Abstract
Context. Impulsive radio and hard X-ray emission from large solar flares are usually attributed to a hard distribution of high-energy electrons accelerated in the energy dissipation process of magnetic reconnection. Aims. We report the detection of impulsive radio and hard X-ray emissions produced by a population of energetic electrons with a very soft distribution in an M-class flare: SOL2015-08-27T05:45 . Methods. The absence of impulsive emission at 34 GHz and hard X-ray emission above 50 keV and the presence of distinct impulsive emission at 17 GHz and lower frequencies and in the 25–50 keV X-ray band imply a very soft distribution of energetic electrons producing the impulsive radio emission via the gyro-synchrotron process, and impulsive X-rays via bremsstrahlung. Results. The spectrum of the impulsive hard X-ray emission can be fitted equally well with a power-law model with an index of ∼6.5 or a super-hot thermal model with a temperature as high as 100 MK. Imaging observations in the extreme-UV and X-ray bands and extrapolation of the magnetic field structure using a nonlinear force-free model show that energetic electrons trapped in coronal loops are responsible for these impulsive emissions. Conclusions. Since the index of the power-law model is nearly constant during the impulsive phase, the power-law distribution or the super-hot component should be produced by a bulk energization process such as the Fermi and betatron acceleration of collapsing magnetic loops.
Highlights
Solar flares are triggered by magnetic reconnection on small kinetic scales, where magneto-hydrodynamic (MHD) description of magnetized plasmas is not appropriate
Such smallscale processes can lead to change in the topology of largescale magnetic field structures whose evolution can be accurately modelled with MHD and may dominate the release of magnetic energy stored in nonpotential fields (Musset et al 2015)
It has been recognized that the released magnetic energy can be converted into the kinetic energy of high-energy particles and/or hot plasmas via a variety of processes that are closely coupled to the magnetic field structure (Somov & Kosugi 1997) and can be probed via observations of radiative characteristics of these highenergy particles
Summary
Solar flares are triggered by magnetic reconnection on small kinetic scales, where magneto-hydrodynamic (MHD) description of magnetized plasmas is not appropriate. Since the adiabatic approximation is assumed, the injected particles can be energized at most by the factor of the transverse contraction (Bogachev & Somov 2005) It has difficulties in accelerating particles to very high energies. Radio and X-ray spectral analyses show that the impulsive emission can be produced by high-energy electrons with either a very soft power-law distribution or a super-hot thermal distribution. 5 to confirm the association of impulsive emission with high-energy electrons trapped in flare loops. We discuss these results and draw conclusions in Sect.
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