SPECTRAL CHARACTERISTICS OF THE FARLEY-BUNEMAN INSTABILITY - SIMULATIONS VERSUS OBSERVATIONS

The Farley-Buneman instability is a collisional two-stream instability observed in the E region ionosphere at altitudes in the range of 95-1 10 km. While linear theory predicts the dominant wavelengths, it canno t fully describe the behavior of this nonlinearly saturated instabilit y as observed by radar and rocket measurements. We simulate the behavi or of this instability in the plane perpendicular to the Earth's magne tic field, using a two-dimensional hybrid code which models electron d ynamics as a fluid and ion dynamics with a particle-in-cell approach. The results show the growth, saturation, and nonlinear behavior of the instability for a much longer period of time than was possible with t he pure particle codes used in previous studies. This paper describes the spectra from these simulations and compares them to the observed s pectra. Both the simulations and observations show that (1) type I spe ctra result from saturated two-stream waves for a broad range of eleva tion angles, (2) the phase velocity of these waves is below that predi cted by linear theory, (3) mode coupling leads to type II-like spectra without the presence of a plasma density gradient as often thought ne cessary, (4) longer wavelengths due to mode coupling develop, and (5) spectral power decreases at a rate of 0.3 dB/degree of elevation angle .