Issue |
Astron. Astrophys. Suppl. Ser.
Volume 123, Number 2, June I 1997
|
|
---|---|---|
Page(s) | 279 - 304 | |
DOI | https://doi.org/10.1051/aas:1997161 | |
Published online | 15 June 1997 |
Spectrographic and imaging observations of solar type U radio bursts
1
Astrophysikalisches Institut Potsdam, Observatorium für solare Radioastronomie, Telegrafenberg A31, D-14473 Potsdam, Germany e-mail: haurass@aip.de
2
DASOP, CNRS-URA 2080, Observatoire de Paris, Section de Meudon, F-92195 Meudon, France e-mail: klein@obspm.fr
Send offprint request to: H. Aurass
Received:
4
March
1996
Accepted:
18
September
1996
A sample of 23 type U bursts is studied by use of spectrographic and imaging
observations at decimetric and metric wavelengths (spectrograph of Potsdam
- Tremsdorf Solar Radio Observatory, Nançay multifrequency
radioheliograph of Paris Observatory). The radio source locations are
compared with Yohkoh soft X-ray or Meudon H images. The main results
are:
The sample is consistent with isotropic emission from U bursts uniformly
distributed over the disk.
The configuration corresponds to bursts being emitted by electron
beams in loop structures: the rising and the descending
branches have spatially separated sources. Under appropriate observing
conditions they
can be shown to project to different sides of the neutral line of the
photospheric magnetic field. Underlying plasma loop structures with the same
topology could in several cases be identified in Yohkoh SXT images.
On many occasions complex source patterns are observed. Type III bursts
and ascending type U branches that occur together within ≤1 s give
evidence for cotemporal beam injection into structures of different
connectivity. Repetitive brightenings near the site of the ascending U branch
require repeated beam injection or the formation of counterstreaming electron
beams in coronal loops.
A majority of the events has higher starting frequency, higher flux
density and a smaller source in the ascending branch than in the descending
branch, but examples of the opposite situation are also found. The degree of
circular polarization is generally low and difficult to measure. No systematic
behaviour is recognized.
Simultaneous type U bursts were found in one case with a frequency ratio
near 1.7. The location of the sources during the two lanes excludes
their interpretation as a fundamental - harmonic pair.
Type U bursts are found in transient large-scale structures as well as
in configurations that are stable over several hours. The repetitive
occurrence of type U bursts with a homologous source pattern (in space and
time) has been detected during up to 4 hours. Such stable conditions of the
beam injection and the propagation path may be in conflict with stochastic or
chaotic beam injection dynamics.
Key words: Sun: activity / Sun: corona / Sun: flares / Sun: magnetic field / Sun: particle emission / Sun: radio radiation
© European Southern Observatory (ESO), 1997