electrons are accelerated in the current sheet and trapped in the magnetic arch. Pulsations are due to MHD waves affecting the whole arch and electron beams as well. We confront two existing theories to these new observational features. The l+w ) t model (Chernov 1976, 1989), based on Langmuir wave - whistler coupling at normal and anomalous Doppler resonance, can account for all the observed fine struc- tures of zebra patterns, and gives a plausible magnetic field of 11 × 10 4 T in the source. The Winglee & Dulk (1986) model, based on electron-cyclotron maser emission of upper-hybrid waves at double plasma resonance, seems the most adequate to account for the evolving emission line, with its source in the dense current sheet.