First solar radar observations in microwaves

The first microwave radar experiment probing solar radio emission source regions is presented. The 300-m dish telescope at Arecibo was used to scatter 2380-MHz pulsed signals on coronal Langmuir waves with frequencies from 170 to 270 MHz in source regions of type I emission, and the radio signal produced by the nonlinear interaction of the two wave modes was monitored at 2600 MHz at a bandwidth of 100 MHz. The experiment has performed as expected, with a threshold sensitivity on the order of 1 Jy, however no echo has yet been detected. An upper limit to the Langmuir wave energy density of 0.0005 nKT is thus obtained from the wave-wave interaction coefficient for a source temperature of 5,000,000 K, size of 10 to the 28th cu cm and an isotropic and flat Langmuir wave spectrum.