New insights from a nonlocal generalization of the Farley-Buneman instability problem at high latitudes

Abstract

Abstract. When their growth rate becomes too small, the Eregion Farley-Buneman and gradient-drift instabilities switch from absolute to convective. The neutral density gradient is what gives the instabilities their convective character. At high latitudes, the orientation of the neutral density gradient is close to the geomagnetic field direction. We show that this causes the wave-vector component along the geomagnetic field to increase with time. This in turn leads to wave stabilization, since the increase goes hand-in-hand with an increase in parallel electric fields that ultimately short-circuits the irregularities. We show that from an equivalent point of view, the increase in the parallel wave vector is accompanied by a large upward group velocity that limits the time during which the perturbations are allowed to grow before escaping the unstable region. The goal of the present work is to develop a systematic formalism to account for the propagation and the growth/decay of high-latitude Farley-Buneman and gradient-drift waves through vertical convective effects. We note that our new formalism shies away from a plane wave decomposition along the magnetic field direction. A study of the solution to the resulting nonlinear aspect angle equation shows that, for a host of initial conditions, jump conditions are often triggered in the parallel wave-vector (defined here as the vertical derivative of the phase). When these jump conditions occur, the waves turn into strongly damped ionacoustic modes, and their evolution is quickly terminated. We have limited this first study to Farley-Buneman modes and to a flow direction parallel to the electron E × B drift. Our initial findings indicate that, irrespective of whether or not a jump in aspect angle is triggered by initial conditions, the largest amplitude modes are usually near the ion-acoustic speed of the medium (although Doppler shifted by the ion motion), unless the growth rates are small, in which case the waves tend to move at the same drift as the ambient electrons.

A first comparison of irregularity and ion drift velocity measurements in the E-region

“…Hamza and St-Maurice, 1995). Of special interest for this study is the recent work by Drexler and St.-Maurice (2005) who proposed that what appears as large aspect angle echoes in the HF radar data can in fact be successfully interpreted using the nonlocal formulation of Drexler et al (2002). Furthermore, Drexler and St.-Maurice (2005) have derived a formula for the phase velocity (rather than simply adopting the one from the linear theory) that only contains the ion drift term, thus providing a theoretical basis for the interpretation used in Milan et al (2004).…”

“…Furthermore, Drexler and St.-Maurice (2005) have derived a formula for the phase velocity (rather than simply adopting the one from the linear theory) that only contains the ion drift term, thus providing a theoretical basis for the interpretation used in Milan et al (2004). Even though the nonlocal theory of Drexler et al (2002) and Drexler and St.-Maurice (2005) strictly speaking applies only to small flow angles, it provides additional insights as to why irregularities at large flow and aspect angles are observed at all as well as why their Doppler velocities appear to be close to that of the ion motion in the E-region.…”

A possible origin for large aspect angle "HAIR'' echoes seen by SuperDARN radars in the E region

“…2.1 Nonlocal generalization of the dispersion equation Drexler et al (2002) have presented a nonlocal description of Farley-Buneman waves based on a standard linearization procedure which can be found in many papers (e.g. Sudan, 1983).…”

The influence of non-isothermal electrons and neutral wind structures on the doppler properties of vertical m-size field-aligned irregularities in the low latitude E-region

“…A problem with this mechanism, however, is that the diffusion is from the electron drifts and therefore at right angles to the direction needed for diffusion to affect the wave amplitude (St.-Maurice, 1990). Alternatively, a monotonic growth in the aspect angle has been shown by Drexler et al (2002) to be inevitable at high latitudes and to certainly be able to affect slow growing modes by not only limiting their amplitude but also by eradicating them (through damping by large aspect angles). However, for fast growing m-size modes, the growth in aspect angles should normally be too slow to matter.…”