Abstract
Abstract. By three-dimensional hybrid simulations, proton heating is investigated starting from a monochromatic large-amplitude Alfvén wave with left-handed circular polarization launched along the mean magnetic field in a low-beta plasma. We find that the perpendicular scattering is efficient in three dimensions and the protons are heated by the obliquely propagating waves. The thermal core proton population is heated in three dimensions as well in the longitudinal and parallel directions by the field-aligned and obliquely propagating sound waves out of the parametric decay. The astrophysical context is discussed.
Highlights
In situ measurements at 1 AU from the VELA satellite (Bame et al, 1975) reveal that the velocity distribution function of solar wind protons is broader in the direction perpendicular to the mean magnetic field than in the parallel direction
According to numerical simulation studies (e.g., Araneda et al, 2007), the field-aligned part describing the tail or the proton beam of the velocity distribution functions can originate in the parametric decay of the Alfvén waves, a process predicted by theories and supported by observations
Most of the peaks observed in the magnetic field spectrum correspond to the wave–wave couplings of the pump wave with the fundamentals and the harmonics of the daughter compressional wave driven by the decay instability
Summary
In situ measurements at 1 AU from the VELA satellite (Bame et al, 1975) reveal that the velocity distribution function of solar wind protons is broader in the direction perpendicular to the mean magnetic field (hereafter the z direction) than in the parallel direction. This velocity anisotropy indicates a higher perpendicular temperature than the parallel one. More recently, Gao et al (2013) confirm in the 2-D hybrid simulation that obliquely propagating Alfvén waves are excited by the field-aligned parametric decay, and propose a heating mechanism of the ambient plasma in a stochastic fashion. Our finding that the particles can be more quickly heated by the 3-D parametric decay can be tested by in situ measurements by the upcoming heliospheric missions such as Parker Solar Probe (Fox et al, 2016) and Solar Orbiter (Müller et al, 2013)
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