Earth's magnetic dipole field deflects the solar wind — a supersonic plasma stream from the Sun. The interaction compresses field lines on the day side into a bow shock and stretches them into a magnetotail on the night side. The IMF Bz component controls magnetic reconnection.
Bow shock standoff distance: ~10 R_E. Magnetopause: ~8-12 R_E. Negative Bz = southward = reconnection opens, aurora intensifies.