Speaker
Description
Though decades of studies of coherent pulsar radiation, the physical mechanism of the radio emission at kinetic micro-scales is still under investigation. One of the proposed mechanisms is the linear acceleration emission that is coherent antenna-type of radiation. The mechanism is based on plasma particles oscillating along magnetic field lines and producing linear acceleration emission.
We studied how plasma bunches/clouds of electron-positron pairs created during spark events in the polar gap region evolve as a function of the plasma temperature and drift velocity between electrons and positrons and how the bunches radiate by the linear acceleration emission. We utilized particle-in-cell simulations of relativistically hot bunches to investigate the non-linear evolution of the bunches. Also, we have implemented a novel treatment by utilizing the plasma currents in the simulations to obtain properties of the coherent radio emission mechanism. Our treatment does not require tracking individual plasma macro-particles but can directly utilize aggregated information from the currents about the collective particle motion.
We found that the initial drift velocity between electrons and positrons is the main parameter influencing the bunch evolution. For zero drift, the bunches can expand and overlap in the phase space and form relativistic streaming instability. Otherwise, the bunches are constrained from expansion by ambipolar diffusion effects, oscillating electrostatic fields are formed, and the plasma is strongly heated. Furthermore, we calculated the radio emission properties of both types of the bunch evolution. We found that bunches constrained from expansion have similar observational characteristics as observed for pulsars. The radiation power oscillates at micro-second time scales, and the spectrum contains a flat part for low frequencies and power-law profiles for higher frequencies. Also, the emitted radiation is relativistically beamed along the pulsar dipole axis.
