Niels F. Otani

Saturation of the Farley-Buneman instability via nonlinear electron E×B drifts

The Farley-Buneman instability is a collisional two-stream instability observed in the E region ionosphere at altitudes in the range 90–120 km. While linear theory predicts the dominant wavelengths, it cannot fully describe the behavior of this nonlinearly saturated instability, as observed by radar and rocket measurements. This paper explores the nonlinear behavior of this phenomenon and the resulting waves through simulations and theory. Our two-dimensional simulations model wave behavior in the plane perpendicular to the Earths magnetic field, applying a fluid model to describe the electron dynamics and either a particle or a fluid model to describe ion behavior. The results show the growth, saturation, and nonlinear behavior of the instability for a much longer period of time than was possible with the pure particle codes used in previous studies. These simulations show (1) growth of Farley-Buneman waves, (2) the development of secondary waves which propagate along the extrema and perpendicular to the Farley-Buneman waves, (3) turning of the primary waves away from the electron drift direction, (4) a saturated wave phase velocity below the one predicted by linear theory but above the acoustic speed and (5) nonlinear electron E×Bo drifting dominates the behavior of the saturated waves. This paper describes both the simulation techniques and fundamental results. Additionally, this paper outlines a theory explaining the dominant nonlinear process seen in this instability.

Current sheets in the solar corona

Coronal magnetic fields are twisted up by motion of their footpoints in the photosphere. When the twist exceeds a critical amount, kink-ballooning instabilities occur. These instabilities are studied numerically, in long, thin, axially bounded magnetic fields. Nonlinearly, the three-dimensional kinking motion compresses magnetic flux, forming a current sheet. Magnetic energy can be dissipated at a rate orders of magnitude greater than without the current sheets. The energy of footpoint motion can then go into coronal heating, via Ohmic dissipation in the current sheets. 33 references.

A saturation mechanism for the Farley-Buneman instability

Studies with a reduced-mode two-fluid model have revealed a promising candidate for the saturation mech- anism of the Farley-Buneman instability in the daytime equatorial electrojet. The mechanism operates by redis- tributing the zero-order electron E B flow eld. Sec- ondary waves generated nonlinearly by the instability are responsible for the flow eld modication. Saturation oc- curs because the modied flow reduces the principal charge transport responsible for the growth of the primary wave. Two-dimensional particle simulations of the instability ex- hibit saturation via the same mechanism.

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 95–110 km. While linear theory predicts the dominant wavelengths, it cannot fully describe the behavior of this nonlinearly saturated instability as observed by radar and rocket measurements. We simulate the behavior of this instability in the plane perpendicular to the Earths magnetic field, using a two-dimensional hybrid code which models electron dynamics 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 the pure particle codes used in previous studies. This paper describes the spectra from these simulations and compares them to the observed spectra. Both the simulations and observations show that (1) type I spectra result from saturated two-stream waves for a broad range of elevation angles, (2) the phase velocity of these waves is below that predicted by linear theory, (3) mode coupling leads to type II-like spectra without the presence of a plasma density gradient as often thought necessary, (4) longer wavelengths due to mode coupling develop, and (5) spectral power decreases at a rate of 0.3 dB/degree of elevation angle.

Hybrid simulations of the saturated Farley-Buneman instability in the ionosphere

Numerical simulations of the Farley-Buneman instability in 2–1/2 dimensions using particle ions and fluid electrons show the growth, saturation and nonlinear behavior of two-stream waves. This hybrid technique models the saturated state of the instability for a much longer period of time than the pure particle codes that preceded it. While focusing principally on modeling the topside E region equatorial electrojet, many of these results apply to the auroral two-stream instability as well. The following features are seen in all our hybrid simulations: (1) wave growth at an angle offset from the electron drift direction where the angle depends on the strength of the driving electric field, (2) nonlinear coupling to waves traveling perpendicular to the propagation direction of the principal two-stream waves, (3) a saturated wave phase velocity at or above the sound speed but well below the velocity predicted by linear theory and (4) phase velocities which remain almost constant as a simulated radar sweeps from a horizontal direction to nearly vertical. The nonlinear electron motion dominates the behavior of these waves. Further, these simulations indicate that ion kinetic effects are not essential for the saturation of the instability and that electron temperature effects have a minor impact on the final saturated state.

