On the possible excitation of electromagnetic ion cyclotron waves in solar ejecta

Abstract

We study the possibility of exciting electromagnetic ion cyclotron waves (EICWs) in solar ejecta (CMEs) by a kinetic instability driven by ion temperature anisotropies. Our approach is to vary key parameters about assumed baseline values. Since Tp,‖ > Tp,⊥ in most solar ejecta, the polarization of the unstable waves is right‐handed. If the average proton beta is low (βp ≤ 0.3), the activity is negligible for moderate temperature ratios, Tp,‖/Tp,⊥. Increasing βp increases both the frequency range and the instability growth rate. Increasing the temperature anisotropy brings about qualitatively similar effects as increasing βp, with comparable growth rates. Increasing the relative alpha‐to‐proton density ratio η has two effects: the active frequency range is shifted toward lower frequencies and the growth rate increases. Between η = 0 and η = 0.15, the maximum growth rate increases by a factor of ∼20, highlighting the importance of the alphas for generating this instability. A case that may represent some magnetic clouds with exceptional parameters, βp = 0.2, Tp,‖/Tp,⊥ = 10, and η = 0.08 − 0.15, is considered. The maximum growth rate is found to be twice the reference CME case, while the active frequency range is 3 times wider. We conclude that EICWs should be present in some ejecta and possibly also in those magnetic clouds with relatively weak magnetic field, high He++ content, and large Tp,‖/Tp,⊥ ratios, and whose βp is high, for example, through interaction with a succeeding fast stream. We also suggest that substantial changes with respect to normal conditions should occur in the power spectrum of EICWs in the terrestrial plasma depletion layer when a CME, or a magnetic cloud, with negative anisotropy passes Earth.