The opposite evolution of spectral index of microwave and HXR emission in solar flare

This paper presents an unusual phenomenon for the evolution of spectral index of non-thermal electrons related to HXR and microwave emission in a M1.5 class flare. It is found that the light curves of both microwave and HXR bursts has similar two rise-peak-decay phases. The spectral index deduced from HXR emission evolves as S-H-S pattern in each phase, while the spectral index relative to microwave emission presents S-H-H pattern in the first rise-peak-decay phase and H-S-H pattern in second one. Weak diffusion is proposed to explain the different evolution in the first rise-peak-decay phase. For the second period, microwave spectrum is softened, which imply one new process for pitch angle scattering occur. Strong diffusion of trapped electrons could be the available cause, which result in the decreasing number of trapped electrons. In additional, the change of magnetic structure of flare loop may shift the location of magnetic mirrors and the upward shifting of magnetic mirrors could reduce the number of microwave emission at 17 GHz and soften the microwave spectrum.