An estimation of the shape and temporal variation of the solar wind sonic, Alfvénic and fast magnetosonic surfaces

Abstract

In this work we find the shape and dimension of the solar wind transition surfaces, namely the slow (sonic), the fast magnetosonic and the Alfvén surface, as a function of heliospheric latitude using actual experimental data from Ulysses spacecraft during its first pole to pole journey. We also give the temporal variation of the radius of these surfaces using measurements from spacecraft at 1 AU. All transition surfaces are equatorial elongated. The sonic surface is the most asymmetric surface with polar radius ~ 1.5 R_sun and equatorial radius ~ 2 R_sun. The Alfvén and the fast magnetosonic surfaces seems to be less asymmetric. The Alfvén polar and equatorial radius is ~ 14 R_sun and ~ 17 R_sun respectively while the fast magnetosonic polar and equatorial radius is ~ 15 R_sun and ~ 18 R_sun respectively. The temporal variation of the transition surfaces radii follows the 11-year solar cycle except for the sonic radius which seems not to vary remarkably with time.