Previous
abstract 
Next abstract
We have compared the soft X-ray/microwave ratio of solar and stellar flares with the ratio of the corresponding `quiescent' emissions of active M and K stars and other active stars. Solar flare X-ray observations by the GOES satellite have been converted into total luminosities (erg/s) using the inferred temperature and emission measure, and standard X-ray model spectra. Microwave luminosities (erg/sHz) near the spectral peak of gyrosynchrotron emission (5-10 GHz) have been selected. The average ratio is 10$^{15.9\pm 0.2}$ Hz for impulsive and gradual flares, and slightly more for microflares. Highly polarized stellar flare microwave emission is probably of different origin and cannot be compared. The only simultaneous observation of stellar flare X-rays and unpolarized microwaves in the literature has a luminosity ratio of 10$^{15.5}$ Hz.
The average ratio between `quiescent' X-ray and microwave luminosities of young, rapidly rotating M and K stars has previously been reported to be 10$^{15.5}$ Hz. It is only slightly smaller for Algols, RS CVn binaries and post T Tauri stars. The observation of comparable ratios between thermal X-rays and gyrosynchrotron emission in the `quiescent' active coronae and solar/stellar flares suggests that the coronal heating mechanism and the flare energy release are similar physical processes. In particular, the heating process of active stellar coronae seems to be associated by acceleration of electrons.
This research is supported by the Swiss National Science Foundation, NASA, the University of Colorado, and NIST.
