When a super-Alfvénic electron beam propagates along an ambient magnetic field, the left-hand circularly polarized Alfvén wave is Cherenkov emitted (two stream instability). This instability results in a spontaneous conversion of the background plasma helicity to the wave helicity. The background helicity induces a frequency (energy) shift in the eigenmodes, which changes the critical velocity for Cherenkov emission, and it becomes possible for a sub-Alfvénic electron beam to excite a nonsingular Alfvén mode.

• Received 29 January 1994

DOI:https://doi.org/10.1103/PhysRevLett.72.3989