Electromagnetic beam modes driven by anisotropic electron streams

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Abstract
We present a theory which provides an explanation for recent laboratory observations of a ‘‘new electromagnetic mode in a non-Maxwellian high-beta plasma’’ and suggests a likely relevance of these modes to space plasmas. We find unstable parallel-propagating waves, which can be beam modes or whistlers, depending mainly on the degree of velocity anisotropy in a weak electron beam. In the beam-mode limit relevant to the experiment, this is a kinetic Weibel instability, enhanced by a parallel magnetic field. The frequency of the mode can be above or below the electron cyclotron frequency.