Generation of relativistic electrons at the termination shock in the solar flare region

Abstract

Context. Solar flares are accompanied by an enhanced emission of electromagnetic waves from the radio up to the γ-ray range. The associated hard X-ray and microwave radiation is generated by energetic electrons. These electrons play an important role, since they carry a substantial part of the energy released during a flare. The flare is generally understood as a manifestation of magnetic reconnection in the corona. The so-called standard CSHKP model is one of the most widely accepted models for eruptive flares. The solar flare event on September 10, 2017 offers us a unique opportunity to study this model. The observations from the Expanded Owens Valley Solar Array (EOVSA) show that ≈1.6 × 104 electrons with energies > 300 keV are generated in the flare region. Aims. There are signatures in solar radio and extreme ultraviolet (EUV) observations as well as numerical simulations that a “termination shock” (TS) appears in the magnetic reconnection outflow region. Electrons accelerated at the TS can be considered to generate the loop-top hard X-ray sources. In contrast to previous studies, we investigate whether the heating of the plasma at the TS provides enough relativistic electrons needed for the hard X-ray and microwave emission observed during the solar X8.2 flare on September 10, 2017. Methods. We studied the heating of the plasma at the TS by evaluating the jump in the temperature across the shock by means of the Rankine–Hugoniot relationships under coronal circumstances measured during the event on September 10, 2017. The part of relativistic electrons was calculated in the heated downstream region. Results. In the magnetic reconnection outflow region, the plasma is strongly heated at the TS. Thus, there are enough energetic electrons in the tail of the electron distribution function (EDF) needed for the microwave and hard X-ray emission observed during the event on September 10, 2017. Conclusions. The generation of relativistic electrons at the TS is a possible mechanism of explaining the enhanced microwave and hard X-ray radiation emitted during flares.