A new parametric reflection mechanism for ducted whistlers and an explanation of precursors

Abstract

Abstract A new reflection mechanism of ducted whistler-mode waves in the magnetosphere, which can be defined as a parametric reflection, is discussed. Unlike the well-known process of backscattering, parametric reflection occurs when the scale length of electron density inhomogeneities is much less than the whistler wave-length; the reflected whistler wave appears simultaneously with the excitation of short-scale lower hybrid resonance (LHR) plasma waves. The efficiency of such a parametric reflection is higher than that of traditional backscattering. Two cases are analysed. The first is when a reflected whistler wave arises via the process of the parametric decay instability of a sufficiently intense whistler wave into two quasielectrostatic short-scale waves, that is a LHR wave and an ionic (ion cyclotron or sound) wave. The second case is when the initial whistler interacts with naturally occurring ionic waves. Quantitative estimates show that both cases can exist in the magnetosphere. In the case of ducted whistler, generated by a lightning discharge, reflected signals at a given frequency are formed near two points along the magnetic field line where this frequency coincides with the local LHR frequency. This leads to appearance on a spectrogram of signals with a hook-like shape, which precede the whistler. It is shown that these signals can be associated with different types of whistler precursors.