Sideband instabilities of a large-amplitude whistler wave due to phase-bunching of resonant particles with small perpendicular velocities

New types of sideband instabilities associated with a large-amplitude whistler wave propagating parallel to an external magnetic field B0 are proposed and investigated. The instabilities are due to a spiral beam consisting of resonant particles of small velocities, vsub(perpendicular), perpendicular to B0. It is shown analytically that in collisionless plasmas the angles zeta between the wave magnetic field and vsub(perpendicular) of these particles, when started from a random distribution, bunch quickly to π/2. A spiral beam is formed when there exist collisions which are insufficient to destroy this phase bunching, but are sufficient to prevent any other systematic variation of zeta. Under these conditions, three different whistler modes are found unstable in the immediate vicinity of the carrier wave frequency. Their growth rates can become greater than those of previously investigated sideband instabilities if the resonance velocity substantially exceeds the thermal velocity. (auth.)