  In the proton counterflow model of a pulsar magnetosphere that we have
recently proposed, non-relativistic protons are supplied from the magnetosphere
to flow toward the pulsar surface and screen an electric field above the polar
cap region. In this Letter, we show that the proton counterflow is also
suitable for the bunching of pair plasma. The two-stream instability is easily
excited and can produce bunches of pairs with a relevant length scale to emit
coherent curvature radiation.
