Solar type IV burst spectral fine structures

Abstract

The fine structures (FS) of solar type IV radio bursts are of principal interest for flare plasma diagnostics in the low corona. In this paper we give an observational (Part I) and theoretical (Paper II) treatment of broad band radio pulsations (BBP) and zebra patterns (ZP) in a well observed flaring sigmoidal loop system of AR 7792 on 25 October 1994. We present comprehensive meter-decimeter radio spectral (Astrophysical Institute Potsdam, AIP) and meter wave heliographic (Nancay Radio Heliograph of Paris-Meudon Observatory, NRH) observations. Spectral and spatial properties of FS elements (one pulsation pulse, one single zebra stripe) as well as a statistical analysis for the whole fine structure event are presented. The source sites are compared with soft X-ray images of the flare, and with force-free extrapolated coronal magnetic fields. Both FS sources occur in a common diverging loop structure with a turning height of about 70 Mm. The BBP source is shown to appear (if seen along the loop axis) nearer to the injection site of the electrons than the ZP source. BBP do show high frequency drift (-250 MHz s - 1 ). At a given frequency, the projected source speed is 1.1 x 10 5 km s - 1 . For ZP, we find a good correlation between the inclination of a single zebra stripe to the heliographic observing frequency level in the dynamic spectrogram, and the speed of the simultaneously observed projected source motion at this frequency. The direction of the source motion at a given frequency is on average found to be perpendicular between BBP and ZP sources. During a time interval of 90 s the BBP source consists in its lower part (higher observing frequencies) of a widely spaced double source. Despite a source distance of 360 Mm both subsources are highly correlated and thus probably simultaneously driven. We come to the conclusion that both fine structures are emitted during repeated electron beam injection into an asymmetric magnetic trap configuration between a footpoint in the leading spot of AR 7792 and a trailing more dispersed footpoint. In Paper II we will show that the specifications derived from the data allow for selecting one out of several competing fine structure models which explains the simultaneous formation of BBP and ZP in the same loop structure.