Dan Winske

A cross‐field current instability for substorm expansions

We investigate a cross-field current instability (CFCI) as a candidate for current disruption during substorm expansions. The numerical solution of the linear dispersion equation indicates that (1) the proposed instability can occur at the inner edge or the midsection of the neutral sheet just prior to the substorm expansion onset although the former environment is found more favorable at the same drift speed scaled to the ion thermal speed, (2) the computed growth time is comparable to the substorm onset time, and (3) the excited waves have a mixed polarization with frequencies near the ion gyrofrequency at the inner edge and near the lower hybrid frequency in the midtail region. On the basis of this analysis we propose a substorm development scenario in which plasma sheet thinning during the substorm growth phase leads to an enhancement in the relative drift between ions and electrons. This results in the neutral sheet being susceptible to the CFCI and initiates the diversion of the cross-tail current through the ionosphere. Whether or not a substorm current wedge is ultimately formed is regulated by the ionospheric condition. A large number of substorm features can be readily understood with the proposed scheme. These include (1) precursory activities (pseudobreakups) prior to substorm onset, (2) substorm initiation region to be spatially localized, (3) three different solar wind conditions for substorm occurrence, (4) skew towards evening local times for substorm onset locations, (5) different acceleration characteristics between ions and electrons, (6) tailward spreading of current disruption region after substorm onset, and (7) local time expansion of substorm current wedge with possible discrete westward jump for the evening expansion.

The proton cyclotron instability and the anisotropy/β inverse correlation

Spacecraft observations in the strongly compressed subsolar magnetosheath show an inverse correlation between the proton temperature anisotropy (T{sub {perpendicular}p}/T{sub {parallel}p} > 1 where {perpendicular} and {parallel} denote directions perpendicular and parallel to the background magnetic field) and the parallel proton {beta}({beta}{sub {parallel}p}). This manuscript uses one-dimensional hybrid simulations of the proton cyclotron anisotropy instability in homogeneous electron-proton plasmas to study this correlation which may represent a limited closure relation for fluid theories of anisotropic space plasmas. The emphasis is on driven simulations which increase the temperature anisotropy by periodically reducing the magnetic-field-aligned velocities of the protons. The late-time states from ensembles of both initial value and driven simulations yield very similar expressions for the proton anisotropy/{beta}{sub {parallel}p} inverse correlation, and provide a basis for explaining differences between sheath observations from different spacecraft. The driven simulations also yield expressions for the maximum instability growth rate and the fluctuating field energy as functions of {beta}{sub {parallel}p} and a parameter characterizing the anisotropy driver. 50 refs., 5 figs.

Kinetic simulations of the Kelvin‐Helmholtz instability at the magnetopause

Two-dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the kinetic evolution of the Kelvin-Helmholtz instability for a magnetopauselike configuration. The unidirectional magnetic field is essentially transverse to the plasma flow velocity, which is the most unstable case according to linear theory and models the flow dynamics in the subsolar region of the magnetopause for northward interplanetary magnetic field. We recover effects analogous to those found in MHD simulations, including a mode cascade to longer wavelengths. The boundary layer consists of coherent structure and is not well described by a diffusive process. Isolated structures on the order of the ion gyroradius are formed which can cross the boundary in either direction. We describe how the time evolution of these structures represents transport across boundary layers, and we consider the possible connection of these entities to flux transfer events and other structure seen in the low-latitude boundary layer at the Earths magnetopause as well as to flux ropes commonly observed near the ionopause of Venus. We also discuss the relation of the hybrid calculations to previous MHD simulations and to observations.

Electromagnetic proton cyclotron instability: Interactions with magnetospheric protons

The temperature anisotropy of the hot (keV) protons of the outer magnetosphere can excite the electromagnetic proton cyclotron instability. Wave-particle scattering by this instability not only maintains the hot proton anisotropy at or below an upper bound but also imparts energy to the cool (eV) proton component. This instability and its consequences are examined through the use of linear and second-order Vlasov theory and one-dimensional hybrid simulations in a homogeneous plasma model which represents these two protonic constituents. A different form for the upper bound on the hot proton anisotropy is derived from linear theory and the simulations; comparison against plasma observations from Los Alamos magnetospheric plasma analyzers in geosynchronous orbit shows very good agreement. Second-order theory and simulations are used to obtain a different scaling for the late-time apparent temperature of the cool proton component Tc which indicates that it is inversely correlated with the cool proton density nc. Observations in both the plasmasphere and the outer magnetosphere sometimes show a similar inverse correlation, suggesting that the electromagnetic proton cyclotron instability is an important contributor to cool proton heating in these regions.

Hybrid simulation of the formation of a hot flow anomaly

Results are presented from two-dimensional hybrid simulations of the interaction of a supercritical quasi-perpendicular collisionless shock wave with current sheets embedded in the upstream flow. The current sheet normals are perpendicular to the shock normal. When the motional electric field is directed toward the discontinuity, the interaction leads to the generation of a region with high ion temperature, low magnetic field, and low density, which has many properties of a class of events variously referred to as active current sheets, hot diamagnetic cavities, or hot flow anomalies (HFAs). The simulations demonstrate that the HFA results from the interaction of ions reflected at the shock with the current sheet. The interaction does not involve an instability and is not caused by the collision of the current sheet and the shock but rather is a property of a shock with an embedded current sheet. The simulations suggest that the HFAs formed in this way are always attached to the shock and extend into both the upstream and downstream regions.

