Saturation of the Farley‐Buneman instability via three‐mode coupling

Abstract

Characteristics of type‐1 radar echoes obtained from the E region ionosphere have yet to be conclusively explained. The dynamical properties of the saturated state of the Farley‐Buneman instability are widely thought responsible for these type‐1 echoes. In this paper we present a different perspective and new details on a previously proposed three‐wave coupling mechanism (Otani and Oppenheim, 1998) for the saturation of the Farley‐Buneman instability. A novel method is presented for the analysis of general, three‐wave systems with stationary, sinusoidal solutions. Computer simulations of the three‐wave system produce steady states in close agreement with those obtained from the method. Despite the fact that the system only contains three modes, a number of features also agree well with observation, including density fluctuation magnitudes (∣δn∣/n0 = 5%), propagation speeds (clustered around the sound speed), and power falloff as a function of elevation angle. We demonstrate how the spatial distributions of the phases of the electron advection term and electron E × B velocity for the secondary modes lead to the partial cancellation of the destabilizing zero‐order electron drift, thereby saturating the Farley‐Buneman instability. The mechanism is consistent with the one previously advanced, which described saturation of the instability in terms of the diversion of electron flow around density peaks.