Triggered Emissions of Kinetic Alfvén Waves and Parallel Ion Cyclotron Waves by Ion Beam Mode in the Solar Wind

Kinetic Alfvén waves (KAWs) and ion cyclotron waves (ICWs) are believed to play a significant role in the solar wind heating and acceleration and are pervasive in space and astrophysical plasmas. However, the generation mechanism for the coexistence of both wave modes remains unclear. The present work proposes a novel generation scenario of KAWs and parallel ICWs in a homogeneous solar wind plasma using hybrid simulations. Our numerical study reveals that the emissions of KAWs and parallel ICWs can be triggered by the ion beam (IB) mode in a homogeneous alpha or proton beam plasma with a beam velocity of ≥1.2vA for alpha particles or ≥1.6vA for proton beams, where vA represents the local Alfvén velocity. The growth rates of both KAWs and parallel ICWs are significantly higher than that of the IB mode. Moreover, the initial background proton beta exhibits an inverse correlation with the growth of KAWs. Ultimately, the saturation energy of these triggered emissions could rival that of the IB mode. Given the prevalent occurrence of IBs in the solar wind, this triggering process provides a credible explanation for the origin of KAWs and parallel ICWs and their coexistence within the beam plasma environments.