Open‐closed field line boundary position: A parametric study using an MHD model

Abstract

In this paper we investigate the effect of changes in the interplanetary magnetic field (IMF), solar wind dynamic pressure, and dipole tilt angle on the position of the ionospheric projection of the open‐closed field line boundary (OCB) in a magnetohydrodynamic (MHD) model. We carry out a large number of steady state global MHD simulations in order to parameterize the OCB as a function of the solar wind By and Bz which we find to have the largest effect on the OCB location. We interpolate between the values produced by the simulations, which allows us to evaluate the location of the OCB projection into the ionosphere for any values of ∣By∣ < 10 nT and ∣Bz∣ < 10 nT. It is found that, particularly on the nightside, the OCB position is very sensitive to changes in the northward IMF component Bz, but it is much less sensitive to changes in Bz when it is southward. The response of the OCB location to changes in By also depends greatly on whether Bz is northward or southward, being much larger in situations in which Bz > 0 nT. We also find that the polar cap area increases with the increasing solar wind dynamic pressure. The Bx component of the IMF and the dipole tilt angle are found to have relatively small effects on the location of the OCB.