An analytic solar magnetic field model

We describe a simple analytic model for the magnetic field in the solar corona and interplanetary space which is appropriate to solar minimum conditions. The model combines an azimuthal current sheet in the equatorial plane with an axisymmetric multipole field representing the internal magnetic field of the Sun. The radial component of the field filling interplanetary space is approximately monopolar at large heliocentric distances as observed. These open field lines connect to the polar regions of the Sun and define the polar coronal holes which are prevalent at solar minimum and which are the source of the fast solar wind. By including both dipole and a quadrupole terms at the origin it is possible to construct a good representation of the coronal magnetic field in such conditions. We also note that the Parker spiral will be underwound relative to the case of the monopole because the open field Lines emanate from solar latitudes in excess of 60 degrees.