On the background-gyroresonant character of the Bell instability. (arXiv:1811.05666v1 [physics.plasm-ph])
<a href="http://arxiv.org/find/physics/1/au:+Weidl_M/0/1/0/all/0/1">Martin S. Weidl</a>, <a href="http://arxiv.org/find/physics/1/au:+Winske_D/0/1/0/all/0/1">Dan Winske</a>, <a href="http://arxiv.org/find/physics/1/au:+Niemann_C/0/1/0/all/0/1">Christoph Niemann</a>
We show that the Bell instability, which is widely considered potentially
important for cosmic-ray acceleration, is the low-frequency limit of a
gyroresonant interaction between the protons of the interstellar medium and
shear-Alfv’en waves. At large cosmic-ray current densities, its growth rate is
therefore limited by the proton gyrofrequency, and two modes emerge from the
cold-beam dispersion relation. A third mode driven by electron gyroresonance is
only weakly unstable at low current densities. We discuss implications for
magnetic-field amplification and its saturation in the vicinity of supernova
remnants.
We show that the Bell instability, which is widely considered potentially
important for cosmic-ray acceleration, is the low-frequency limit of a
gyroresonant interaction between the protons of the interstellar medium and
shear-Alfv’en waves. At large cosmic-ray current densities, its growth rate is
therefore limited by the proton gyrofrequency, and two modes emerge from the
cold-beam dispersion relation. A third mode driven by electron gyroresonance is
only weakly unstable at low current densities. We discuss implications for
magnetic-field amplification and its saturation in the vicinity of supernova
remnants.
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