Perpendicular Ion Heating by Low-Frequency Alfven-Wave Turbulence in the Solar Wind (Invited)

A number of observations suggest that low-frequency Alfven-wave (AW) and kinetic-Alfven-wave (KAW) turbulence is an important, and even dominant, heating mechanism in the solar wind. However, it has for some time been unclear whether such turbulence can explain the perpendicular ion heating that is observed in coronal holes and low-beta fast-wind streams. (i.e., heating that preferentially enhances thermal motions in directions perpendicular to the local magnetic field). In this presentation, I describe recent work that derives an analytic formula for the perpendicular ion heating rate in a low-beta plasma containing low-frequency AW/KAW turbulence, and that tests this formula using numerical simulations of test particles interacting with a spectrum of randomly phased AWs and KAWs. I also argue that low-frequency AW/KAW turbulence offers a promising explanation for several detailed features of the perpendicular ion temperature profiles observed in coronal holes, including the preferential heating of minor ions, the rapid radial increase in the OVI temperature between 1.6Rs and 1.9Rs, and the abrupt flattening of the OVI temperature profile as the heliocentric distance r increases above 1.9Rs.