Parametric Interaction of VLF and ELF Waves in the Ionosphere

Generation of Very Low Frequency (VLF) electromagnetic whistler waves due to the parametric interaction of quasi-electrostatic VLF waves known as Lower Oblique Resonance (LOR) waves and Extremely Low Frequency (ELF) waves are analyzed in the frame of two possible scenarios. In the first scenario, quasi-electrostatic LOR waves and ELF waves are excited by conventional loop and dipole antennas. In the second one, they are naturally excited by plasma instabilities in the plasmasphere boundary layer. We present the results of numerical simulations of nonlinear coupling between LOR and Fast Magnetosonic (MS) waves, which was suggested to explain the observations of VLF electromagnetic emissions at frequencies well above the lower hybrid resonance frequency in the Turbulent Plasmasphere Boundary Layer (TPBL). These emissions represent a distinctive subset of the substorm/storm-related VLF whistler activity contributing to the alteration of the outer radiation belt boundary. As the TPBL is interior to the plasma sheet inner boundary and thus devoid of substorm-injected kiloelectronvolt electrons, the standard whistler generation mechanism must be excluded. Numerical solution of the derived nonlinear equations show that parametric coupling of LOR and ELF waves creates VLF electromagnetic emissions with the spectral characteristics consistent with the observations.