The magnetopause as a source of nonthermal continuum radiation
Second only to auroral kilometric radiation as the Earth's most prominent radio emission, magnetospheric nonthermal continuum radiation has been the subject of numerous observational and theoretical investigations. It has been well established that nonthermal continuum radiation which exists in both ordinary (O) and extraordinary (X) electromagnetic modes, results from the frequency smearing of low frequency Terrestrial myriametric radiation (TMR) due to multiple reflections within the magnetospheric cavity; higher frequency TMR penetrates the magnetosheath and propagates away through the solar wind. TMR is inherently narrow banded and is produced by the mode conversion of electrostatic upper hybrid (ESUH) waves, with density gradients, possible normal to the magnetic field, playing an important part in the conversion process. Up to the present, most attention has been focussed on the plasmapause as the major source region. It will be shown, however, that the magnetopause far from being only a passive reflector of TMR is itself an important source region. This is not entirely unexpected since it has the three basic ingredients for TMR production – plasma frequencies greater than the cyclotron frequency, the presence of ESUH waves, and regions where the density gradient is large and perpendicular to the magnetic field. Data from satellites within the magnetospheric cavity are presented to illustrate that the magnetopause could well be as active a TMR source as the plasmapause. Results from the linear mode conversion theory will be reported which show that, in contrast to the plasmapause from which the TMR is in the O-mode, the TMR from the magnetopause can appear in both O and X modes.