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

Solar flares are accompanied by an enhanced emission of electromagnetic waves from the radio up to the gamma-ray range. The associated hard X-ray (HXR) and microwave radiation is generated by energetic electrons, which 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 on September 10, 2017 offers a unique opportunity to study this model. The observations from the Expanded Owens Valley Solar Array (EOVSA) show that 1.6x10^4 electrons with energies >300 keV were generated in the flare region. There are signatures in solar radio and extreme ultraviolet 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 HXR sources. In contrast to previous studies, we investigate whether the heating of the plasma at the TS provides enough relativistic electrons needed for the HXR and microwave emission observed during the X8.2 solar flare on September 10, 2017. 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 that event. The part of relativistic electrons was calculated in the heated downstream region. 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 needed for the microwave and HXR emission observed during that event. The generation of relativistic electrons at the TS is a possible mechanism to explain the enhanced microwave and HXR radiation emitted during flares.
Comment: Accepted for publication in A&A