V.F. Melnikov^1,2 and A.V.R. Silva ²
1 Radiophysical Research Institute, B.Pecherskaya St. 25, 603600, Nizhnii Novgorod, Russia

2 CRAAE, Mackenzie, R. da Consolação, 896, 01302-000, São, Paulo, Brazil

In this work the relation and temporal evolution of spectral indices of relatively weak impulsive HXR- and microwave bursts are investigated. The study has been done using the data obtained in 1991-1994 with BATSE and OVRO. The frequency range covered by OVRO is 1-18 GHz. Hard X-ray emission above the background for most of the events ranges from 20 to 200 keV. An interesting new result is that most of the events presents continuous microwave spectral flattening (in the optically thin region (at f>f_p)) simultaneously with softening of HXR-spectrum during the decay phase. In the case of hardening or no changes in the HXR-spectrum the microwave spectrum flattens. On the other hand, almost all events which display a steepening of the microwave spectrum also display a softening of the HXR-spectrum. It is shown that in the majority of the events during the rise phase and at the time of maximum of the bursts the difference  = _x-_µ between electron spectral indices derived from HXR-spectra and microwave spectra is in the range 0.5-1.5. However, in some cases  can reach 2-3. In most of the events  increases in the decay phase, reaching in some events the value of 4-6. We show that the differences between _x and _µ  and their temporal change during the decay phase can be naturally explained mainly by the dynamics of the energy spectrum of trapped electrons due to Coulomb collisions and wave-particle interactions. We simulated the microwave and HXR spectrum evolution assuming that the electron injection spectrum hardens during the rise phase and softens during the decay phase (`soft-hard-soft' behavior). Simulations show that when the spectrum of accelerated electrons during the decay phase softens at all energies, the spectrum of trapped electrons softens only at low energies. At high energies the trapped electron spectrum hardens with time causing the continuous spectral flattening of microwave emission at high frequencies (in optically thin region). The value of the `break-up' energy depends on the injection duration and conditions in a flare loop.