Issue |
A&A
Volume 410, Number 3, November II 2003
|
|
---|---|---|
Page(s) | 1001 - 1010 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20031249 | |
Published online | 17 November 2003 |
Solar type IV burst spectral fine structures
I. Observations
1
Astrophysical Institute Potsdam, 14482 Potsdam, Germany
2
Paris-Meudon Observatory, LESIA, CNRS-UMR 8109, 92195 Meudon, France
3
Institute of Applied Physics RAS, 603600 Nizhny Novgorod, Russia
Corresponding author: H. Aurass, haurass@aip.de
Received:
2
June
2003
Accepted:
28
July
2003
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 (Nançay 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 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.
Key words: Sun: flares / Sun: corona / Sun: radio radiation / Sun: magnetic fields
© ESO, 2003
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