Generation of Proton Beams by Non-uniform Solar Wind Turbulence
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Recent theoretical advances and observations indicate that magneto-hydrodynamic (MHD) Alfvénic turbulence is anisotropic and cascades mainly toward small scales perpendicular to the mean magnetic field. Eventually, the turbulence cascade reaches the ion-gyroradius scales where Alfvénic turbulent fluctuations possess parallel electric fields. We show that the local enhancements of the solar-wind turbulence can generate proton beams running ahead of these enhancements. The basic process leading to the beam formation is proton reflections off the turbulent fluctuations at the MHD/kinetic spectral break. With the turbulence amplitudes observed in the solar wind, theory predicts beam number densities of about 0.1 of the background number density and beam velocities of about 1.3 of the Alfvén velocity. These values fit the beam parameters measured in the solar wind well. In general, the more energetic proton beams with higher densities and velocities originate from higher turbulence levels and/or a hotter proton background. The higher the spectral break wavenumber, the faster the generated proton beam. These trends are to be examined by future solar wind observations.
CitationVoitenko, Y.; Pierrard, V. (2015). Generation of Proton Beams by Non-uniform Solar Wind Turbulence. , Solar Physics, Vol. 290, Issue 4, 1231-1241, DOI: 10.1007/s11207-015-0661-8.