Non-field-aligned Proton Beams and Their Roles in the Growth of Fast Magnetosonic/Whistler Waves: Solar Orbiter Observations

Abstract

The proton beam is an important population of the non-Maxwellian proton velocity distribution in the solar wind, but its role in wave activity remains unclear. In particular, the velocity vector of the proton beam and its influence on wave growth/damping have not been addressed before. Here we explore the origin and the associated particle dynamics of a kinetic wave event in the solar wind by analyzing measurements from Solar Orbiter and comparing them with theoretical predictions from linear Vlasov theory. We identify the waves as outward-propagating circularly polarized fast magnetosonic/whistler (FM/W) waves. The proton’s velocity distribution functions can destabilize FM/W waves. According to linear Vlasov theory, the velocity fluctuations of the core and the beam associated with FM/W waves render the original field-aligned background drift velocity non-field-aligned. This non-field-aligned drift velocity carrying the information of the velocity fluctuations of the core and the beam is responsible for the wave growth/damping. Specifically, for the FM/W waves we analyze, the non-field-aligned fluctuating velocity of the beam population is responsible for the growth of these unstable waves in the presence of a proton beam. In contrast, the core population plays the opposite role, partially suppressing the wave growth. Remarkably, the observed drift velocity vector between the core and the beam is not field aligned during an entire wave period. This result contrasts the traditional expectation that the proton beam is field aligned.

Comparative Study of the Kinetic Properties of Proton and Alpha Beams in the Alfvénic Wind Observed by SWA-PAS On Board Solar Orbiter

“…The meticulous analysis performed by these authors revealed that the peak of the probability density function of the angle between the proton beam drift velocity and the local field was a few degrees, remarkably in agreement with our findings. This observation is interesting because, despite the 5°angular resolution of PAS, we can discern plasma parameters with an uncertainty of under 5°, as remarked by Zhu et al (2023). It would be intriguing to explore whether these fluctuations have a comparable impact on the misalignment between the alpha core and alpha beam velocity drifts.…”

“…According to Zhu et al (2023), linear Vlasov theory posits that the presence of fast magnetosonic and/or whistler waves induces fluctuations in the velocities of both the proton core and beam, consequently disrupting the alignment of their velocity drift with the magnetic field. Subsequent analysis by the same authors, leveraging Solar Orbiter observations, revealed distinct time intervals characterized by the prevalence of circularly polarized fast magnetosonic/whistler waves.…”

Evolution of the Interplanetary Turbulence and the Associated Turbulence Anisotropy in the Outer Heliosphere: VOYAGER 2 Observations

“…The SOP is inversely correlated with k B , 0 ( ) q . This indicates that the orthogonal components have a higher coherence at small propagation angles, and thus the quasi-parallel-propagating fluctuations are more wave-like (He et al 2022;Zhu et al 2023). At small propagation angles, the SOP is slightly greater at τ < 100 s than at τ > 100 s. The ellipticity distribution is isotropic at a constant period.…”