St. Suess et Ej. Smith, LATITUDINAL DEPENDENCE OF THE RADIAL IMF COMPONENT - CORONAL IMPRINT, Geophysical research letters, 23(22), 1996, pp. 3267-3270
Measurements by Ulysses have confirmed that there is no significant gr
adient with respect to heliomagnetic latitude in the radial component,
B-r, of the interplanetary magnetic field. In the corona, the plasma
beta is << 1, except directly above streamers, so longitudinal and lat
itudinal gradients in field strength will relax due to the transverse
magnetic pressure gradient force as the solar wind carries magnetic fl
ux away from the Sun. This happens quickly enough so that the field is
essentially uniform by 5 - 10 R., apparently remaining so as it is ca
rried to beyond 1 AU. Here, we illustrate the coronal relaxation with
a qualitative physical argument and by reference to a detailed MHD sim
ulation.