44 research outputs found
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 on the barrier. The magnetic field of the barrier is uniform and
transverse to the beam velocity. When the energy densities of the beam and the
magnetic field are comparable, , 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 . The
beam is found to penetrate deep into the barrier provided that , where is about 0.4. The speed of such a penetration
is subrelativistic and depends on . 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 . 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 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