什么是 Main geomagnetic field; geocentric axial dipole？

The main geomagnetic field is the part of the Earth’s magnetic field produced inside the Earth, in its liquid iron outer core; it changes over years to millions of years, unlike the fields of the ionosphere and the magnetosphere, which change over seconds to days. The geocentric axial dipole is the field of a magnetic dipole placed at the centre of the Earth and aligned with its rotation axis, pointing south, so that its field lines leave the southern hemisphere and enter the northern one.