Spectrographic and imaging observations of solar type U radio bursts

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, NanCay multifrequency radioheliograph of Paris Observatory). The radio source locations are compared with Yohkoh soft X-ray or Meudon H-alpha images. The main results are: 1. The sample is consistent with isotropic emission from U bursts uniformly distributed over the disk. 2. 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. 3. On many occasions complex source patterns are observed. Type III bursts and ascending type U branches that occur together within less than or equal to 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. 4. 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. 5. 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. 6. 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.