Are coronal holes the only source of fast solar wind at solar minimum? art. no. 1377

A two-dimensional (2-D) MHD model of an Alfven-wave-driven solar wind in the heliospheric meridional plane is presented whereby the consequences of a magnetic field distribution at the Sun that digresses from the standard dipole structure, often used in 2-D MHD models, are explored. The latitudinal distribution of open magnetic flux is specified at the solar surface, with a clear distinction between the polar coronal hole and the neighboring quiet Sun. The region of strictly closed magnetic structures at the coronal base is limited in latitude to +/-30degrees. Alfven waves are assumed to emanate from the solar surface above 30degrees latitude so as to open the field lines anchored there, as well as heat and accelerate the solar wind. The field lines anchored at the solar surface below 30degrees latitude, where the plasma is thermally conductive, remain closed, resulting in the formation of the helmet streamer astride the solar equator. The solution obtained is characterized by a helmet streamer in magnetostatic equilibrium, centered at the solar equator, and a steady solar wind outside. With the assumption that the contribution of the polar coronal hole and the quiet Sun to the interplanetary magnetic flux is evenly split between them during solar minimum, a fast wind solution is obtained that matches the latitudinal distribution of the solar wind parameters observed by Ulysses and SOHO and extends from the pole to 9degrees latitude. The larger fraction (approximate to60%) of the fast wind (above 30degrees latitude at 1 AU) emerges from the polar coronal hole, while the remainder is associated with the quiet Sun. While the actual fraction of the fast solar wind originating from polar coronal holes relies on an accurate estimate of the contribution of the open magnetic flux from polar coronal holes to the total IMF flux, this model shows that extending the source of open magnetic flux at the Sun beyond the traditionally assumed polar coronal holes can readily account for the latitudinal distribution of fast wind measured by Ulysses at solar minimum.