Electron trapping and acceleration by kinetic Alfvén waves in solar flares

Abstract

Context. Theoretical models and spacecraft observations of solar flares highlight the role of wave-particle interaction for non-local electron acceleration. In one scenario, the acceleration of a large electron population up to high energies is due to the transport of electromagnetic energy from the loop-top region down to the footpoints, which is then followed by the energy being released in dense plasma in the lower atmosphere. Aims. We consider one particular mechanism of non-linear electron acceleration by kinetic Alfvén waves. Here, waves are generated by plasma flows in the energy release region near the loop top. We estimate the efficiency of this mechanism and the energies of accelerated electrons. Methods. We use analytical estimates and test-particle modelling to investigate the effects of electron trapping and acceleration by kinetic Alfvén waves in the inhomogeneous plasma of the solar corona. Results. We demonstrate that, for realistic wave amplitudes, electrons can be accelerated up to 10−1000 keV during their propagation along magnetic field lines. Here the electric field that is parallel to the direction of the background magnetic field is about 10 to 10 3 times the amplitude of the Dreicer electric field. The acceleration mechanism strongly depends on electron scattering which is due to collisions that only take place near the loop footpoints. Conclusions. The non-linear wave-particle interaction can play an important role in the generation of relativistic electrons within flare loops. Electron trapping and coherent acceleration by kinetic Alfvén waves represent the energy cascade from large-scale plasma flows that originate at the loop-top region down to the electron scale. The non-diffusive character of the non-linear electron acceleration may be responsible for the fast generation of high-energy particles.

Comparative study of electric currents and energetic particle fluxes in a solar flare and Earth magnetospheric substorm

“…The most relevant to the solar flares, the filamentation of field-aligned currents due to transformation of Alfven waves (carrying such currents) to kinetic or inertial Alfven waves (Lysak 2004;Lysak et al 2013;Chaston et al 2014). Such transformation is quite important, because it opens a door for collisionless dissipation of currents by thermal electrons accelerated by field-aligned electric fields of kinetic/inertial Alfven waves (see discussion of these processes in application to the solar corona in, e.g., Fletcher & Hudson 2008;Haerendel 2012;Artemyev et al 2016). Therefore, some elements of kinetic physics from substorm observations (e.g., efficiency of electron acceleration/field-aligned current damping, see Damiano et al (2016); Sharma Pyakurel et al ( 2018)) may be implemented into solar flare models.…”

Solar Plasma Radio Emission in the Presence of Imbalanced Turbulence of Kinetic-Scale Alfvén Waves

“…Other possible sources for the imbalanced KAW turbulence in the solar corona include phase mixing (Voitenko and Goossens, 2000) and magnetic reconnection (Voitenko, 1998). In solar flares these KAWs can lead to impulsive plasma heating (Voitenko, 1998) and non-local electron acceleration to relativistic energies (Artemyev, Zimovets, and Rankin, 2016). Contrary to the balanced IAW turbulence, which is symmetric with respect to k → −k , the wave amplitudes in the imbalanced IAW turbulence are much smaller at k < 0 than at k > 0.…”

“…Other possible sources for the imbalanced KAW turbulence in the solar corona include phase mixing (Voitenko and Goossens, 2000) and magnetic reconnection (Voitenko, 1998). In solar flares these KAWs can lead to impulsive plasma heating (Voitenko, 1998) and non-local electron acceleration to relativistic energies (Artemyev, Zimovets, and Rankin, 2016).…”

“…Note that DAWs can resonantly exchange energy with particles in the velocity range between the Alfvén velocity V A and the electron thermal velocity V T e . As in the solar atmosphere usually V A < V T e , and DAWs are usually not efficient agents for resonant acceleration of coronal electrons to suprathermal energies (see however Artemyev, Zimovets, and Rankin, 2016). The favourable conditions for the suprathermal electron acceleration are in the regions with strong magnetic fields and/or low temperatures, where the electron thermal speed drops under the Alfvén speed, V A > V T e .…”

Numerical and Analytical Study of Electron Plasma Waves in Nonthermal Vasyliunas‐Cairns Distributed Plasmas

“…The observed nonthermal distributions of energetic particles in the tail are best modeled through power‐law‐like distributions. These power‐law‐like distributions are observed in the space plasma environments like galactic cosmic rays, plasmas in the super‐thermal radiation fields, the solar wind, the solar flares, the solar corona, also present around and inside the Earth's foreshock and bowshock, ionosphere and around the moon (Achterberg & Norman, 1980; Asbridge et al., 1968; Bame et al., 1967; Feldman et al., 1983; Hansen & Emslie, 1988; Hasegawa et al., 1985; Jones & Ellison, 1991; Lundin et al., 1989). The presence of such distributions is also extensively validated by the measurements of spacecraft on different space plasma missions (Feldman et al., 1975; Maksimovic et al., 1997; Montgomery et al., 1968; Pierrard & Lazar, 2010; Pilipp et al., 1987; Zouganelis, 2008).…”