Characteristics of big solar flares observed with RHESSI and Nobeyama Radioheliograph

During the impulsive phase of a solar flare, a large amount of electrons is accelerated. The most direct diagnostics of electron acceleration are provided through radio and hard X-ray observations where we observe synchrotron emissions in the GHz range and non-thermal bremsstrahlung emissions above typically 10 keV. The best pair to observe these two emissions are the Nobeyama Radioheliograph (NoRH) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). We present a statistical study of 35 jointly observed big (>GOES M7-class) flares. Initial results reveal a linear good correlation between the hard X-ray flux above 50 keV and the microwave fluxes at 17 and 34 GHz. This result indicates that the population of accelerated electrons emit both of hard X-rays and microwaves. However, this correlation is too good, considering the difference of the emission mechanisms. Since gyro-synchrotron emission is very sensitive to magnetic field strength, the microwave becomes intense if the source is located at a strong magnetic field region like a footpoint region while hard X-ray intensity does not depend on magnetic field strength. Our event list includes two types of flares; one is a flare with a microwave source around the looptop and the other is a flare with a microwave source at footpoint or leg of the flare loop. However, still there is a good correlation. Thus, at first, we investigated the ratio between hard X–ray and microwave peak flux for these two groups. The results showed that there is only a weak dependence between these two groups. To further understand this situation, we need event studies with other observations such as Hinode/XRT, SDO/AIA and so on. Some preliminary results of event studies are also presented. The RHESSI operation was finished in 2018 and NoRH operation will be finished in 2020. In such a situation, this kind of studies will become one of scientific targets for a new solar radio telescope, MUSER in China.