Diagnostic Analysis of the Solar Proton Flares of September 2017 by Their Radio Bursts

The powerful solar flares that occurred on September 4-10, 2017 are analyzed based on a quantitative diagnostics method for proton flares developed at the Institute of Terrestrial Magnetism, the Ionosphere and Radio-Wave Propagation (IZMIRAN) in the 1970-1980s. We show that the fluxes and energy spectra of the protons reached the Earth with the energies of tens of MeV qualitatively and quantitatively correspond to the intensity and frequency spectra of the microwave radio bursts in the range of 2.7-15.4 GHz. Specifically, the flare of September 4 with a peak radio flux S similar to 2000 sfu at the frequency f similar to 3 GHz (i.e., with the soft radio spectrum) was accompanied by a significant proton flux J (> 10 MeV) similar to 100 pfu and a soft energy spectrum with the index gamma similar to 3.0, while the strong flare on September 10 with S similar to 21000 sfu at f similar to 15 GHz (i.e., with the hard radio spectrum) led to a very intense proton event with J (> 10 MeV) similar to 1000 pfu with a hard spectrum (gamma similar to 1.4), including the ground level enhancement (GLE72). This is further evidence that microwave radio data can be successfully used in diagnostics of proton flares independently of a specific source of particle acceleration at the Sun, in particular, with the IZMIRAN method.