Microwave and Hard X-Ray Flare Observations by NoRH/NoRP and RHESSI: Peak-flux Correlations

This paper presents initial results from a statistical study of solar microwave and hard X-ray flares jointly observed over the past two solar cycles by the Nobeyama Radio Polarimeters, the Nobeyama Radio Heliograph, and the Reuven Ramaty High Energy Solar Spectroscopic Imager. As has been previously demonstrated, the microwave (17 GHz and 34 GHz) peak flux shows a linear correlation with the nonthermal hard X-ray bremsstrahlung peak emission seen above 50 keV. The correlation holds for the entire rise phase of each individual burst, while the decay phases tend to show more extended emission at microwaves than is generally attributed to particle trapping. While the correlation is highly significant (coefficient of 0.92) and holds over more than four orders of magnitude, individual flares can be above or below the fitted line by an average factor of about 2. By restricting the flare selection to source morphologies with the radio emission from the top of the flare loop, the correlation tightens significantly, with a correlation coefficient increasing to 0.99 and the scatter reduced to a factor of 1.3. These findings corroborate the assumption that gyrosynchrotron microwave and hard X-ray bremsstrahlung emissions are produced by the same flare-accelerated electron population. The extent of the linear correlation over four orders of magnitude suggests that magnetic field strengths within nonthermal 17 GHz sources are surprisingly similar over a wide range of flare sizes.