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The GOES X1.5 class flare that occurred on August 30,2002 at 1327:30 UT isone of the few events detected so far at submillimeter wavelengths. We presenta detailed analysis of this flare combining radio observations from 1.5 to 212GHz (an upper limit of the flux is also provided at 405 GHz) and X-ray.Although the observations of radio emission up to 212 GHz indicates thatrelativistic electrons with energies of a few MeV were accelerated, nosignificant hard X-ray emission was detected by RHESSI above ~ 250 keV. Imagesat 12--20 and 50--100 keV reveal a very compact, but resolved, source of about~ 10" x 10". EUV TRACE images show a multi-kernel structure suggesting acomplex (multipolar) magnetic topology. During the peak time the radio spectrumshows an extended flatness from ~ 7 to 35 GHz. Modeling the optically thin partof the radio spectrum as gyrosynchrotron emission we obtained the electronspectrum (spectral index delta, instantaneous number of emitting electrons). Itis shown that in order to keep the expected X-ray emission from the sameemitting electrons below the RHESSI background at 250 keV, a magnetic fieldabove 500 G is necessary. On the other hand, the electron spectrum deduced fromradio observations >= 50 GHz is harder than that deduced from ~ 70 - 250 keVX-ray data, meaning that there must exist a breaking energy around a fewhundred keV. During the decay of the impulsive phase, a hardening of the X-rayspectrum is observed which is interpreted as a hardening of the electrondistribution spectrum produced by the diffusion due to Coulomb collisions ofthe trapped electrons in a medium with an electron density of n_e ~ 3E10 - 5E10cm-3.

Submillimeter and X-ray observations of an X Class flare