engleski

On the magnetic field evolution during a radio pulsation event

We reanalyzed radio observations, magnetograms, H&alpha data, and Yohkoh soft X-ray images of NOAA AR 7792 for the 25 October 1994 flare event. The active region is characterized by sigmoid shaped soft X-ray loop systems. Force-free extrapolated magnetic field line maps have been calculated for different height ranges of the active region corona. Our aim was to identify those parts of the field line maps which become activated during the flare. We find evidence for at least three interacting acceleration sites of nonthermal electrons. Two of them are situated within a sheared field arcade which forms two expanding ribbons in flaring. A third acceleration site is associated with an area of emerging magnetic flux of parasitic polarity near the main sunspot of the active region. At this site, soft X-ray jet activity was seen already days before the flare. A confined flare topology is identified there in heights below 10000 km. The arcade flare and the confined flare regions get more and more interacting due to the arcade flare ribbons expansion. In this interaction stage a reconnection process in the confined flare structure seems to become more and more driven. This leads to a quasiperiodic injection of streams of nonthermal electrons observed as a decimetric - metric radio pulsation event. The analysis illuminates the association between the evolution of the soft X-ray loop morphology and the dynamics of nonthermal electron acceleration during this event.

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