On the relative speed and temperature ratio of solar wind alpha particles and protons: Collisions versus wave effects

Abstract

We study the relative flow speed and the temperature ratio of alpha particles and protons and their connections to the helium ion abundance, the collisional age, and the power of transverse fluctuations within the inertial range. It is found that the alpha-to-proton temperature ratio, T α/Tp, anti-correlates with the helium ion abundance. Despite a relatively high collisional age and small wave power, the ratio Tα/Tp can reach comparatively high values (even above 2) whenever the helium ion abundance is below about 0.02. In contrast, the differential speed of alpha particles with respect to protons is correlated with the total wave power and anti-correlated with the collisional age. Ultimately, the individual heating of each ion species is positively correlated with the total wave power. Our findings suggest that a high-friction collision could be efficient in reducing the differential speed between alpha particles and protons, but appears not to be sufficient to equalize the alpha and proton temperatures, i.e., to make Tα ≃ Tp. This is a hint that the local wave heating process is acting on a timescale shorter than the collision time. © 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.