Due to the propagation of the emitter along magnetic field lines, the density dependence of their emission frequency and the velocity dependence of their spectral shape type U bursts provide unique diagnostics of coronal plasma - magnetic field structures and their dynamic behaviour, provided sensitive spectral and imaging observations with subsecond time resolution are available. Depending on their frequency, the bursts trace loops seen in soft X-rays (decimetric U bursts) or at greater altitude (metric bursts), where the only other diagnostic is coronographic observation at the limb.
Acknowledgements
The authors are highly obliged to the YOHKOH community for its open data policy, and to R.D. Bentley (Mullard Space Science Laboratory, U.K.) and his group from the YOHKOH Data Archive Center for support in software and data transfer. They are grateful for the generous supply of GOES soft X-ray observations by the NASA/GSFC Solar Data Analysis Center. We thank our colleagues running the daily observations at Potsdam - Tremsdorf, Nançay and Meudon. Discussions with M. Karlicky, A. Klassen, G. Mann, P.C.H. Martens, M. Pick, A. Raoult, A. Tlamicha, G. Trottet, A.V. Stepanov, L. van Driel-Gesztely, P. Zlobec and Y. Zlotnik are acknowledged, as well as M. Aschwanden's constructive criticism of the manuscript. We thank H. Detlefs and P. Hackenberg for their help in solving software problems, and D. Scholz and G. Servajean for preparing illustrations. The Nançay Radio Observatory is funded by the French Ministry of Education, the CNRS and the Région Centre. This paper became possible due to travel grants 312/pro-bmft-gg (DAAD) and 94053 (MAE) within the German-French PROCOPE programme.