Computer simulation of Alfven waves and double layers along auroral magnetic field lines

A plasma simulation has been developed to model interactions between inertial Alfven waves and double layers and to investigate their relative contributions to auroral particle acceleration. We use a novel one-dimensional particle-in-cell code, with periodic boundary conditions, to model the nonlinear excitation of current-driven weak double layers via the free energy supplied by an inertial Alfven wave. Analysis of the simulation output shows that double layers are not the agent primarily responsible for electron acceleration. Rather, the inertial Alfven wave accelerates groups of electrons into a steepening beam as it encounters them. As the beam electrons reenter the main distribution, decelerated by anomalous resistive effects, they are replaced by electrons farther downstream. Hence, the particles do not free-stream over the length of the channel. Furthermore, this wave action persists even when the system is linearly stable to ion-acoustic modes, precluding the possibility that this behavior is brought about by the formation of ion-acoustic double layers.

Heating of the solar wind by pickup ion driven Alfvén ion cyclotron instability

Pickup ions in a ring velocity distribution are unstable to several kinetic plasma instabilities. At large heliocentric distances where the overall plasma β (ratio of kinetic to magnetic energy) is dominated by the energy density of interstellar pickup ions and pickup is perpendicular to the interplanetary magnetic field, the dominant of these is the Alfven ion cyclotron instability (AIC). We demonstrate by hybrid particle simulation that, for conditions where the solar wind β is low, AIC driven by the pickup ions couples to the solar wind. The result is perpendicular heating, leading to an anisotropic solar wind distribution. This process may contribute to enhanced solar wind temperatures at large heliocentric distances and may allow for indirect measurement of interstellar pickup ions.

Four‐field model for dispersive field‐line resonances: Effects of coupling between shear‐Alfvén and slow modes

A new theoretical model is proposed for dispersive field-line resonances in collisionless magnetospheric plasmas on the basis of reduced four-field equations. The model improves upon the predictive capabilities of earlier two-field models. In particular, due to the coupling of the shear-Alfven mode to the slow mode in the four-field system, it is now possible to account for the observed low frequencies of field-line resonances. Furthermore, parallel electric fields can be large without requiring the field-aligned current density to be unrealistically large. Qualitative implications for recent FAST and ground-based observations are discussed.

A unified model of acoustic and lattice waves in a one-dimensional strongly coupled dusty plasma

An exact dispersion relation is obtained for linear dust-compressional waves in a one-dimensional plasma crystal consisting of cold dust particles located at lattice points uniformly spaced in equilibrium. This general dispersion relation reduces in asymptotic limits to the dust-acoustic and dust-lattice wave dispersion relations. Implications for experiments are discussed.

Current-driven resistive ballooning modes in axially bounded solar flare plasmas

The most unstable current-driven resistive modes of an axially bounded coronal loop are found in computer simulations to exhibit the spatial structure of ballooning modes. The observed modes are not confined to mode rational surfaces, but instead have broad radial extent. A theory assuming ballooning mode spatial structure predicts that a minimum current should be required for linear instability, and that, when the mode is unstable, the linear growth rate scales linearly with the resistivity eta below a critical resistivity, and scales as cu root of eta for larger resistivities. Both predictions are borne out by simulation results. Both theory and simulation analyses of the mode suggest that the strong radial structure of the mode near the ends of the system is the primary contributing factor to the instability of the mode. A helical current sheet is formed in the nonlinear evolution of the mode near the edge of the current channel and is accompanied by a strong radial gradient in the current and partial current reversal. 17 references.