The relatively cold core plasma density in the region where the ionosphere mixes with the plasmasphere is difficult to probe experimentally. It is shown that resonances of ULF magnetohydrodynamic waves can provide unique information on plasma mass density in the region where the ionosphere and plasmasphere merge. The technique depends on the detection of harmonics of ULF field line resonances and allows the determination of plasma mass densities at discrete locations along a magnetic field line without assuming any function for plasma distribution along the field. The plasma mass densities obtained from ULF harmonics at L=1.8 are compared with several plasma mass density models of this region. In addition to describing and illustrating the technique, the method is critically examined for its limitations. These include the finite number of ULF harmonics that can be detected, the complexity of the nonlinear solving scheme, the consequences of using the method at larger L values, and approximating the system of equations with a finite difference scheme. It is shown that the technique can provide independent estimates of plasma mass densities at discrete, magnetic field-aligned altitudes, which gives an additional experimental check on theoretical models that predict the dynamics of plasma from the topside ionosphere to the inner plasmasphere. ¿ 1999 American Geophysical Union