On the nature of pulsar radio emission

Abstract

A theory of pulsar radio emission generation, in which the observed waves are produced directly by maser-type plasma instabilities operating at the anomalous cyclotron-Cherenkov resonance $\omega- k_{\parallel} v_{\parallel} + \omega_B/ \gamma_{res}=0$ and the Cherenkov-drift resonance $\omega- k_{\parallel} v_{\parallel} - k_{\perp} u_d =0$, is capable of explaining the main observational characteristics of pulsar radio emission. The instabilities are due to the interaction of the fast particles from the primary beam and the tail of the distribution with the normal modes of a strongly magnetized one-dimensional electron-positron plasma. The waves emitted at these resonances are vacuum-like, electromagnetic waves that may leave the magnetosphere directly. In this model, the cyclotron-Cherenkov instability is responsible for core emission pattern and the Cherenkov-drift instability produces conal emission. The conditions for the development of the cyclotron-Cherenkov instability are satisfied for both typical and millisecond pulsars provided that the streaming energy of the bulk plasma is not very high $\gamma_p \approx 10$. In a typical pulsar the cyclotron-Cherenkov and Cherenkov-drift resonances occur in the outer parts of magnetosphere at $r_{res} \approx 10^9 cm$. This theory can account for various aspects of pulsar phenomenology including the morphology of the pulses, their polarization properties and their spectral behavior. We propose several observational tests for the theory. The most prominent prediction are the high altitudes of the emission region and the linear polarization of conal emission in the plane orthogonal to the local osculating plane of the magnetic field.