Effect of superthermal particles on the quiet Sun radio emission

The bremsstrahlung emissivity and absorption coefficient in the radiofrequency range are derived under the assumption that the electron population is not purely thermal, but presents a tail of high energy particles. This population is approximated by a two-component Maxwellian distribution and by the kappa-functions of different (integer) index. It is shown that, if the temperature ratio of the two Maxwellians is larger than 10, the absorption coefficient and the effective temperature (the quantities entering the radio transfer equation) depend only on the fraction R of particles in the highest temperature Maxwellian. In the case of kappa-functions the above quantities depend on the index n of the functions.
The microwave radio spectrum is computed for different values of R and for , finding, in all cases, brightness temperatures lower than those computed with a pure thermal distribution. This could explain some inconsistencies found between radio and EUV observations.