Simulations of Electron Acceleration at Collisionless Shocks: The Effects of Surface Fluctuations

Energetic electrons are a common feature of interplanetary shocks and planetary bow shocks, and they are invoked as a key component of models of nonthermal radio emission, such as solar radio bursts. A simulation study is carried out of electron acceleration for high Mach number, quasi-perpendicular shocks, typical of the shocks in the solar wind. Two dimensional self-consistent hybrid shock simulations provide the electric and magnetic fields in which test particle electrons are followed. A range of different shock types, shock normal angles, and injection energies are studied. When the Mach number is low, or the simulation configuration suppresses fluctuations along the magnetic field direction, the results agree with theory assuming magnetic moment conserving reflection (or Fast Fermi acceleration), with electron energy gains of a factor only 2 - 3. For high Mach number, with a realistic simulation configuration, the shock front has a dynamic rippled character. The corresponding electron energization is radically different: Energy spectra display: (1) considerably higher maximum energies than Fast Fermi acceleration; (2) a plateau, or shallow sloped region, at intermediate energies 2 - 5 times the injection energy; (3) power law fall off with increasing energy, for both upstream and downstream particles, with a slope decreasing as the shock normal angle approaches perpendicular; (4) sustained flux levels over a broader region of shock normal angle than for adiabatic reflection. All these features are in good qualitative agreement with observations, and show that dynamic structure in the shock surface at ion scales produces effective scattering and can be responsible for making high Mach number shocks effective sites for electron acceleration.Comment: 26 pages, 12 figure

Supermagnetosonic jets behind a collisionless quasi-parallel shock

The downstream region of a collisionless quasi-parallel shock is structured containing bulk flows with high kinetic energy density from a previously unidentified source. We present Cluster multi-spacecraft measurements of this type of supermagnetosonic jet as well as of a weak secondary shock front within the sheath, that allow us to propose the following generation mechanism for the jets: The local curvature variations inherent to quasi-parallel shocks can create fast, deflected jets accompanied by density variations in the downstream region. If the speed of the jet is super(magneto)sonic in the reference frame of the obstacle, a second shock front forms in the sheath closer to the obstacle. Our results can be applied to collisionless quasi-parallel shocks in many plasma environments.Comment: accepted to Phys. Rev. Lett. (Nov 5, 2009

Electron Injection at High Mach Number Quasi-Perpendicular Shocks : Surfing and Drift Acceleration

Electron injection process at high Mach number collisionless quasi-perpendicular shock waves is investigated by means of one-dimensional electromagnetic particle-in-cell simulations. We find that energetic electrons are generated through the following two steps: (1) electrons are accelerated nearly perpendicular to the local magnetic field by shock surfing acceleration at the leading edge of the shock transition region. (2) the preaccelerated electrons are further accelerated by shock drift acceleration. As a result, energetic electrons are preferentially reflected back to the upstream. Shock surfing acceleration provides sufficient energy required for the reflection. Therefore, it is important not only for the energization process by itself, but also for triggering the secondary acceleration process. We also present a theoretical model of the two-step acceleration mechanism based on the simulation results, which can predict the injection efficiency for subsequent diffusive shock acceleration process. We show that the injection efficiency obtained by the present model agrees well with the value obtained by Chandra X-ray observations of SN 1006. At typical supernova remnant shocks, energetic electrons injected by the present mechanism can self-generate upstream Alfven waves, which scatter the energetic electrons themselves.Comment: 35 pages, 9 figures, accepted by Ap

Ion heating resulting from pickup in magnetic reconnection exhausts

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95415/1/jgra19672.pd

Temperature Anisotropy in a Shocked Plasma: Mirror-Mode Instabilities in the Heliosheath

We show that temperature anisotropies induced at a shock can account for interplanetary and planetary bow shock observations. Shocked plasma with enhanced plasma beta is preferentially unstable to the mirror mode instability downstream of a quasi-perpendicular shock and to the firehose instability downstream of a quasi-parallel shock, consistent with magnetic fluctuations observed downstream of a large variety of shocks. Our theoretical analysis of the solar wind termination shock suggests that the magnetic holes observed by Voyager 1 in the heliosheath are produced by the mirror mode instability. The results are also of astrophysical interest, providing an energy source for plasma heating.Comment: 11 pages, 2 figures, accepted for publication in ApJ Letter

Wide ultrarelativistic plasma beam -- magnetic barrier collision and astrophysical applications

The interaction between a wide ultrarelativistic fully-ionized plasma beam and a magnetic barrier is studied numerically. It is assumed that the plasma beam is initially homogeneous and impacts with the Lorentz factor $\Gamma_0\gg 1$ on the barrier. The magnetic field of the barrier $B_0$ is uniform and transverse to the beam velocity. When the energy densities of the beam and the magnetic field are comparable, $\alpha = 8\pi n_0m_pc^2(\Gamma_0-1)/B^2_0\sim 1$, the process of the beam -- barrier interaction is strongly nonstationary, and the density of reversed protons is modulated in space by a factor of 10 or so. The modulation of reversed protons decreases with decrease of $\alpha$. The beam is found to penetrate deep into the barrier provided that $\alpha > \alpha_{cr}$, where $\alpha_{cr}$ is about 0.4. The speed of such a penetration is subrelativistic and depends on $\alpha$. Strong electric fields are generated near the front of the barrier, and electrons are accelerated in these fields up to the mean energy of protons, i.e. up to $\sim m_pc^2\Gamma_0$. The synchrotron radiation of high-energy electrons from the front vicinity is calculated. Stationary solutions for the beam -- barrier collision are considered. It is shown that such a solution may be only at $\alpha \lesssim 0.2 - 0.5$ depending on the boundary conditions for the electric field in the region of the beam -- barrier interaction. Some astrophysical applications of these results are briefly discussed.Comment: 11 pages, Latex (revtex), 12 postscript figures, submitted to Phys. Rev.

Recent Advances in Understanding Particle Acceleration Processes in Solar Flares

We review basic theoretical concepts in particle acceleration, with particular emphasis on processes likely to occur in regions of magnetic reconnection. Several new developments are discussed, including detailed studies of reconnection in three-dimensional magnetic field configurations (e.g., current sheets, collapsing traps, separatrix regions) and stochastic acceleration in a turbulent environment. Fluid, test-particle, and particle-in-cell approaches are used and results compared. While these studies show considerable promise in accounting for the various observational manifestations of solar flares, they are limited by a number of factors, mostly relating to available computational power. Not the least of these issues is the need to explicitly incorporate the electrodynamic feedback of the accelerated particles themselves on the environment in which they are accelerated. A brief prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares, inspired by RHESSI observations. The individual articles are to appear in Space Science Reviews (2011

The Diffusion Region in Collisionless Magnetic Reconnection

A review of present understanding of the dissipation region in magnetic reconnection is presented. The review focuses on results of the thermal inertia-based dissipation mechanism but alternative mechanisms are mentioned as well. For the former process, a combination of analytical theory and numerical modeling is presented. Furthermore, a new relation between the electric field expressions for anti-parallel and guide field reconnection is developed