Mode Conversion of Waves in the Ion-Cyclotron Frequency Range in Magnetospheric Plasmas

Waves in the ion-cyclotron range of frequencies with linear polarization detected by satellites can be useful for estimating the heavy ion concentrations in planetary magnetospheres. These waves are considered to be driven by mode conversion (MC) of the fast magnetosonic waves at the ion-ion hybrid resonances. In this Letter, we derive analytical expressions for the MC efficiency and tunneling of waves through the MC layer. We evaluate the particular parallel wave numbers for which MC is efficient for arbitrary heavy ion/proton ratios and discuss the interpretation of the experimental observations.

Figure 1

(a) Sketch of the wave dispersion: because of the radial inhomogeneity of the magnetic field, the IIH resonance is accompanied by a cutoff layer at the low field side, which form together the MC layer. (b) Scattering coefficients as a function of the tunneling factor for the isolated MC layer for waves incident from the outer magnetosphere.

Figure 2

(a) Tunneling factor as a function of the ${\mathrm{He}}^{+}$ concentration at the geostationary orbit for $k_{\parallel }=0$ and $n_e=10{\mathrm{cm}}^{-3}$. (b) Tunneling factor ${\eta }_{20}$ (log scale) for various plasma densities. Note that in all cases, MC can only be efficient for $X[{\mathrm{He}}^{+}]$ less than a few percent.

Figure 3

Normalized IIH resonance and $L$-cutoff frequencies as a function of the FW parallel wave number.