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

B. J. Fraser, J. L. Horwitz, J. A. Slavin, Z. C. Dent, and I. R. Mann 


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 (r) profiles. To illustrate this we investigate mass 
loading through radial profile variations in the Alfven velocity (VA). Results 
show that the gradient in r and VA 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. 

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Geophysical Research Letters, 32, L04102, doi:10.1029/2004GL021315, 2005