William Daughton

Electromagnetic properties of the lower-hybrid drift instability in a thin current sheet

The linear and nonlinear properties of the lower-hybrid drift instability are examined in a thin current sheet with thickness comparable to a thermal ion gyroradius ρi∼L. The linear Vlasov stability is calculated using a formally exact technique in which the orbit integrals are treated numerically and the eigenvalue problem for the resulting system of integrodifferential equations is solved using a finite element representation of the eigenfunction. For the fastest growing lower-hybrid modes with wavelength on the electron gyroscale (kyρe∼1), the resulting mode structure is localized on the edge of the current sheet. However, for modes with wavelengths intermediate between the electron and ion gyroscale kyρiρe∼1, the lower-hybrid instability has a significant electromagnetic component to the mode structure which is localized in the central region of the sheet. The addition of a weak guide field complicates the mode structure and gives rise to fluctuations in all three components of the magnetic field. The...

Coherent structures, intermittent turbulence, and dissipation in high-temperature plasmas

An unsolved problem in plasma turbulence is how energy is dissipated at small scales. Particle collisions are too infrequent in hot plasmas to provide the necessary dissipation. Simulations either treat the fluid scales and impose an ad hoc form of dissipation (e.g., resistivity) or consider dissipation arising from resonant damping of small amplitude disturbances where damping rates are found to be comparable to that predicted from linear theory. Here, we report kinetic simulations that span the macroscopic fluid scales down to the motion of electrons. We find that turbulent cascade leads to generation of coherent structures in the form of current sheets that steepen to electron scales, triggering strong localized heating of the plasma. The dominant heating mechanism is due to parallel electric fields associated with the current sheets, leading to anisotropic electron and ion distributions which can be measured with NASAs upcoming Magnetospheric Multiscale mission. The motion of coherent structures also generates waves that are emitted into the ambient plasma in form of highly oblique compressional and shear Alfven modes. In 3D, modes propagating at other angles can also be generated. This indicates that intermittent plasma turbulence will in general consist of both coherent structures and waves. However, the current sheet heating is found to be locally several orders of magnitude more efficient than wave damping and is sufficient to explain the observed heating rates in the solar wind.

The unstable eigenmodes of a neutral sheet

The linear stability of a Harris current sheet is examined using the Vlasov description for both ions and electrons. Orbit integrals are treated numerically using the exact particle orbits and including the global structure of the perturbation inside the integral. Both electromagnetic and electrostatic contributions to the field perturbation are retained and the eigenvalue problem for the system of integro-differential equations is solved using a Hermite expansion of the eigenfunction. For the tearing mode, results are in excellent agreement with established theory. For the recently discovered kink mode, results are consistent with kinetic simulations at low mass ratio mi/me⩽16. However, in the limit of realistic electron mass, the growth rate of the kink mode is substantially reduced in contrast to results from kinetic simulations. It is demonstrated that a background population may dramatically alter the growth rate of the kink mode at realistic values of the mass ratio. This result may have relevance t...

Collisionless magnetic reconnection in the presence of a guide field

The results of kinetic simulations of magnetic reconnection in Harris current sheets are analyzed. A range of guide fields is considered to study reconnection in plasmas characterized by different β values, β>me/mi. Both an implicit particle-in-cell (PIC) simulation method and a parallel explicit PIC code are used. Simulations with mass ratios up to the physical value are performed. The simulations show that the reconnection rate decreases with the guide field and depends weakly on the mass ratio. The off-diagonal components of the electron pressure tensor break the frozen-in condition, even in low β plasmas. In high β plasmas, evidence is presented that whistler waves play a key role in the fast reconnection physics, while in low β plasmas the kinetic Alfven waves are important. The in-plane and the out-of-plane ion and electron motion are also considered, showing that they are influenced by the mass ratio and the plasma β.

