On the Influence of the Earth's Magnetic Dipole Eccentricity and Magnetospheric Ring Current on the Magnetopause Location
Abstract
We investigate the influence of two typically unconsidered parameters-Earth's magnetic dipole eccentricity and ring current-on the location of the magnetopause. Although empirical magnetopause models generally assume the Earth's magnetic field to be axially symmetric, the terrestrial magnetic dipole is shifted by as much as 500 km out of the Earth's center.
Additionally, the magnetic field at the magnetopause is further modified by magnetospheric currents, most importantly by the ring current. In order to quantify the effects related to these phenomena on the magnetopause location, we compare observed magnetopause distances with model distances calculated using a model, which does not take them into account.
International Geomagnetic Reference Field is used to describe the dipole eccentricity and relevant multipoles, which results in a statistically observable magnetopause displacement (approximate to 0.2R(E)) at locations where the magnetic field strength increases/decreases as compared to the dipole field. Additionally, the magnetic field at the magnetopause is modified by the ring current, becoming stronger at the times of a stronger ring current.
We use the corrected Dst* index to describe the ring current strength, and we demonstrate that its variation results in a change of the magnetopause stand-off distance as large as about 0.8R(E). We further use the T96 magnetic field model to estimate the total contribution of magnetospheric currents to the magnetic field at the magnetopause and their effects on the magnetopause location.
We suggest empirical relations that can be used to incorporate the obtained dependences into existing empirical models of magnetopause location.