Narrow-band structure and amplitude of terrestrial myriametric radiation
The narrow-band structure of terrestrial myriametric radiation (TMR) is investigated by ray tracing electrostatic waves in a hot plasma which contains a loss cone distribution. Path-integrated growth of electrostatic waves which refract to the radio window, where mode conversion into TMR is assumed to take place, show that wave amplitudes at the window are approximately 20–30 µV m−1. This is more than sufficient to account for the observed TMR amplitudes of 2–10 µV m−1. During propagation, if the wave frequency becomes close to nfce on the forward electrostatic branch then the waves are heavily damped and no energy is transferred to the radio window. It is shown that the narrow-band frequency bandwidth is less than the source electron gyrofrequency fce, but the separation between spectral peaks across several bands is equal to or slightly greater than fce. Increasing the density gradient reduces the wave amplitudes by reducing the resonant parallel wave number so that waves no longer resonate with the most unstable part of the loss cone distribution. Therefore there is an upper limit to the density gradient for efficient generation of TMR.