102 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
Physics of Boundaries and their Interactions in Space Plasmas
This final report describes a brief summary of our accomplishments during the complete contract period. Traditionally, due to computational limitations, it has been impossible to obtain a global view of the magnetosphere on ion time and spatial scales. As a result, kinetic-simulations have concentrated on the local structure of different magnetospheric discontinuities and boundaries. However, due to the emergence of low cost supercomputers, as well as by taking full advantage of latest advances in data mining and visualization technology, we were able to bypass our planned (proposed) regional simulations and proceed to large-scale 3-D and 2-D global hybrid simulations of the magnetosphere. As a result, although we are only finishing the second year of the proposed activity, much of the original scientific objectives have been surpassed and new avenues of investigation have been opened. Such simulations have led us to possible explanations of some long-standing issues in magnetospheric physics. They have also enables us to make a number of important discoveries predictions, which need to be looked for in satellite data. Examples include the finding that the bow shock can become unstable to the Kelvin-Helmholtz (KH), (2) the discovery of a mechanism for intermittent reconnection due to ion physics which may be relevant to the explanation of the recurrence rate of flux transfer events (FTEs), and (3) this finding that the current sheet in the near-Earth magnetotail region can become unstable to KH with detectable, unique ionospheric signatures. Further, we demonstrated a viable mechanism for the onset of reconnection at the magnetopause, examined the detailed structure of the boundary layer incorporating curvature effects, and provided an explanation for the large core fields observed within FTEs as well as flux ropes in the magnetotail
Physics of Boundaries and Their Interactions in Space Plasmas
This report describes the work done by SciberNet, Inc. during the month of January. During this time, we primarily worked on further analysis of the results presented at the AGU as well as writing them up for publication. Using large scale simulations, we showed that the magnetopause during the southward IMF case is quite irregular with varying thickness, and has a complex flow pattern owing to the nonlinear effects of the convective flow superimposed on the flows generated in the reconnection layer. We used inflow-outflow boundary conditions to examine the kinetic nature of the discontinuities that are formed in the reconnection layer and concluded that nonlocal effects play a major role in the formation of such discontinuities and can alter their properties from the usual structures expected from 1-D simulations or from fluid theories. Finally, we used our 3-D simulations to examine the nonlinear interaction of the tearing mode with the Kelvin-Helmholtz instability. We showed that this interaction leads to the generation of a large core field which is observed both in the magnetotail as well as the magnetopause
Physics of Boundaries and Their Interactions in Space Plasmas
In this report, we provide a summary of our most significant research accomplishments resulting from this contract. For the sake of brevity, most of the projects are explained in a paragraph length, highlighting only pertinent results
Physics of Boundaries and their Interactions in Space Plasmas
This report describes the work done by SciberNet, Inc. during the month of October. We are working on the further refinement of the model used in our large-scale hybrid simulations of the magnetopause. Specifically, we are experimenting with several ways of modeling the effects of cold magnetospheric ions into our simulations. In addition, we are preparing two presentations for the upcoming Fall AGU highlighting the results of these simulations. We have also made progress in our development of a new kinetic linear code which we are using to study the linear properties of the Kelvin-Helmholtz instability at the magnetopause. We have extended the code from the electrostatic limit to the fully electromagnetic regime and are currently in the process of debugging and testing the code. Finally, we have made several test runs with our 2-D hybrid code for the magnetopause. The inflow-outflow boundary conditions are working properly. However, there are issues related to the setup and evolution of the original equilibrium that we are still trying to resolve. Finally, we are preparing several presentations for the upcoming Fall AGU
Physics of Boundaries and their Interactions in Space Plasmas
This report describes the work done by SciberNet, Inc. during the month of August. We have resolved the issues associated with the implementation of the dipole field in our large scale hybrid simulations of the magnetopause. We have setup several runs and will spend the next several months analyzing the data. The results will be presented at the Fall AGU. We are also continuing our analysis of the 3-D simulations of thin current sheets at the magnetopause, paying special attention to the conditions under which Kelvin-Helmholtz would lead to sizable perturbations of the magnetopause. In a related study, we are in the process of developing a new kinetic linear code that would for the first time enable us to examine the linear properties of the Kelvin-Helmholtz instability in the fully kinetic regime. Finally, we are continuing our code development to include inflow-outflow boundary conditions in our 2-D and 3-D hybrid codes. We are also comparing the different methods of code parallelization in order to extend the limits of our calculations
The Acceleration of Ions in Solar Flares During Magnetic Reconnection
The acceleration of solar flare ions during magnetic reconnection is explored
via particle-in-cell simulations that self-consistently follow the motions of
both protons and particles. We demonstrate that the dominant ion
heating during reconnection with a guide field (a magnetic component
perpendicular to the reconnection plane) results from pickup behavior during
the entry into reconnection exhausts. In contrast with anti-parallel
reconnection, the temperature increment is dominantly transverse, rather than
parallel, to the local magnetic field. The comparison of protons and alphas
reveals a mass-to-charge () threshold in pickup behavior that favors
heating of high ions over protons, which is consistent with impulsive
flare observations.Comment: Revised based on reviewer's comments; text clarified and references
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Non adiabatic electron behavior through a supercritical perpendicular collisionless shock: Impact of the shock front turbulence
International audienceAdiabatic and nonadiabatic electrons transmitted through a supercritical perpendicular shock wave are analyzed with the help of test particle simulations based on field components issued from 2 − D full-particle simulation. A previous analysis (Savoini et al., 2005) based on 1 − D shock profile, including mainly a ramp (no apparent foot) and defined at a fixed time, has identified three distinct electron populations: adiabatic, overadiabatic, and underadiabatic, respectively, identified by μds/μus ≈ 1, >1 and <1, where μus and μds are the magnetic momenta in the upstream and downstream regions. Presently, this study is extended by investigating the impact of the time evolution of 2 − D shock front dynamics on these three populations. Analysis of individual time particle trajectories is performed and completed by statistics based on the use of different upstream velocity distributions (spherical shell of radius vshell and a Maxwellian with thermal velocity vthe). In all statistics, the three electron populations are clearly recovered. Two types of shock front nonstationarity are analyzed. First, the impact of the nonstationarity along the shock normal (due to the front self-reformation only) strongly depends on the values of vshell or vthe. For low values, the percentages of adiabatic and overadiabatic electrons are almost comparable but become anticorrelated under the filtering impact of the self-reformation; the percentage of the underadiabatic population remains almost unchanged. In contrast, for large values, this impact becomes negligible and the adiabatic population alone becomes dominant. Second, when 2 − D nonstationarity effects along the shock front (moving rippling) are fully included, all three populations are strongly diffused, leading to a larger heating; the overadiabatic population becomes largely dominant (and even larger than the adiabatic one) and mainly contributes to the energy spectrum
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
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