Unequal compression of the geomagnetic field by the solar wind

The total pressure is balanced at the magnetopause where separates the shocked solar wind and the compressed geomagnetic field. In this study, we use the magnetic fields from five THEMIS probes and the theoretical dipole magnetic field model to calculate the ratio of the compressed geomagnetic field just inside the magnetopause to the dipole magnetic field. Here we find that the ratio is linearly proportional to the subsolar standoff distance. In addition, we also study the relation between the compressed geomagnetic field and the subsolar distance. Here we also find that the compressed geomagnetic field varies with the subsolar distance in a power law of -2.3, which is different from that estimated from the dipole magnetic field (-3.0). Furthermore, this relation can be turned into a function of the compressed geomagnetic field in terms of the subsolar distance. The function can be served as a boundary condition in the global modeling of the magnetosphere and can advance our understandings of the loss of electrons of the radiation belt at the magnetopause and the interaction between the solar wind and the magnetosphere.