The response of the coupled magnetosphere-ionosphere-thermosphere system to a 25% reduction in the dipole moment of the Earth's magnetic field

We study the response of the magnetosphere-ionosphere-thermosphere system to a 25% reduction in magnetic field intensity, using the coupled magnetosphere-ionosphere-thermosphere (CMIT) model. Simulations were performed with a dipole moment of 8 × 10²² A m², close to the present-day value, and a dipole moment of 6 × 10²² A m², both under the same solar wind conditions, intermediate solar activity (F10.7 = 150), and for March equinox and June solstice.

The 25% reduction in field strength causes the magnetosphere to shrink and the polar cap to expand, in agreement with theory. The Pedersen and the Hall ionospheric conductances increase by 50%--60% and 60%--65%, respectively. This causes a ~9%--12% decrease in electric potential and a ~20% increase in field-aligned currents during equinox.