Why BZ?

Abstract:

Magnetic reconnection is the mechanism by which magnetic field lines change partners, enabling a low stress magnetic configuration to be converted into a high-stress state. An example of this can be found in the Earth’s magnetotail lobes in which the two oppositely directed bundles of magnetic flux that constitute the lobes join up in the center of the tail, creating magnetic stress toward the Earth on the Earthward side of the point of linkage and anti-Earthward stress beyond that point. Antiparallel magnetic fields can also occur at the magnetopause, but the magnetic configuration of the solar wind-magnetosphere boundary is complex. The external field is bent and the internal field is bent as well, but in a different configuration. A variable amount of magnetic flux is close to being antiparallel across the magnetopause as the interplanetary magnetic field varies its orientation, especially its clock angle about the solar wind flow.

If we model the interaction and look for the area of maximum anti-parallelness, we find that this area is sensitive to the clock angle of the magnetic field around the point of intersection of the solar wind with the center of the magnetopause, i.e. the stagnation point. It is also sensitive to the tilt angle of the dipole. If the solar wind flowed along the direction to the Sun, this stagnation point would be the subsolar point and the best coordinate system to use would be the solar magnetospheric coordinate system and the effective magnetic field component would be the Z component.