Proton temperature anisotropy upper bound

The electromagnetic proton cyclotron instability and the mirror instability are driven by the proton temperature anisotropy T⊥p/T‖p > 1, where ⊥ and ‖ denote directions relative to the background magnetic field. Linear theory and one-dimensional hybrid simulations imply that the former mode grows more rapidly over 0.05 ≤ β‖p ≤ 5 and that wave-particle scattering by its enhanced fluctuations imposes an upper bound on the temperature anisotropy of the form where and B0 is the background magnetic field. Here Sp and αp are fitting parameters, and . This paper describes results from more general two-dimensional hybrid simulations, which permit both instabilities to grow simultaneously. These simulations confirm the one-dimensional results on the initial domain ; enhanced fluctuations display the properties of the proton cyclotron instability and αp ≃ 0.4. On this domain the two-dimensional simulations also yield an upper bound for the fluctuating field energy density of the form with fitting parameter . The simulations on the initial domain 10 ≤ β‖p≤100 show spectral characteristics of both instabilities and exhibit a more stringent bound on the proton anisotropy, in agreement with observations in the terrestrial magnetosheath.

Electron dissipation in collisionless magnetic reconnection

A study of the electron dynamics in the dissipation region of collisionless magnetic reconnection is presented. The investigation is based on a new 2.5-dimensional electromagnetic particle-in-cell simulation code. This code is applied to the problem of reconnection in two differently sized current sheets: one with a thickness of the ion inertial length and the other with electron inertial length thickness. The complete set of contributions to the reconnection electric field is calculated directly from the particle information. The two cases lead to quite different results. In the ion scale, sheet reconnection is significantly slower, and the dissipation is provided virtually exclusively by electron quasi-viscous effects. The electron scale sheet reconnects much faster, involving a bifurcation of the reconnection region and the formation of a magnetic island. In this latter case, dissipation appears to be primarily provided by electron inertial effects and here foremost by bulk electron acceleration. Finally, an attempt to represent the effects of electron pressure-based dissipation in a transport model is presented also.

Collisionless reconnection supported by nongyrotropic pressure effects in hybrid and particle simulations

This paper presents the detailed comparative analysis of full particle and hybrid simulations of collisionless magnetic reconnection. The comprehensive hybrid simulation code employed in this study incorporates essential electron kinetics in terms of the evolution of the full electron pressure tensor in addition to the full ion kinetics and electron bulk flow inertia effects. As was demonstrated in our previous publications, the electron nongyrotropic pressure effects play the dominant role in supporting the reconnection electric field in the immediate vicinity of the neutral X point. The simulation parameters are chosen to match those of the Geospace Environmental Modeling (GEM) “Reconnection Challenge.” It is that these comprehensive hybrid simulations perfectly reproduce the results of full particle simulations in many details. Specifically, the time evolutions of the reconnected magnetic flux and the reconnection electric field, as well as spatial distributions of current density and magnetic field at all stages of the reconnection process, are found to be nearly identical for both simulations. Comparisons of variations of characteristic quantities along the x and z axes centered around the dominating X points also revealed a remarkable agreement. Noticeable differences are found only in electron temperature profiles, i.e., in the diagonal electron pressure tensor components. The deviation in the electron heating pattern in hybrid simulations from that observed in particle simulations, however, does not affect parameters essential for the reconnection process. In particular, the profiles of the off-diagonal components of the electron pressure tensor are found to be very similar for both runs and appear unaffected by heat flux effects. Both simulations also demonstrate that the Ey component of the electric field is nearly constant inside the diffusion region where ions are nonmagnetized. We demonstrate that the simple analytical estimate for the reconnection electric field as a convection electric field at the edge of the diffusion region very well reproduces the reconnection electric field observed in the simulations.

Nonlinear evolution of the lower‐hybrid drift instability

The results of simulations of the lower‐hybrid drift instability in a neutral sheet configuration are described. The simulations use an implicit formulation to relax the usual time step limitations and thus extend previous explicit calculations to weaker gradients, larger mass ratios, and long times compared with the linear growth time. The numerical results give the scaling of the saturation level, heating rates, resistivity, and cross‐field diffusion and a demonstration by comparison with a fluid electron model that dissipation in the lower‐hybrid drift instability is caused by electron kinetic effects.

Proton resonant firehose instability: Temperature anisotropy and fluctuating field constraints

The electromagnetic proton firehose instability may grow in a plasma if the proton velocity distribution is approximately bi-Maxwellian and T‖p > T⊥p, where the directional subscripts denote directions relative to the background magnetic field. Linear Vlasov dispersion theory in a homogeneous electron-proton plasma implies an instability threshold condition at constant maximum growth rate 1 − T⊥p/T‖p = Sp/β‖pαp over 1 < β‖p ≤ 10 where and Bo is the background magnetic field. Here Sp and αp are fitting parameters and αp ≃ 0.7. One- and two-dimensional initial value hybrid simulations of this growing mode are carried out under proton cyclotron resonant conditions in a homogeneous plasma on the initial domain 2 ≲ β‖p ≤ 100. The two-dimensional simulations show that enhanced fluctuations from this instability impose a bound on the proton temperature anisotropy of the form of the above equation with the fluid theory result αp ≃ 1.0. On this domain both one- and two-dimensional simulations yield a new form for the upper bound on the fluctuating field energy density from the proton resonant firehose instability where SB and αB are empirical parameters which are functions of the initial growth rate. This logarithmic behavior is qualitatively different from a fluid theory prediction and, like the anisotropy bound, should be subject to observational verification in any sufficiently homogeneous plasma in which the proton velocity distribution is approximately bi-Maxwellian.