The evolution of the electron-beam distribution and of instabilities in the magnetized plasma of the auroral region are closely coupled. Aside from the Langmuir wave instability, the excitation of the highly obliquely propagating lower hybrid instability is of particular interest, because of the potential of ion heating perpendicular to the ambient magnetic field (ion conic generation). Nonlinear turbulence effects, flattening of the beam distribution by the initially dominant Langmuir instability and propagation effects have been invoked for a significant excitation of lower hybrid waves. In order to differentiate these effects, an analysis of wave growth for the actual evolving distribution function is necessary. A code which can evaluate the dispersion relation of a magnetized plasma for actual (usually strongly non-Maxwellian) distribution functions obtained from observation or simulation has been developed and applied to simulation studies.
The figure shows
the modification of the electron distribution by the Langmuir and lower
hybrid instability. The latter can be excited by the anomalous Doppler
effect, even after the initially Maxwellian beam (at speed vpar=1) has
been completely flattened.