Constraints on the O+5 anisotropy in the solar corona

Velocity distributions of O+5 ions derived from Ultraviolet Coronagraph Spe ctrometer (UVCS) observations in coronal holes indicate that the O+5 ions a re highly anisotropic (T-perpendicular toi/T-parallel toi approximate to 30 -300 at 3.5 R-.). The observations provide empirical values for the electro n density and the ion temperatures. It is well known that the electromagnet ic ion cyclotron instability is driven by temperature anisotropy. The insta bility leads to the rapid decrease of anisotropy and transfer of part of th e kinetic energy of the particles into the magnetic field fluctuations. Her e we use linear theory and hybrid simulations combined with the empirical v alues of the densities and the temperatures to investigate the ion cyclotro n 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 magnitud e within similar to 300-900 proton cyclotron periods. Thus, the ion cyclotr on instability constrains the anisotropy of O+5 ions that can be sustained in the solar corona without continuous perpendicular heating.