A technique to investigate plasma mass density in the topside ionosphere using ULF waves

The relatively cold core plasma density in the region where the ionosphere mixes with the plasmasphere is difficult to probe experimentally. It is sho wn that resonances of ULF magnetohydrodynamic waves can provide unique info rmation on plasma mass density in the region where the ionosphere and plasm asphere merge. The technique depends on the detection of harmonics of ULF f ield line resonances and allows the determination of plasma mass densities at discrete locations along a magnetic field line without assuming any func tion for plasma distribution along the field. The plasma mass densities obt ained from ULF harmonics at L=1.8 are compared with several plasma mass den sity models of this region. In addition to describing and illustrating the technique, the method is critically examined for its limitations. These inc lude the finite number of ULF harmonics that can be detected, the complexit y of the nonlinear solving scheme, the consequences of using the method at larger L values, and approximating the system of equations with a finite di fference scheme. It is shown that the technique can provide independent est imates of plasma mass densities at discrete, magnetic field-aligned altitud es, which gives an additional experimental check on theoretical models that predict the dynamics of plasma from the topside ionosphere to the inner pl asmasphere.