A Z-MODE ELECTRON-CYCLOTRON MASER MODEL FOR BOTTOMSIDE IONOSPHERIC HARMONIC RADIO EMISSIONS

A model is proposed for recent ground-based observations of auroral ro ar emissions, detected at 2 Omega(e), and 3 Omega(e), where Omega(e) i s the local electron cyclotron frequency in the source region, between 200 and 500 km above the Earth's surface. Electron cyclotron maser em ission is a likely mechanism to account for these emissions because it naturally produces coherent radiation at harmonics of Omega(e). A the ory for auroral roar emissions has already been proposed, whereby mase r-generated second (X2) and third (X3) harmonic x mode radiation is am plified in the source region by multiple reflections off the walls of the density cavity in which they are produced. After many reflections the X2 and X3 waves propagate along the density cavity to a ground-bas ed observer. However, it is demonstrated here with ray-tracing calcula tions that it is highly probable that maser-generated X2 and X3 radiat ion is reabsorbed at lower altitudes and thus cannot be detected at th e ground. An indirect maser mechanism is proposed instead, where maser -generated z mode waves at Omega(e) grow to high levels in the source region and then undergo repeated nonlinear wave-wave coalescence to pr oduce second-and third-harmonic waves that propagate directly to the g round. The z mode waves must satisfy the necessary kinematic constrain ts to produce observable second-and third-harmonic radiation. The depe ndence of the z mode maser on the temperature and functional form of t he unstable electron distribution is discussed, along with the conditi ons required for the coalescence processes to proceed and produce the observed levels of radiation.