A New Perspective on Non-field-aligned Proton Beams and Their Roles in Cyclotron Wave Growth: Solar Orbiter Observations

Proton beam is an important population for solar wind kinetic physics but remains unclear for its roles in wave activity. Especially, the full vector of the proton beam bulk velocity and its influence on wave growth/dissipation has not been addressed before. The new cutting-edge mission, Solar Orbiter (SolO), has revealed many ion-scale cyclotron waves prevailing in the solar wind. This work targets to investigate the origin of these waves by analyzing the measurements from SolO and comparing them with plasma wave theory. We identify an unstable left-handed ion-scale cyclotron wave event persisting about 30 minutes. Contrary to the traditional perspective, the drift velocity vector between core (c) and beam (b) does not align with the local magnetic field during wave activity. Furthermore, we find that the fluctuating bulk velocity of the beam is responsible for the partial suppressing of unstable waves, while field-core interaction plays the opposite role in boosting the wave growth. The core and the beam also have different Alfven properties. Specifically, at periods from 8 s to 100 s, the velocity fluctuations (V) and the Alfven velocity fluctuations (VA) have opposite correlations (CC) for core and beam, where CC(Vc, VA,c) >0 and CC(Vb, VA,b)<0. We elucidate unprecedentedly that the non-field-aligned beams exist due to the differences in bulk velocity fluctuations between the core and the beam. The core and the beam have opposite influences on the wave growth: bolstering vs. restraining. The new perspective proposed in this study sheds new light on the energy conversion of coherent waves or turbulence by emphasizing the concerns of core and beam separately.