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Magnetic-field-aligned Electric Fields and Nonresonant Acceleration in Alfvénic Turbulence

Backward-propagating or reverse fluctuations in Alfvénic turbulence are shown to produce magnetic-field-aligned (MFA) electric fields capable of highly intermittent acceleration of particles along the local mean magnetic field. Probability distribution functions (PDFs) for the angles between magnetic and electric local mean fields in the plane perpendicular to the background magnetic field are calculated both analytically and through Monte Carlo simulations as functions of the fraction of reverse fluctuations. The PDFs peak at but quickly broaden as increases, up to the limit of a uniform PDF for or zero cross-helicity. Energy from a mixture of forward- and backward-propagating Alfvén waves can easily be transferred to the plasma, through the intermittent MFA electric fields, on a timescale much shorter than the Kolmogorov timescale for turbulence cascade. In such a mixture, for typical 1 au solar wind turbulence parameters, nonresonant interaction through the MFA electric fields rather than gyroresonance controls the energy exchanges between turbulent fields and particles. Possible consequences of the nonresonant interaction through the MFA fields are further suggested, from the observed fast variations of solar wind speed and resulting spectral flattening above 10−2 Hz, and the turbulence level variability/intermittency near 1 au, to the powering of chromospheric jets/spicules in the upper chromosphere and heating of the chromosphere, transition region, and corona, due to the high reflection rate of Alfvén waves in the upper chromosphere. Conditions for the direct proton acceleration (jet formation) in the chromosphere include a temperature ≤104 K and a magnetic field between about 10 and 100 G.