Alpha-Particle/Proton Differential Flow in the Solar Wind: Implications for Plasma 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 temperature anisotropies and relative drifts along the direction of the background magnetic field. Two 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 - for example, the Alfvén/ion-cyclotron and fast-magnetosonic/whistler instabilities limit the drift velocity to a value comparable to the Alfvén speed, which decreases with increasing heliocentric distance r. However, while plasma instabilities transform bulk-flow kinetic energy into heat and plasma waves, the rotational force does not.

We present an analytic expression for the rate Q_flow at which energy is released when alpha particles are decelerated by instabilities. We find that Q_flow becomes zero at a critical radius r=r_crit, where r_crit is between 1.5 AU and 2 AU in the fast solar wind in the ecliptic plane, and r_crit increases with increasing heliographic latitude. 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. We compare the value of Q_flow at r<r_crit with the empirical heating rates for protons and alpha particles deduced from in-situ measurements of fast-wind streams from the Helios and Ulysses spacecraft. We find that Q_flow exceeds the empirical heating rate for alpha particles at r<1 AU.

We conclude that the continuous energy input from alpha-particle deceleration at r<r_crit makes a considerable contribution to the heating of the fast solar wind in addition to other local heating mechanisms such as the dissipation of waves and turbulence. 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.