On the Origin of the Dominant Waves in the Extended Solar Corona

Abstract The extended solar corona at 10–30 solar radii is essentially devoid of all waves below 100 kHz other than triggered ion acoustic waves (TIAWs), which consist of a low-frequency (LF) electromagnetic wave at a frequency of a few Hz coupled to one or more electrostatic waves at a few hundred Hz, such that the amplitudes of the high-frequency (HF) waves peak at a fixed phase of each LF wave period. All the waves in a TIAW event travel at the same phase speed, which is found to be 150 km s −1 (the ion acoustic speed was about 100 km s −1 ). It has not been possible to explain the TIAW as a resonant wave–wave interaction, so a nonresonant interaction has been considered in which the loss of energy by the LF wave is used to both heat the electrons and grow the HF waves. Evidence in support of this explanation is described, and a particle-in-cell simulation that discusses this process is summarized. This simulation demonstrates wave–particle interactions driven by the enhanced LF fluctuations, which subsequently modify the proton velocity distribution function and create conditions favorable for the growth of HF waves. This interplay between a pair of waves, mediated by modifications of plasma parameters and energy conversion, represents a significant nonlinear process in plasma physics, the study of which will deepen the understanding of energy transfer, wave generation, and plasma dynamics in diverse astrophysical environments.