Hybrid Plasma Simulations of Farley‐Buneman Instabilities in the Auroral E‐Region

Abstract

Coupling between the magnetosphere and the high latitude ionosphere through energetic particles and electromagnetic fields results in the production of Hall currents that drive Farley-Buneman instabilities (Farley, 1963) which generate a spectrum of field-aligned plasma density irregularities (Rojas et al., 2016). Numerous studies have shown that these irregularities can modify the mean state of the ionosphere through wave heating (Bahcivan, 2007; St.Maurice, 1990). Furthermore, by affecting the local temperature, several other parameters can also be modified: plasma density, composition, conductivity, and transport. Consequently, neutral wind surges, gravity waves, and traveling atmospheric and ionospheric disturbances can be produced which can ultimately affect ionospheric stability at lower latitudes (Fuller-Rowell et al., 1994). Recently, studies have suggested that these instabilities can change the evolution of magnetospheric dynamics by changing the conductivity of the ionosphere (Wiltberger et al., 2017).Farley-Buneman waves belong to the family of two-stream instabilities. They develop at altitudes between 95 and 120 km in the auroral and equatorial E region and to lesser extent at midlatitudes (Sahr & Fejer, 1996). In these regions, due to their different mobilities and collision rates, magnetized Hall-drifting electrons induce polarization drifts on the unmagnetized ions. Because of their inertia, ions tend overshoot the polarization field recovery and accumulate in the crests of the local density irregularities faster than diffusion opposes them (Hysell et al., 2013). As a result, longitudinal density waves are formed. The propagation of these waves is nearly in the direction of the electron drifts, and their dominant wavelengths are in the order of few meters. In contrast to the equatorial case, in the auroral electrojet, wave heating plays an important role. Electric fields parallel and perpendicular (in lesser extent) to the background magnetic field have been shown to be important in explaining the heating observed in nature (Hysell, 2015). Moreover, these changes in temperature will influence the dynamics of the instabilities by changing some of the state parameters such as the ion-acoustic speed.Linear, local fluid theory of Farley-Buneman instabilities, although limited, has produced some important, verified predictions. For instance, it gives a reasonable estimate for the threshold electric field (E th ) required to trigger the instabilities: the electron convection driven by this threshold field has to be larger than the ion-acoustic speed to produce wave growth. Linear analysis of the full 5-moment system of equations has modified the initial estimates of E th to take into account the role of thermal instabilities, which also produce a change in the direction of the waves . Although linear fluid theory predicts incorrectly the rapid growth of very short wavelengths, linear kinetic theory shows that ion Landau damping effectively suppresses this growth (Schmidt & Gary, 1973). Using these line...

Fluid Models Are Probably Capable To Simulate Farley-Buneman Instabilities

“…Furthermore, the time series of both the regularized and, to a lesser extent, the unregularized simulations present an overshoot just before saturation. This behavior has been documented in hybrid (Rojas & Hysell, 2021) and fluid (Hassan et al, 2015) simulations, but it does not seem to be present in PIC simulations.…”

“…The Farley-Buneman instability is an electrostatic process with the dominant dynamics mostly restricted to a 2D plane perpendicular to B. Both the magnetized electrons and unmagnetized ions collide predominantly with neutral particles (Rojas & Hysell, 2021). Therefor, assuming both species are locally Maxwellian, the following five moment fluid system should be capable of capturing most of the important physics (Schunk & Nagy, 2009):…”

The future of auroral E-region plasma turbulence research

“…However, the model was only suited to simulations near the Farley-Buneman instability threshold. Subsequent simulations (Kovalev et al, 2009) improved on the previous results by accounting for electron thermal effects and more recent work by Rojas and Hysell (2021) presented significant advances to the numerical implementation of the hybrid continuous approach while reproducing Farley-Buneman irregularity growth, wave turning, saturation, and mode coupling.…”

Simultaneous Occurrence of Traveling Ionospheric Disturbances, Farley Buneman and Gradient Drift Instabilities Observed by the Zhongshan SuperDARN HF Radar

“…As a result, heat flow, inelastic collisions with neutrals, and heating modulation would affect the waves. We can neglect the term ( 2αn e ) in Equation 4, because its contribution to the growth rate is very small at decameter and smaller sizes (Rojas & Hysell, 2021). The unitless constant ψ is defined by:…”

“…There is a role played by ions, electrons, magnetic and electric fields to generate the E ‐region FBI and GDI. In the lower part of the ionospheric E ‐region, ions are unmagnetized, that is, ν in /Ω i > 1, where ν in is the ion‐neutral collision frequency and Ω i is the ion gyro‐frequency (Kovalev et al., 2008; Rojas & Hysell, 2021). They move with the neutral atmosphere which sometimes is assumed to be stationary (Milan & Lester, 2001, and references therein), but the electrons are magnetized, that is, ν en /Ω e ≪ 1, where ν en is the electron‐neutral collision frequency and Ω e is the electron gyro‐frequency, and their motions are mainly governed by the electric and magnetic fields (Kovalev et al., 2008; Rojas & Hysell, 2021).…”