NARROWBAND dm-SPIKES OBSERVED DURING THE 15 JUNE 1991 FLARE

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Abstract

We performed a statistical analysis of the dm-spikes that were present for an exceptionally long period (more than 10 min) during the 15 June 1991 flare. We realized that the polarization degree, the duration and the mutual delay of the R- and L-components were nearly the same for both CW and ACW cases. CW (ACW) means the clockwise (anti-clockwise) sense of the loop in the R−L versus L+R plots, when the data of a single spike are considered according to the time sequence. The presence of such a loop is determined by the delay of the weaker (stronger) polarimetric component in respect to the other one. The increase of the polarization percentage started first at 610, then at 408 and afterwards at 327 MHz. It was found that the duration of spikes was almost completely independent from the polarization degree. The mean duration of spikes at different frequencies corresponds to that computed using the formula of Güdel and Benz. Like the mean duration, also the mean delay decreased with increasing frequency. The CW/ACW ratio varied simultaneously for the frequencies we recorded during an interval of about three minutes. Spikes were considered as the radio manifestation of superthermal electrons accelerated in the MHD cascading waves. The evolution of spikes and their parameters were qualitatively interpreted within the flare reconnection model with turbulent plasma outflows.

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