The polar cusps define regions of significant transformation in Earth's protective magnetic shield. Within this region energy, momentum, and mass are transferred from the solar wind to the magnetosphere (Fritz & Zong, 2005). Definitions of the location of the cusp region vary, but a similar theme relates the location of the cusp region to the region where magnetic field lines diverge, or transition from being closed to open (Johnsen, Lorentzen, Holmes, & Løvhaug, 2012; Prölss, 2006; Fritz & Zong, 2005; Russell, 2000). Closed field lines have two foot points, one in each hemisphere; open field lines have one foot point at Earth, the other mapping to the solar wind (SW). Charged particles in the SW are able to follow these open field lines into Earth's upper atmosphere. It has been shown that the open/closed boundary (OCB) is directly equatorward of the cusp and that the cusp can serve as a reasonable proxy for determining the location of the OCB (Johnsen, Lorentzen, Holmes, & Løvhaug, 2012). Therefore, it is reasonable to assert that in modeling studies, the OCB can serve as a proxy for the cusp region. The latitudinal extent of the cusp is dynamic (Russell, 2000). Studies have shown that the geomagnetic pole tilt angle, SW pressure, and the interplanetary magnetic field (IMF) all contribute to a dynamic cusp (i.e. Russell, 2000). The purpose of this talk/poster is to present the findings of our recently submitted manuscript, "Model-based properties of the open/closed boundary: Is there a UT-dependence?" Using this study, we will show that the cusp experiences a diurnal variation that is UT-dependent and that this dependence is of sufficient magnitude to warrant inclusion in studies of the cusp region and the OCB.

References

Fritz, T. A., & Zong, Q. G. (2005). The magnetospheric cusps: A summary. Surveys in Geophysics, 409-414. doi:10.1007/s10712-005-1904-2

Johnsen, M. G., Lorentzen, D. A., Holmes, J. M., & Løvhaug, U. P. (2012). A model based method for obtaining the open/closed field line boundary from the cusp auroral 6300 Å [OI] red line. Journal of Geophysical Research. doi:10.1029/2011JA016980

Prölss, G. W. (2006). Electron temperature enhancement beneath the magnetospheric cusp. Journal of Geophysical Research. doi:10.1029/2006JA011618

Russell, C. T. (2000). The polar cusp. Adv. Space Res., 25(7/8), 1413-1424.