Latitudinal solar wind velocity variations from polar coronal holes: A self‐consistent MHD model

Abstract

Coronal holes are regions of the solar corona magnetically open to the interplanetary medium where the plasma flows outward to form the solar wind. An axisymmetric magnetohydrodynamic model has been developed that describes both the magnetic structure of the solar corona and the outward flow of coronal plasma in a self‐consistent manner, considering a dipolar field at the solar surface. Equilibrium solutions were found for a field at the solar pole of 10 G and a variety of plasma boundary conditions at 1 AU in order to explore the latitudinal dependence of the solar wind velocity from polar coronal holes. As expected, two large polar coronal holes were found, extending down to 32° latitude, separated by a belt of closed coronal helmets. The coronal helmet belt extended to 1.7 Rs, and above this height all flux lines were open and connected to one of the polar coronal holes, from which all solar wind flowed. Three cases of latitudinal velocity variation at 1 AU were considered: (1) a sharp transition between a slow wind at low latitudes and a fast wind at high latitude (in accordance with Ulysses' observations), (2) a linear latitudinal increase of velocity from slow wind at the equator to fast wind over the pole, and (3) a constant velocity irrespective of latitude. In all cases a fast wind flowed from the poles, but there always existed a low‐latitude band of slow solar wind close to the Sun. This slow wind flowed out from the boundary of both coronal holes and formed a band of slow wind on either side of the heliospheric current sheet. The transition between slow and fast wind close to the Sun was sharply defined in all cases.