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 simultaneously by 3 spectrographs, ARTEMI S (100-500 MHz), OSRA (200-400 MHz) and IZMIRAN (180-270 MHz), and by the Nancay radioheliograph. For approximate to 2 hours, the event exhi bited strong pulsations on various time scales and ''zebra patterns'' with new features: sudden frequency shifts of the whole pattern, split ting of individual zebra stripes into two stripes, structuration of th e upper-frequency split stripes into emission dots in phase with appro ximate to 0.2 s pulsations. Another new and spectacular feature was a approximate to 10 MHz bandwidth emission at the high frequency cut-off of the whole event, oscillating between 350 and 450 MHz in phase with approximate to 3 min pulsations, and itself structured by the approxi mate to 0.2 s pulsations. Another property observed for the first time was that the circular polarization of zebra patterns changed sign dur ing the event, possibly due to magnetic field reversal at some point o f a long-lasting magnetic reconnection process in the upper corona. Ac cording to a classical picture, electrons are accelerated in the curre nt sheet and trapped in the magnetic arch. Pulsations are due to MHD w aves affecting the whole arch and electron beams as well. We confront two existing theories to these new observational features. The l + w d ouble right arrow t model (Chernov 1976, 1989), based on Langmuir wave - whistler coupling at normal and anomalous Doppler resonance, can ac count for all the observed fine structures of zebra patterns, and give s a plausible magnetic field of 11 x 10(-4) T in the source. The Wingl ee & 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 den se current sheet.