Determination of the solar wind angular momentum flux from the HELIOS data - an observational test of the Weber and Davis theory

Abstract

In situ attempts to measure the sun's angular momentum loss in the solar wind and thereby to test the Weber and Davis description of the magnetic coupling between stellar rotation and winds have thus far produced widely divergent and inconclusive results. A new estimate for the solar loss rate in the ecliptic plane has been derived from the Helios spacecraft data. By intercomparing measurements made by the twin probes over the full 0.3-1.0 AU baseline of their orbits, it is possible to eliminate the systematic instrumental offsets from the true radial direction that have plagued previous efforts. The main observational findings are that (1) the total angular momentum flux loss rate (field + particles) near the solar equator is ˜0.2-0.3 × 1030 dyn cm sr-1, about one-quarter the Weber and Davis prediction and much lower than previous spacecraft estimates, and (2) the distribution of that flux between particles and field stresses is very near the 1:3 ratio of the model, when an important contribution from the heretofore neglected solar wind alpha-particles is accounted for. Though few by number in the solar wind, the alpha-particles' flow speed and direction in general differ from that of the protons, largely offsetting the latters' angular momentum content (+0.15-0.2 × 1030 for the protons, -0.1 × 1030 for the alpha-particles, plus being in the direction of corotation with the Sun). As to the small value reported for the total flux, theory and observation can be reconciled by moving the mean Alfvén radius, rA, in to 12 Rsun, a figure that is consistent with coronal models more realistic than the single polytrope formulation used by Weber and Davis. There is a distinct tendency for slow solar wind to carry positive total flux and for fast wind, negative; this can probably be explained in terms of stream-interaction dynamics in the super-Alfvénic region. It thus appears that the Weber and Davis theory adequately describes angular momentum loss in solar-type winds, insofar as simple magnetic stresses are taken as the dominant coupling mechanism. However, in the general astrophysical application, it is suggested that a more accurate treatment of coronal acceleration be incorporated to properly locate rA (and hence fix the total loss rate) and that some allowance for three-dimensional effects be made. Also, should large speed differentials between alpha-particles and protons occur well inside rA, a three-fluid version of the Weber and Davis model may be in order.