The link between shocks, turbulence, and magnetic reconnection in collisionless plasmas

Global hybrid (electron fluid, kinetic ions) and fully kinetic simulations of the magnetosphere have been used to show surprising interconnection between shocks, turbulence, and magnetic reconnection. In particular, collisionless shocks with their reflected ions that can get upstream before retransmission can generate previously unforeseen phenomena in the post shocked flows: (i) formation of reconnecting current sheets and magnetic islands with sizes up to tens of ion inertial length. (ii) Generation of large scale low frequency electromagnetic waves that are compressed and amplified as they cross the shock. These “wavefronts” maintain their integrity for tens of ion cyclotron times but eventually disrupt and dissipate their energy. (iii) Rippling of the shock front, which can in turn lead to formation of fast collimated jets extending to hundreds of ion inertial lengths downstream of the shock. The jets, which have high dynamical pressure, “stir” the downstream region, creating large scale disturbances ...

Electromagnetic proton/proton instabilities in the solar wind

Electromagnetic proton/proton instabilities are excited by the relative streaming parallel to the magnetic field of two distinct proton components, the more dense core and the more tenuous beam. Here linear Vlasov theory is used to study these instabilities in a homogeneous plasma model. Under conditions often observed in the high-speed solar wind, both magnetosonic and Alfven modes become proton/proton unstable; for various parameter domains there are two unstable regimes of the magnetosonic mode and three unstable regimes of the Alfven mode. In a dimensionless parameter model representing typical high-speed solar wind conditions, the most strongly unstable modes are the magnetosonic instability with maximum growth rate in the direction of the background magnetic field and the Alfven mode at propagation oblique to that field. Although the former mode has been regarded by several previous researchers to be the dominant proton/proton instability in the solar wind, the results described here indicate that the strongly unstable regime of the Alfven mode, which previously has not been studied in a solar wind model, has the lower threshold at sufficiently large beam density and/or sufficiently small core β. Earlier studies of proton/proton instabilities in the solar wind based on the assumption that the magnetosonic mode is most important may need reconsideration.

Formation of hard power laws in the energetic particle spectra resulting from relativistic magnetic reconnection.

Using fully kinetic simulations, we demonstrate that magnetic reconnection in relativistic plasmas is highly efficient at accelerating particles through a first-order Fermi process resulting from the curvature drift of particles in the direction of the electric field induced by the relativistic flows. This mechanism gives rise to the formation of hard power-law spectra in parameter regimes where the energy density in the reconnecting field exceeds the rest mass energy density σ ≡ B(2)/(4πnm(e)c(2))>1 and when the system size is sufficiently large. In the limit σ ≫ 1, the spectral index approaches p = 1 and most of the available energy is converted into nonthermal particles. A simple analytic model is proposed which explains these key features and predicts a general condition under which hard power-law spectra will be generated from magnetic reconnection.

Kinetic theory of the drift kink instability in a current sheet

The linear stability of a current sheet is of interest in connection with the dynamics of the magnetotail and in particular as a triggering mechanism for substorms. Until recently, most theoretical work in this area has concentrated on the collisionless tearing mode. Recent simulations suggest that for thin current sheets another long wavelength electromagnetic mode, the so-called drift kink instability, may also be of importance. The linear stability analysis for a Harris-type equilibrium is formulated using a kinetic description for both ions and electrons. The orbit integrals are treated numerically using the exact unperturbed particle orbits and including the global structure of the perturbation inside the integral. It is found that the drift kink has a significant electrostatic component which strongly alters the mode structure and real frequency relative to the case of a purely electromagnetic mode. The resulting growth rates, mode structure and parametric dependences are presented and compared with previous results. The effect of finite parallel wavelength is considered, and the relevance of the drift kink mode to the magnetotail is discussed.

PARTICLE ACCELERATION AND PLASMA DYNAMICS DURING MAGNETIC RECONNECTION IN THE MAGNETICALLY DOMINATED REGIME

Magnetic reconnection is thought to be the driver for many explosive phenomena in the universe. The energy release and particle acceleration during reconnection have been proposed as a mechanism for producing high-energy emissions and cosmic rays. We carry out two- and three-dimensional kinetic simulations to investigate relativistic magnetic reconnection and the associated particle acceleration. The simulations focus on electron-positron plasmas starting with a magnetically dominated, force-free current sheet (

Electromagnetic proton/proton instabilities in the solar wind: Simulations

\sigma \equiv B^2/(4\pi n_e m_e c^2) \gg 1