A NEW PARAMETRIC REFLECTION MECHANISM FOR DUCTED WHISTLERS AND AN EXPLANATION OF PRECURSORS

A new reflection mechanism of ducted whistler-mode waves in the magnet osphere, which can be defined as a parametric reflection, is discussed . Unlike the well-known process of backscattering, parametric reflecti on 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 ref lection is higher than that of traditional backscattering. Two cases a re analysed. The first is when a reflected whistler wave arises via th e process of the parametric decay instability of a sufficiently intens e whistler wave into two quasielectrostatic short-scale waves, that is a LHR wave and an ionic (ion cyclotron or sound) wave. The second cas e is when the initial whistler interacts with naturally occurring ioni c waves. Quantitative estimates show that both cases can exist in the magnetosphere. In the case of ducted whistler, generated by a lightnin g discharge, reflected signals at a given frequency are formed near tw o points along the magnetic field line where this frequency coincides with the local LHR frequency. This leads to appearance on a spectrogra m of signals with a hook-like shape, which precede the whistler. It is shown that these signals can be associated with different types of wh istler precursors. (C) 1997 Elsevier Science Ltd.