DEKA-KEV X-RAY-EMISSION ASSOCIATED WITH THE ONSET OF RADIO NOISE STORMS

Radio noise storms show that suprathermal electrons (a few tens of keV ) are present in the vicinity of active regions during several hours o r even a few days. Where and how these electrons are energized is not yet well known. A flare-like sudden energy release in the active regio n is in general observed at the onset of noise storms, either as a ful ly developed flare or, more often, as a soft X-ray brightening without conspicuous Her signature. In order to investigate to what extent ele ctrons energized in the active region contribute to the noise-storm em ission in the overlying coronal structures, we combine radio imaging ( Nancay radioheliograph) with X-ray spectral observations at photon ene rgies of a few keV (GOES) and - for the first time - around 10 keV (WA TCH/GRANAT). In two of four studied events the WATCH data show a signi ficant excess of the deka-keV count rate above the expectation from an isothermal fit to the GOES fluxes. Although the electron population p roducing the deka-keV X-ray emission would be energetic enough to powe r the simultaneous radio noise storm, the much longer duration of the radio emission requires time-extended particle acceleration. The accel eration probably occurs in the corona overlying the X-ray emitting reg ion, triggered by the processes which give rise to the X-ray brighteni ngs.