Observation of Alfvén Ion Cyclotron Waves in ICME Magnetic Clouds at 1 au

Abstract

Waves in plasma play an essential role in the energy transfer and plasma-heating processes. This article discusses the in situ observation of Alfvén ion cyclotron (AIC) waves and their characteristics within interplanetary coronal mass ejection (ICME) flux ropes. We analyzed 401 ICME flux ropes, observed by WIND spacecraft from 1995 to 2021 at 1 au. We found only five ICME flux ropes that show an explicit presence of AIC waves; two have normalized magnetic helicity σ m ≤ −0.5, and the remaining three show σ m ≥ 0.5 polarization. The angle between velocity and magnetic field (θ VB) for σ m ≤ −0.5 is <40°, whereas for σ m ≥ 0.5, θ VB > 140°. This result supports the existence of quasi-parallel and quasi-antiparallel left-handed polarized AIC waves within ICME flux ropes. We suggest that AIC waves are possibly triggered by (i) proton temperature anisotropy Tp ⊥/Tp ∥ > 1 driven by cyclotron instability and (ii) low-frequency Alfvén waves through the magnetohydrodynamic turbulent cascade. This study shows evidence of fluid and kinetic scales coupling in the ICME flux rope.