Path‐integrated growth of electrostatic waves: The generation of terrestrial myriametric radiation

Abstract

It is generally accepted that electrostatic wave energy is the source of terrestrial myriametric radiation (TMR), but there are several theories to suggest how this energy is converted into TMR. The linear mode conversion “window” theory of Jones (1976, 1980), which in the past has been considered by some to be too inefficient to account for the observed wave amplitudes, is considered here. First, the ray tracing program HOTRAY is described. This program is used to trace electromagnetic and electrostatic waves in a hot magnetized plasma and to calculate the path‐integrated growth rates for a realistic unstable particle distribution function. A density model is constructed from wave observations made by DE 1 of an event where TMR was beamed to northern and southern latitudes from a source very close to the magnetic equator. Ray tracing shows that backward propagating electrostatic waves can refract into electromagnetic Z mode waves and transport energy to the so‐called radio window at the equator. At this point, mode conversion of energy into O mode radiation is assumed to take place. Ray tracing of O mode radiation from the radio window shows that TMR is beamed to northern and southern latitudes as observed and as predicted by the theory. Path‐integrated growth rates show that the electrostatic waves amplify by a factor ≥42 from the background fluctuation level before reaching the window. This is sufficient to account for the observed TMR wave amplitudes which require the waves to amplify by a factor ≥20. Increasing the depth of the loss cone, or increasing the hot plasma density, to within observed limits, increases the wave amplification up to a factor of 104. Strong Landau and cyclotron damping from the hot plasma component restricts the efficient transfer of energy to the radio window to within a few tenths of a degree in latitude about the magnetic equator. Thus the strongest TMR is emitted from the equator. In general, TMR beamed to northern latitudes from radio windows north of the equator is stronger than that beamed to southern latitudes; conversely, TMR beamed to northern latitudes from radio windows south of the equator is generally weaker than that beamed to southern latitudes. It is shown that electrostatic wave energy can still be transported efficiently to the radio window for magnetic field intensity variations of 1 or 2%. Thus the generation of TMR is not sensitive to variations in the magnetic field strength of this magnitude.