A global MHD and empirical magnetic field model investigation of the magnetospheric cusp

Magnetic field configurations, cusp locations, merging, and interconnection regions for the Lyon-Fedder global magnetohydrodynamic (MHD) simulation mo del and the Tsyganenko 96 (T96_01) empirical model are presented and compar ed for three different interplanetary magnetic field (IMF) directions (nort h, south, and dusk directed). Field line configurations indicate a signific antly larger magnetosphere in the T96_01 model compared to the MHD model fo r all IMF directions. In each of the models the polar cusp is identified by depressions in the magnetic field residual and by the boundary between day side closed and open field lines. It is found that the MI-ID model exhibits a much stronger residual magnetic field depression in the cusp region than does the T96-01 model. The MHD field depression is associated with the pre sence of diamagnetic magnetosheath plasma that enters the cusp via the MHD merging process. In the northward IMF case, MHD merging above both poles re sults in an MHD open-closed field line boundary located at 89 degrees invar iant latitude with MHD cusp plasma and field depressions centered on closed field lines near 81 degrees. The T96_01 model is very different from this with its north IMF open-closed boundary located at 79 degrees invariant lat itude. For the southward IMF case the T96_01 open-closed boundary at 74 deg rees invariant latitude lies slightly poleward of the MHD open-closed bound ary at 73 degrees. In this case the MHD cusp plasma is centered near 75 deg rees invariant latitude. In the dusk-directed IMF case the MHD cusp plasma shifts similar to2 hours in magnetic local time (MLT). toward dusk and rema ins poleward of the MHD open-closed boundary. In this case the MHD dayside open-closed boundary extends from dawn to dusk, while the T96_01 dayside op en-closed boundary extends only from dawn to similar to 13 MLT and near loc al noon is coincident with the MHD boundary at similar to 74 degrees invari ant latitude. Differences between the two models are due in large part to d ifferences in the form of their interconnectivity with the interplanetary m agnetic field lines. For each IMF direction the MHD merging locations and T 96_01 strongest interconnection regions are identified and are found to be very different. On the basis of comparisons with numerous case and statisti cal studies of the cusp it is concluded that the MHD model provides a fairl y realistic representation of the cusp and dayside magnetic field configura tion, while the T96_01 model is limited in its ability to model the cusp re gion.