DECELERATION OF ALPHA PARTICLES IN THE SOLAR WIND BY INSTABILITIES AND THE ROTATIONAL FORCE: IMPLICATIONS FOR HEATING, AZIMUTHAL FLOW, AND THE PARKER SPIRAL MAGNETIC FIELD

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 Q(flow) 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 r < r(crit), and the rotational force controls the deceleration of alpha particles at r > r(crit), where r(crit) similar or equal to 2.5 AU in the fast solar wind in the ecliptic plane. We find that Q(flow) is positive at r < r(crit) and Q(flow) = 0 at r >= rcrit, consistent with the previous finding that the rotational force does not lead to a release of energy. We compare the value of Q(flow) at r < r(crit) with empirical heating rates for protons and alpha particles, denoted Q(p) and Q(alpha), deduced from in situ measurements of fast-wind streams from the Helios and Ulysses spacecraft. We find that Q(flow) exceeds Q(alpha) at r < 1 AU, and that Q(flow)/Q(p) 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 r < r(crit) 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.