Preferential positron heating and acceleration by synchrotron maser instabilities in relativistic positron–electron–proton plasmas

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In this paper, a new process of the preferential strong heating of positrons through the ion synchrotron maser instability in positron–electron–proton (e⁺–e⁻–p⁺) magnetized plasmas is studied, using particle-in-cell simulations. It is found that the positrons form a nonthermal power-law-like energy distribution through their gyroresonant interaction with the extraordinary modes emitted by the ions. This process may be important for the shock excitation of the nonthermal synchrotron radiation observed from astrophysical systems powered by relativistic outflows from compact central objects (e.g., supernova remnants powered by pulsars and jets from active galactic nuclei). When the initial particle distributions are cold rings in momentum space for all three species, there are two stages of instability. The electrons and positrons first thermalize through the emission of collective extraordinary modes. This phase is followed by the emission of collective proton synchrotron radiation, a process with a slower growth rate than the maser in the e⁻ and e⁺. When the gyrational energy density of the protons exceeds that of the e⁺, a significant amount of energy can be transferred to the positrons through resonant interaction between the e⁺, which gyrate in the same sense as the p⁺, and the elliptically polarized proton synchrotron modes whose frequency is at high harmonics of the fundamental proton gyrofrequency and whose electric field vector has the same sense of rotation as the protons and positrons. The final spectrum of the positrons has a power-law distribution in energy space: The number of positrons with relativistic energy between E and E+dE is N(E)dEE⁻². The electrons, which have the opposite sense of gyromotion to the protons, are not accelerated and retain the relativistic Maxwellian distribution that results from the synchrotron maser instability in the pairs alone. A brief discussion of the relevance of these results to the structure of collisionless relativistic shock waves in astrophysical sources of synchrotron radiation is presented. Physics of Fluids B: Plasma Physics is copyrighted by The American Institute of Physics.