SPECTROGRAPHIC AND IMAGING OBSERVATIONS OF SOLAR-TYPE-U RADIO-BURSTS

A sample of 23 type U bursts is studied by use of spectrographic and i maging observations at decimetric and metric wavelengths (spectrograph of Potsdam - Tremsdorf Solar Radio Observatory, NanCay multifrequency radioheliograph of Paris Observatory). The radio source locations are compared with Yohkoh soft X-ray or Meudon H-alpha images. The main re sults are: 1. The sample is consistent with isotropic emission from U bursts uniformly distributed over the disk. 2. The configuration corre sponds to bursts being emitted by electron beams in loop structures: t he 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 fi eld. Underlying plasma loop structures with the same topology could in several cases be identified in Yohkoh SXT images. 3. On many occasion s complex source patterns are observed. Type III bursts and ascending type U branches that occur together within less than or equal to 1 s g ive evidence for cotemporal beam injection into structures of differen t connectivity. Repetitive brightenings near the site of the ascending U branch require repeated beam injection or the formation of counters treaming electron beams in coronal loops. 4. A majority of the events has higher starting frequency, higher flux density and a smaller sourc e in the ascending branch than in the descending branch, but examples of the opposite situation are also found. The degree of circular polar ization is generally low and difficult to measure. No systematic behav iour is recognized. 5. Simultaneous type U bursts were found in one ca se with a frequency ratio near 1.7. The location of the sources during the two lanes excludes their interpretation as a fundamental - harmon ic pair. 6. Type U bursts are found in transient large-scale structure s as well as in configurations that are stable over several hours. The repetitive occurrence of type U bursts with a homologous source patte rn (in space and time) has been detected during up to 4 hours. Such st able conditions of the beam injection and the propagation path may be in conflict with stochastic or chaotic beam injection dynamics.