Results 1 - 1 of 1
Results 1 - 1 of 1. Search took: 0.012 seconds
[en] Protons and alpha particles in the fast solar wind are only weakly collisional and exhibit a number of non-equilibrium features, including relative drifts between particle species. Two non-collisional mechanisms have been proposed for limiting differential flow between alpha particles and protons: plasma instabilities and the rotational force. Both mechanisms decelerate the alpha particles. In this paper, we derive an analytic expression for the rate at which energy is released by alpha-particle deceleration, accounting for azimuthal flow and conservation of total momentum. We show that instabilities control the deceleration of alpha particles at , and the rotational force controls the deceleration of alpha particles at , where in the fast solar wind in the ecliptic plane. We find that is positive at and at , consistent with the previous finding that the rotational force does not lead to a release of energy. We compare the value of at with empirical heating rates for protons and alpha particles, denoted and , deduced from in situ measurements of fast-wind streams from the Helios and Ulysses spacecraft. We find that exceeds at , and that decreases with increasing distance from the Sun from a value of about one at r = 0.29–0.42 AU to about 1/4 at 1 AU. We conclude that the continuous energy input from alpha-particle deceleration at makes an important contribution to the heating of the fast solar wind. We also discuss the implications of the alpha-particle drift for the azimuthal flow velocities of the ions and for the Parker spiral magnetic field.