Spectrographic and imaging observations of solar type U radio bursts

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A sample of 23 type U bursts is studied by use of spectrographic and imaging observations at deci- metric and metric wavelengths (spectrograph of Potsdam - Tremsdorf Solar Radio Observatory, Nan cay multifre- quency radioheliograph of Paris Observatory). The radio source locations are compared with Yohkoh soft X-ray or Meudon H images. The main results are: 1. The sample is consistent with isotropic emission from U bursts uniformly distributed over the disk. 2. The conguration 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 dierent sides of the neutral line of the photospheric magnetic eld. Underlying plasma loop structures with the same topology could in several cases be identied in Yohkoh SXT images. 3. On many occasions complex source patterns are ob- served. Type III bursts and ascending type U branches that occur together within 1 s give evidence for cotemporal beam injection into structures of dier- ent connectivity. Repetitive brightenings near the site of the ascending U branch require repeated beam in- jection 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 ascend- ing branch than in the descending branch, but exam- ples of the opposite situation are also found. The de- gree of circular polarization is generally low and dif- cult to measure. No systematic behaviour is recog- nized. 5. Simultaneous type U bursts were found in one case with a frequency ratio near 1.7. The location of the