Authors: Nooshin Davis ( University of New Hampshire), Ben Chandran ( University of New Hampshire)

Solar wind observations highlight non-Maxwellian features in proton distribution functions, such as field-aligned beams exhibiting drift velocities comparable to the local Alfven velocity. These differential flows provide a source of free energy that can drive electromagnetic instabilities, notably the fast-magnetosonic/whistler (FM/W) instability. In this study, we focus on the conditions necessary for parallel FM/W waves to be driven unstable due by a proton beam.

We investigate the effects of proton-beam temperature anisotropy on the growth rates of FM/W instabilities, employing numerical solutions to the full hot-plasma dispersion relation.

Furthermore, we explore the constraints on the proton differential flow velocity and its temperature anisotropy due to FM/W instabilities in the solar wind using observations from Parker Solar Probe.