Fast drift bursts are among the best understood features of the nonthermal decimeter/meterwave emission of the solar corona. The most frequent type III bursts are induced by beams of mildly relativistic electrons accelerated in an active region and travelling along open magnetic field lines toward regions of lower densities of the background corona (cf. reviews by Goldman & Smith 1985; Suzuki & Dulk 1985; Pick & van den Oord 1990). The negatively drifting signature in the spectrogram is due to the transformation of plasma waves into electromagnetic waves near the local plasma frequency (fundamental mode, F) and/or twice this value (first harmonic mode, H) possibly leading to simultaneous emission at harmonically related frequencies. Positively drifting type III-like bursts (drifting to higher frequencies, ``reverse drift bursts") are attributed to electron beams propagating toward regions of higher densities.
Type U bursts consist of a combination of a type III burst and a reverse drift burst with a continuous turning region in the spectrum thus leading to an ``inverted U" spectral signature (Maxwell & Swarup 1958) reported from decimeter to dekameter waves (Stone & Fainberg 1971; Aschwanden et al. 1992). The incomplete species with missing descending spectral branch but an emission ``flag" after a turning point is called J bursts. When a characteristic type III-like burst follows the type U signature the whole burst is referred to as type N or U(N) burst (Caroubalos et al. 1987). If continuum emission follows a type III or U burst it is called a type V burst. Occasionally fundamental - harmonic U and J burst patterns (Haddock & Takakura 1965; Labrum & Stewart 1970) have been reported.
Type U bursts are much rarer than type III bursts. The reason is presently unknown, especially since closed field lines inferred from plasma tubes are a usual and typical morphological feature of the solar corona (e.g. Priest 1978; Bray et al. 1991). Early work on type U bursts (Fokker 1970; Labrum & Stewart 1970; Sheridan et al. 1973; Suzuki 1978) showed that the sources of the ascending and descending branch at meter wavelengths trace large scale loop structures which were seen in a few cases to overlie soft X-ray loops observed by Skylab (Stewart & Vorpahl 1977). In case studies, Aurass et al. (1994) and Pick et al. (1994) were able to separate the ascending and descending branch and to localize them on either side of soft X-ray loops seen by the Yohkoh SXT. Aschwanden et al. (1992) observed decimeter type U sources around the summit of extrapolated magnetic field lines. While these results suggest that type U bursts are due to electron beams guided along closed magnetic field structures in the corona, the expected difference between the sense of circular polarization of the ascending and descending U burst branch was only occasionally observed (Sheridan et al. 1973; Benz et al. 1977, 1979; Suzuki 1978).
A comprehensive study of type U burst observations including spectral and spatial resolution as well as polarization data with high time resolution and the comparison with images of coronal structures therefore seems appropriate. We present a sample of events in the attempt to give a full account of the status of available observations and their relation to simple models of beam propagation in the corona. It is also our aim to point out some of the difficulties with the identification of ``true'' type U signatures in spectral records. The data are presented as a catalogue given in the Appendix together with a brief description of the instrumentation. Section 2 summarizes the main content of the catalogue.