A fresh look at the heating mechanisms of the solar corona

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Recently, using particle-in-cell simulations, i.e. in the kinetic plasma description, Tsiklauri et al and Génot et al reported on a discovery of a new mechanism of parallel electric field generation which results in electron acceleration. In this work we show that the parallel (to the uniform unperturbed magnetic field) electric field generation can be obtained in a much simpler framework using an ideal magnetohydrodynamic (MHD) description, i.e. without resorting to complicated wave–particle interaction effects such as ion polarization drift and resulting space charge separation which seems to be an ultimate cause of the electron acceleration. In the ideal MHD the parallel (to the uniform unperturbed magnetic field) electric field appears due to fast magnetosonic waves which are generated by the interaction of weakly nonlinear Alfvén waves with transverse density inhomogeneity. Further, in the context of the coronal heating problem a new two-stage mechanism of the plasma heating is presented by putting emphasis firstly on the generation of parallel electric fields within ideal MHD description directly, rather than focusing on the enhanced dissipation mechanisms of the Alfvén waves and, secondly, dissipation of these parallel electric fields via kinetic effects. It is shown that for a single Alfvén wave harmonic with frequency ν = 7 Hz (which has longitudinal wavelength λA = 0.63 Mm for a putative Alfvén speed of 4328 km s−1), the generated parallel electric field could account for the 10% of the necessary coronal heating requirement. We conjecture that wide spectrum (10−4–103 Hz) Alfvén waves, based on an observationally constrained spectrum, could provide necessary coronal heating requirement. It is also shown that the amplitude of generated parallel electric field exceeds the Dreicer electric field by about four orders of magnitude, which implies realization of the runaway regime with the associated electron acceleration.