J. Y. Lu

Spatiotemporal characteristics of ultra-low frequency dispersive scale shear Alfvén waves in the Earth’s magnetosphere

The high-latitude nightside auroral zone is threaded by geomagnetic field lines supporting localized ultra-low frequency (ULF) shear Alfven waves (SAWs). A particular class of dispersive scale ULF waves can be attributed to resonant mode conversion of global scale near-monochromatic compressional waves that are excited by high-speed solar wind flow past the magnetosphere. In the more distant magnetotail, warm plasma dispersive effects preclude the excitation of latitudinally narrow waveforms. Closer to Earth, flux tubes are loaded with colder plasma that favors inertial scale dispersive SAWs and nonlinear wave processes. The spatiotemporal characteristics of ULF shear Alfven field line resonances near midnight are analyzed, and it is demonstrated that in warm plasma, small-scale nonlinear structuring occurs on field lines where two competing wave dispersion mechanisms cancel. Over realistic time scales, this situation occurs naturally as a result of SAW ponderomotive forces that initiate up-flowing ion mo...

Test kinetic modelling of collisionless perpendicular shocks

Test kinetic simulation results are presented for perpendicular collisionless shocks in magnetized plasmas that are representative of the Earths bow shock. In this approach, particle kinetics are described by tracing particle trajectories in prescribed electromagnetic fields obtained in the MHD approximation, and applying Liouvilles theorem. This provides a first-order description of particle dynamics in complex systems, given approximate fields obtained with a low-level description of the plasma. The method also provides a useful consistency check in assessing the validity of approximate solutions such as those obtained in the ideal MHD approximation. Compared with the more familiar test particle approach, in which trajectories of randomly injected particles are followed in time, the present approach has the advantage of being numerically more efficient, and producing results without statistical errors.

The Transport of Charged Particles in a Flowing Medium

The propagating source method for solving the time-dependent Boltzmann equation describing particle propagation in a magnetically turbulent medium is extended to a more realistic case that includes focusing and adiabatic deceleration. The solutions correspond to beam propagation in the solar wind. Pitch-angle scattering away from 90° is described by standard quasi-linear theory (QLT), while scattering through 90° is approximated by a BGK operator representing a slow mirroring process. The detailed numerical technique for solving the Fokker-Planck equation for two particular spectra is presented. Comparisons are made between our modified QLT (MQLT) model and a BGK model, between highly anisotropic scattering and moderately anisotropic scattering, and between fast particles and slow particles. It is shown that: (1) for moderately anisotropic pitch-angle scattering, the initial ring-beam distribution finally evolves into a broad Gaussian distribution and the QLT isotropic and MQLT anisotropic models could be rather well approximated by the simple relaxation time operator. (2) For highly anisotropic pitch-angle scattering, a moving pulse with a spatially extended flat tail is formed, and there exist some differences between the MQLT and BGK models. Specifically, at a particular pitch angle, the spatial distribution from MQLT model occupies a much wider region than that in the BGK model. (3) In the highly anisotropic scattering medium, more particles are cooled by adiabatic deceleration, some particles move a little faster, and the spatial distribution at a specific pitch angle is much more dispersed than that in the case of moderately anisotropic scattering. (4) Compared with the BGK model, the anisotropy persists for a little longer and some particles move a little slower; consequently, intensity profiles have a greater amplitude at later times in the MQLT model. (5) Finally, fast and slow particles have similar distribution characteristics, except that convection is much more important for slow particles.

Consistency check of a global MHD simulation using the test-kinetic approach

The test-kinetic approach is used to study first order particle kinetic effects in the vicinity of the Earth bow shock, and check consistency with a solution obtained in the MHD approximation. The method, also referred to as reverse particle tracing, consists of integrating particle trajectories backward in time, using electromagnetic fields as obtained from an MHD simulation model. By following particles back to a region where the plasma distribution function is known, and using Liouvilles theorem, it is possible to compute the distribution function at arbitrary points in space. Compared with the more familiar test-particle approach, where large numbers of particles are followed forward in time, the present method has the advantage of producing distribution functions without statistical errors. The test-kinetic method is applied here in combination with magnetospheric fields obtained with the global MHD code BATS-R-US. Consistency between the MHD solution and the first order kinetic solution is checked by comparing moments of the computed ion distribution function directly with corresponding physical parameters obtained in the MHD approximation. From there, the validity and limitations of both the MHD solution and the inferred particle kinetic solution can be assessed.

Reply to comment by J.‐P. St.‐Maurice on “Nonlinear electron heating by resonant shear Alfvén waves in the ionosphere”

[1] St.-Maurice [2005] questions the validity of the resonant shear Alfven wave electron heating mechanism proposed by Lu et al. [2005]. In particular, the following arguments have been raised: (i) the electron cooling time given by Lu et al. [2005] is underestimated because inelastic electron collisions are neglected; (ii) the ionization rate is incorrect; (iii) the electron and ion conductivities are wrong; (iv) the position of the energy deposition layer is incorrect; (v) the energy of precipitating electrons is not well defined, and (vi) the mechanism is simply not applicable. We acknowledge that inelastic collisions with neutrals are important for the electron cooling and ionization processes discussed by Lu et al. [2005]. However, we reject all other allegations. We demonstrate that inelastic cooling leads only to a revision of the threshold current in the nonlinear regime of our theory. However, this does not undermine the overall idea proposed by Lu et al. [2005]. Our improved model makes our results even more consistent with observations. [2] To account for inelastic cooling processes, one has only to modify the electron energy balance equation [Lu et al., 2005, equation (1)]:

MULTISCALE GEOSPACE PHYSICS IN CANADA

Abstract Geospace physics research is entering a new era in Canada. Although this development has been a poorly kept secret among the informed observers, there has not been, to date, an attempt to summarize these changes in a single source and convey to the scientific world the vision and potential of the new “Northern perspective”. In this paper we make a first attempt to fill this gap, without, however, claiming authoritativeness or completeness – such a claim would be defeated by the fast-paced development in Canada on many fronts. The new Canadian perspective is based on a keen awareness of the multiscale character of geospace, and on a realistic yet innovative outlook which integrates Canadas strengths into national efforts emphasizing the use of multi-instrument techniques to attack multiscale complexities in problems. We organize our discussion in four major sections: a description of the plan to enhance Canadas leadership position in ground-based geospace science, centered on the Canadian Geospace Monitoring program; a description of Canadas latest effort to achieve major breakthroughs in our understanding of geospace transition region dynamics, centered on the Canadian small satellite experiment ePOP; a description of Canadas participations in three major international geospace missions (THEMIS, SWARM, and AMISR); and, finally, a description of two potential new Canadian-led geospace missions, Ravens and Orbitals. We will integrate key scientific goals and strategies in our narrative about these missions, and use examples to illustrate the considerable potential of these Canadian efforts.