Diagnostics of flare-accelerated electron beams with X-ray and Radio data from Solar Orbiter

Energetic electrons accelerated by solar flares in the corona may propagate downward, produce X-rays in the chromosphere, and upward, producing coherent type III radio bursts in interplanetary space.  Previous statistical studies of radio and X-ray flare observations have found a good temporal link between the two wavelengths but only a weak correlation between the intensities, in part due to the different emission mechanisms.  Assuming both electron populations share properties from a common acceleration region, theory has predicted a link between the speed of the electron beams travelling outwards (deduced from radio) and the energy density of the electrons travelling downwards (deduced from X-rays). The Solar Orbiter mission is equipped with the STIX and RPW instruments, allowing for simultaneous observations of both X-ray and Radio emissions that can test this theory.   We present results derived from the comparison of 38 radio type III bursts detected by RPW (<10 MHz) associated in time with flares observed by STIX in the 4-150 keV range . From X-ray spectroscopy we obtained the electron spectral index and the electron number of the associated HXR peak, from which the power can be estimated. We derived the Type III exciter speed using the rise and peak times of the time-profiles (V_r and V_p , respectively) in the 0.4-4 MHz range.  We find the observed ratio V_r/V_p is 0.78 +- 0.07, complementing previous similar studies at higher frequencies (30 – 70 MHz) with a ratio of 0.8+-0.06. We report a correlation between the power of all electrons with energies above 30 keV and V_r  (cc=0.47), whilst none is obtained when comparing it with V_p. There is an anticorrelation of the velocities with the electron spectral index as expected, however the anticorrelation coefficients are weak. Relevant correlations are seen when comparing the peak Radio intensity with the electron spectral index (cc=-0.81) and power (E>30keV) (cc=0.59). The energy of the escaping electrons producing the type III radio emission and the ones producing non-thermal HXRs are also compared, showing a significant correlation (cc=0.57). Our results show a clear relation between the most energetic electrons in both populations of beams, supporting the scenario of a common acceleration region. The energy distribution of escaping and confined electrons for some events may depend on other parameters like the geometry of the reconnecting magnetic field.