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

Abstract

Magnetic field configurations, cusp locations, merging, and interconnection regions for the Lyon‐Fedder global magnetohydrodynamic (MHD) simulation model and the Tsyganenko 96 (T96_01) empirical model are presented and compared for three different interplanetary magnetic field (IMF) directions (north, south, and dusk directed). Field line configurations indicate a significantly larger magnetosphere in the T96_01 model compared to the MHD model for 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 dayside closed and open field lines. It is found that the MHD 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 presence of diamagnetic magnetosheath plasma that enters the cusp via the MHD merging process. In the northward IMF case, MHD merging above both poles results in an MHD open‐closed field line boundary located at 89° invariant latitude with MHD cusp plasma and field depressions centered on closed field lines near 81°. The T96_01 model is very different from this with its north IMF open‐closed boundary located at 79° invariant latitude. For the southward IMF case the T96_01 open‐closed boundary at 74° invariant latitude lies slightly poleward of the MHD open‐closed boundary at 73°. In this case the MHD cusp plasma is centered near 75° invariant latitude. In the dusk‐directed IMF case the MHD cusp plasma shifts ∼2 hours in magnetic local time (MLT) toward dusk and remains 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 open‐closed boundary extends only from dawn to ∼13 MLT and near local noon is coincident with the MHD boundary at ∼74° invariant latitude. Differences between the two models are due in large part to differences in the form of their interconnectivity with the interplanetary magnetic field lines. For each IMF direction the MHD merging locations and T96_01 strongest interconnection regions are identified and are found to be very different. On the basis of comparisons with numerous case and statistical studies of the cusp it is concluded that the MHD model provides a fairly realistic representation of the cusp and dayside magnetic field configuration, while the T96_01 model is limited in its ability to model the cusp region.