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,ex . Small-scale Alfvénic wavepackets are promising sources of coherent curvature radiation. When the group Lorentz factor γ gr (k p,ex R c ) 1/3 ∼ 100, where R c is the magnetic curvature radius, a fraction ∼ 10 −3 -10 −2 of the particle kinetic energy is radiated into the extraordinary mode at a peak frequency ∼ 10 −2 ck p,ex . Consistency with observations requires a high pair multiplicity (∼ 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 sub-pulse 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 γ gr ∼ 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.