A global MHD model of the solar wind: Comparison with Ulysses data

Abstract

A global axisymmetric MHD model of the solar wind is developed to study the heliospheric structure during Ulysses’ fast latitude scan in 1994–1995. Ulysses’ observations showed that (1) the heliospheric structure was nearly axisymmetric, steady and dominated by fast solar wind except for an equatorial band ∼40° wide, (2) the fast flows in each hemisphere were found to be relatively structure-free, unipolar and with opposite magnetic field polarities, corresponding to those observed in the polar coronal holes at the Sun, (3) the radial magnetic field in the fast solar wind was nearly independent of heliolatitude. We attempt to reproduce quantitatively these features of Ulysses’ observations and, in particular, to study the transformation of a dipolar magnetic field near the Sun into a latitude-independent one at large heliocentric distances. The governing polytropic single-fluid MHD equations are solved by combining a time-relaxation numerical technique in the inner computational region (1–20 R⊙) and a marching-along-radius numerical method at greater distances out to 5 AU. The boundary conditions are specified at 1 R⊙; the equations include the heat and momentum addition terms due to Alfvén waves, handled by the WKB approximation. We show that the results of our simulation compare favorably with Ulysses observations.