  Narrow-band linearly polarized waves, having a resonant structure and a peak
frequency between the local cyclotron frequency of protons and heavy ions, have
been detected in the magnetospheres of Earth and of Mercury. Some of these wave
events have been suggested to be driven by linear mode conversion (MC) of the
fast magnetosonic waves at the ion-ion hybrid (IIH) resonances. Since the
resonant IIH frequency is linked to the plasma composition, solving the inverse
problem allows one to infer the concentration of the heavy ions from the
measured frequency spectra. In this paper, we identify the conditions when the
MC efficiency is maximized in the magnetospheric plasmas and discuss how this
can be applied for estimating the heavy ion concentration in the magnetospheres
of Earth and Mercury.
