Constraints on the O⁺⁵ Anisotropy in the Solar Corona

Abstract

Velocity distributions of O⁺⁵ ions derived from Ultraviolet Coronagraph Spectrometer (UVCS) observations in coronal holes indicate that the O⁺⁵ ions are highly anisotropic (T_⊥i/T_∥i~30-300 at 3.5 R_solar). The observations provide empirical values for the electron density and the ion temperatures. It is well known that the electromagnetic ion cyclotron instability is driven by temperature anisotropy. The instability leads to the rapid decrease of anisotropy and transfer of part of the kinetic energy of the particles into the magnetic field fluctuations. Here we use linear theory and hybrid simulations combined with the empirical values of the densities and the temperatures to investigate the ion cyclotron instability of the anisotropic minor ions in the coronal hole plasma. We find that an initial O⁺⁵ anisotropy of 50 decreases by an order of magnitude within ~300-900 proton cyclotron periods. Thus, the ion cyclotron instability constrains the anisotropy of O⁺⁵ ions that can be sustained in the solar corona without continuous perpendicular heating.