On the origin of microwave type U-bursts

An analysis is made of the observations of U-type cm-bursts recorded simultaneously with high spatial (Siberian solar radio telescope) and spectral resolution (National Astronomical Observatories/Beijing spectropolarimeters). It is shown that the source positions of increasing and decreasing branches of the U-burst coincide within a few arcsec. The suggestion is made that the occurrence of the observed type U-bursts at microwave frequencies is associated with a pressure and density response to rapid heating pulses in flare loops.