The Open/Closed eld line Boundary (OCB) is the most important boundary in the magnetospheric system. On the dayside, the equatorward edge of the 6300 Å[OI] cusp aurora can be used as a proxy for the OCB. This work, which is a dissertation for the degree of philosophiæ Doctor consists of three scienti c papers focusing on the latitude of the optical cusp OCB and one paper focusing on polar cap patch ...