  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 $k_{p,\rm
ex}$. Small-scale Alfv\'enic wavepackets are promising sources of coherent
curvature radiation. When the group Lorentz factor $\gamma_{\rm gr} \lesssim
(k_{p,\rm ex}R_c)^{1/3} \sim 100$, 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 $\sim 10^{-2}ck_{p,\rm
ex}$. 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, Alfv\'enic 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. Alfv\'en mode growth can track the local plasma
conditions, allowing for lower-frequency emission from the outer magnetosphere.
Beamed radio emission from charged packets with $\gamma_{\rm gr} \sim 50-100$
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.
