It is usually assumed that the Sun/star is surrounded by a vacuum, in which no electrical currents can flow, i.e., ; for an axisymmetric B expressed via Equation (4), this requires
Formulated in this manner, the dynamo solution will spontaneously “pick” its own parity, i.e., its symmetry with respect to the equatorial plane. An alternative approach, popular because it can lead to significant savings in computing time, is to solve only in a meridional quadrant () and impose solution parity via the boundary condition at the equatorial plane ():
Living Rev. Solar Phys. 7, (2010), 3
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