Heavy ion mass loading of the geomagnetic field near the plasmapause and ULF wave implications

[1] The structure of the density discontinuity across the plasmapause is often based on electron and H+ density profiles with the contribution of heavy ions ( He+, O+ etc) neglected. Electron and ion density measurements in this region may differ significantly due to the presence of heavy ions and it is important for the intercomparison of different datasets to understand these differences. Dynamics Explorer (DE-1) magnetic field and plasma composition data have been used to compare heavy ion responses across the plasmapause and to calculate the mass loaded ion density (p) profiles. To illustrate this we investigate mass loading through radial profile variations in the Alfven velocity ( V-A). Results show that the gradient in p and V-A across the plasmapause is modified when mass loading due to multiple heavy ion species is included, particularly in the presence of the O+ torus. Application to ultra-low frequency (ULF) field line resonance is used as an example where the contribution from heavy ions smoothes out the expected ULF wave resonant frequency discontinuity at the plasmapause.