Radio Emission of Pulsars. II. Coherence Catalyzed by Cerenkov-Unstable Shear Alfvén Waves

Abstract

This paper explores small-scale departures from force-free electrodynamics around a rotating neutron star, extending our treatment of resistive instability in a quantizing magnetic field. A secondary, Cerenkov instability is identified: relativistic particles flowing through thin current sheets excite propagating charge perturbations that are localized near the sheets. Growth is rapid at wavenumbers below the inverse ambient skin depth $k_{p,\mathrm{ex}}$. Small-scale Alfvénic wavepackets are promising sources of coherent curvature radiation. When the group Lorentz factor ${\hat{I}}_{\mathrm{gr}}^3\lesssim (k_{p,\mathrm{ex}}{R}_c{)}^{1/3}\sim 100$, where $R_c$ is the magnetic curvature radius, a fraction $\sim 10^{-3}$-$10^{-2}$ of the particle kinetic energy is radiated into the extraordinary mode at a peak frequency $\sim 10^{-2}c{k}_{p,\mathrm{ex}}$. Consistency with observations requires a high pair multiplicity ($\sim 10^{3-5}$) in the pulsar magnetosphere. Neither the primary, slow resistive instability nor the secondary, Alfvénic instability depend directly on the presence of magnetospheric `gaps', and may activate where the mean current is fully supplied by outward drift of the corotation charge. The resistive mode is overstable and grows at a rate comparable to the stellar spin frequency; the model directly accommodates strong pulse-to-pulse radio flux variations and coordinated subpulse drift. Alfvén mode growth can track the local plasma conditions, allowing for lower-frequency emission from the outer magnetosphere. Beamed radio emission from charged packets with ${\hat{I}}_{\mathrm{gr}}^3\sim 50-100$ also varies on sub-millisecond timescales. The modes identified here will be excited inside the magnetosphere of a magnetar, and may mediate Taylor relaxation of the magnetic twist.

32 pages, 8 figures, Astrophysical Journal in press. Minor revisions to description of method