Nongyrotropic Ion Distributions Research Articles

Hybrid Simulations of Positively and Negatively Charged Pickup Ions and Cyclotron Wave Generation at Europa.

In the vicinity of Europa, Galileo observed bursty Alfvén‐cyclotron wave power at the gyrofrequencies of a number of species including K+, O 2+, Na+, and Cl+, indicating the localized pickup of these species. Additional evidence for the presence of chlorine was the occurrence of both left‐hand (LH) and right‐hand (RH) polarized transverse wave power near the Cl+ gyrofrequency, thought to be due to the pickup of both Cl+ and the easily formed chlorine anion, Cl−. To test this hypothesis, we use one‐dimensional hybrid (kinetic ion, massless fluid electron) simulations for both positive and negative pickup ions and self‐consistently reproduce the growth of both LH and RH Alfvén‐cyclotron waves in agreement with linear theory. We show how the simultaneous generation of LH and RH waves can result in nongyrotropic ion distributions and increased wave amplitudes, and how even trace quantities of negative pickup ions are able to generate an observable RH signal. Through comparing simulated and observed wave amplitudes, we are able to place the first constraints on the densities of Chlorine pickup ions in localized regions at Europa.

Ion dynamics associated with Alfven wave in the near-Earth magnetotail: Two-dimensional global hybrid simulation

Ion dynamics in the near-Earth magnetotail region is examined during periods of fast Earthward flow with a two-dimensional (2-D) global-scale hybrid simulation. The simulation shows that shear Alfven waves are generated at x ∼ −10 R E, where the strong earthward flow is arrested by the dipole field, and propagate along field lines from the equator to both southern and northern polar ionosphere. Non-gyrotropic ion velocity distributions occur where the large-amplitude Alfven waves are dominant. The simulation indicates that the Alfven waves are generated by interaction of the fast earthward flow with the stationary near-Earth plasma. Beam ions are found to be pitch-angle scattered and trapped in the wave field, leading to the non-gyrotropic ion distributions in the high-latitude plasma sheet boundary. In addition, significant particle heating and acceleration are found to occur behind the dipolarization front due to the effect of wave turbulence.

Wave‐particle interactions associated with nongyrotropic distribution functions: A hybrid simulation study

Nongyrotropic plasma distribution functions can be formed in regions of space where guiding center motion breaks down as a result of strongly curved and weak ambient magnetic fields. Such are the conditions near the current sheet in the Earth's middle and distant magnetotail, where observations of nongyrotropic ion distributions have been made [Frank et al., 1994]. Here a systematic parameter study of nongyrotropic proton distributions using electromagnetic hybrid simulations is made. We model the observed nongyrotropic distributions by removing a number of arc length segments from a cold ring distribution and find significant differences with the results of simulations that initially have a gyrotropic ring distribution. Model nongyrotropic distributions with initially small perpendicular thermalization produce growing fluctuations that diffuse the ions into a stable Maxwellian‐like distribution within a few proton gyro periods. The growing waves produced by nongyrotropic distributions are similar to the electromagnetic proton cyclotron waves produced by a gyrotropic proton ring distribution in that they propagate parallel to the background magnetic field and occur at frequencies on the order of the proton gyrofrequency. The maximum energy of the fluctuating magnetic field increases as the initial proton distribution is made more nongyrotropic, that is, more highly bunched in perpendicular velocity space. This increase can be as much as twice the energy produced in the gyrotropic case.

Cross‐field diffusion of charged particles and the problem of ion injection and acceleration at quasi‐perpendicular shocks

It is an open question how charged particles are injected at quasiperpendicular shocks into a first‐order Fermi acceleration mechanism. Crossfield diffusion of solar wind ions is a possible injection process. However, in a system with at least one ignorable spatial dimension, charged particles moving in fluctuating fields are tied to the magnetic field lines. We have therefore determined the cross‐field diffusion coefficient of charged particles in self‐consistently generated turbulence by three‐dimensional hybrid simulations. The initial setup consists of a homogeneous magnetic field with an isotropic core plasma plus a second, nongyrotropic ion distribution. The combined distributions resemble the distribution found immediately downstream of the quasi‐perpendicular Earth bow shock: Part of the solar wind is transmitted (core) and part is specularly reflected and subsequently convected downstream (nongyrotropic part). Such a particle distribution excites the Alfvén ion cyclotron and mirror mode instability. The turbulence scatters the nongyrotropic ions both parallel and perpendicular to the field. The perpendicular and the parallel diffusion coefficients have been determined for two values of the density of the nongyrotropic distributions, nb. The ratio of the two diffusion coefficients is smaller than the value predicted by hard sphere scattering theory, i.e., parallel scattering is considerably stronger than scattering perpendicular to the field. The power in the magnetic field fluctuations in the high nb case is comparable to the power obtained in a two‐dimensional quasiperpendicular shock simulation immediately behind the shock ramp. On the basis of perpendicular scattering time it is suggested that cross‐field diffusion in the turbulent wave field generated by the specularly reflected ions is sufficient to inject and accelerate these ions efficiently at quasi‐perpendicular shocks.

Non-gyrotropic ions as evidence for an X-type neutral region

Based on modern satellite observations in the earth's magnetotail and advanced computer simulation studies, it becomes possible to discuss demonstratively a kinetic magnetic reconnection process beyond ion inertia scale. One of the important findings is the observation of non-Maxwellian/non-gyrotropic ion distribution functions in the magnetotail. We discuss that a specific kind of non-gyrotropic ion distribution can be observed in the vicinity of an X-type neutral region.

Nongyrotropy in magnetoplasmas: simulation of wave excitation and phase-space diffusion

Abstract. Nongyrotropic (gyrophase bunched) ion distributions in a magnetoplasma are studied by analytical methods and by two-dimensional hybrid code simulations. Nongyrotropy may not occur in a plasma being simultaneously homogeneous, stationary, and solenoidal in phase space. A detailed study is performed for a homogeneous and stationary plasma with sources and sinks in phase space. The analytical investigation cast in the framework of linearized Maxwell-Vlasov theory yields a coupling of low-frequency left-handed, right-handed, and longitudinal modes. Nongyrotropic ion distributions are unstable; they excite left-handed waves. The growth rate is comparable to that of the ion ring instability. The hybrid code simulation study confirms the expected propagation direction parallel to the background magnetic field. Three diffusion processes are studied: arc lengthening, arc broadening, and arc radius decreasing corresponding to particle energy diffusion. The characteristic diffusion time-scales are found to be of the order of 101 wave cycles.

Ion dynamics of magnetic reconnection in the magnetotail

We discuss the kinetic processes of plasma thermalization, acceleration, and mixing in magnetic reconnection. Non-Maxwellian, gyrotropic ion distribution functions such as anisotropic ion beams in the plasma sheet boundary layer (PSBL) and counter-streaming ions (CSIs) in the plasma sheet are often observed during a plasmoid passage of a satellite in the Earth's magnetotail. Non-gyrotropic ion distribution functions are also sometimes observed just after the passage of the plasmoid. We study the behavior of non-Maxwellian ion distribution functions observed by GEOTAIL. We further study theoretically the ion dynamics by using a particle-in-cell simulation, and discuss the role of non-Maxwellian distribution functions in magnetic reconnection.

Wave excitation due to non-gyrotropic ion distribution by 2-d hybrid code plasma simulation: Time scales & properties

Wave excitation due to non-gyrotropic ion distribution by 2-d hybrid code plasma simulation: Time scales & properties