Adopted meridional magnetic field configuration in the corona. a) A quadrant is shown where the magnetic axis points upward, and the thick contours delineate the lines of force for which the expansion factors f = (A/A₀)(R_⊙/r)² are plotted in b) as a function of heliocentric distance r. The field lines are labeled with the colatitude at which they intersect the Earth orbit (1 AU). Here A denotes the cross-sectional area of the flow tube, the subscript 0 represents values at the inner boundary R_⊙.