Preferential acceleration and perpendicular heating of minor ions in a collisionless coronal hole

Abstract

We incorporate the cyclotron-resonant Fermi heating mechanism of Isenberg & Vasquez into an inhomogeneous, collisionless coronal hole model to investigate this kinetic process in the presence of the other known forces on the coronal hole minor ions. The model includes the effects of gravity, charge-separation electric field, and mirroring in the decreasing magnetic field of a super-radially expanding flux tube. The Fermi process, due to the existence of multiple cyclotron resonances for minor ions, acts preferentially since it is not available to thermal protons in the low-beta coronal hole. The minor ions are treated as test particles, and we consider the specific case of O5+, which is the principal minor ion observed by the UVCS/Solar and Heliospheric Observatory instrument. We estimate an upper limit to the nonlinearly generated resonant wave power by extrapolating from the observed low-frequency fluctuations, and find that only a small fraction of this power is required to provide the observed minor ion energization. The perpendicular heating provided by this Fermi mechanism accelerates the entire minor ion distribution to high speed with respect to the bulk protons, consistent with the differentially streaming minor ion core distributions which are a distinctive property of the in situ fast solar wind. We conclude that this cyclotron-resonant Fermi process is easily capable of providing the observed preferential acceleration and heating of minor ions in the fast solar wind. © 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.