8 research outputs found
Shock Waves in the Large-Scale Structure of the Universe
Cosmological shock waves are induced during hierarchical formation of
large-scale structure in the universe. Like most astrophysical shocks, they are
collisionless, since they form in the tenuous intergalactic medium through
electromagnetic viscosities. The gravitational energy released during structure
formation is transferred by these shocks to the intergalactic gas as heat,
cosmic-rays, turbulence, and magnetic fields. Here we briefly describe the
properties and consequences of the shock waves in the context of the
large-scale structure of the universe.Comment: Submitted to Astrophysics and Space Science (Special Issue for the
proceedings of International Conference on HEDP/HEDLA-08). Pdf with full
resolution Figure 1 can be downloaded from
http://canopus.cnu.ac.kr/ryu/rk.pd
Contributions to the cross shock electric field at supercritical perpendicular shocks: Impact of the pickup ions
A particle-in-cell code is used to examine contributions of the pickup ions
(PIs) and the solar wind ions (SWs) to the cross shock electric field at the
supercritical, perpendicular shocks. The code treats the pickup ions
self-consistently as a third component. Herein, two different runs with
relative pickup ion density of 25% and 55% are presented in this paper. Present
preliminary results show that: (1) in the low percentage (25%) pickup ion case,
the shock front is nonstationary. During the evolution of this perpendicular
shock, a nonstationary foot resulting from the reflected solar wind ions is
formed in front of the old ramp, and its amplitude becomes larger and larger.
At last, the nonstationary foot grows up into a new ramp and exceeds the old
one. Such a nonstationary process can be formed periodically. hen the new ramp
begins to be formed in front of the old ramp, the Hall term mainly contributed
by the solar wind ions becomes more and more important. The electric field Ex
is dominated by the Hall term when the new ramp exceeds the old one.
Furthermore, an extended and stationary foot in pickup ion gyro-scale is
located upstream of the nonstationary/self-reforming region within the shock
front, and is always dominated by the Lorentz term contributed by the pickup
ions; (2) in the high percentage (55%) pickup ion case, the amplitude of the
stationary foot is increased as expected. One striking point is that the
nonstationary region of the shock front evidenced by the self-reformation
disappears. Instead, a stationary extended foot dominated by Lorentz term
contributed by the pickup ions, and a tationary ramp dominated by Hall term
contributed by the solar wind ions are clearly evidenced. The significance of
the cross electric field on ion dynamics is also discussed.Comment: 11 pages, 6 figs and 1 table. This paper will be published in the
journal: Astrophysics and Space Scienc
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Hybrid simulations
The philosophy and numerical implementation of hybrid algorithms are reviewed. In the hybrid approximation, a plasma is described by a set of discrete equations, equivalent to a Vlasov-fluid system. The dynamics of one or more species are modeled using moment equations, while the remaining species are treated as a large number of individual macro-particles. In this paper the hybrid method will be compared to fluid and particle-in-cell algorithms, and the strengths and weaknesses of the various methods will be discussed. A specific limit of the hybrid model, that of macro-particle ions and massless, charge-neutralizing fluid electrons, will be analyzed in detail with particular emphasis on multi-dimensional codes
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Very high Mach number shocks: theory
The theory and simulation of collisionless perpendicular supercritical shock structure is reviewed, with major emphasis on recent research results. The primary tool of investigation is the hybrid simulation method, in which the Newtonian orbits of a large number of ion macroparticles are followed numerically, and in which the electrons are treated as a charge neutralizing fluid. The principal results to be presented are (1) electron resistivity is not required to explain the observed quasi-stationarity of the earth's bow shock, (2) the structure of the perpendicular shock at very high Mach numbers (M/sub A/ approx. = 15 - 20 and ..beta.. approx. = 1, where M/sub A/ is the Alfven Mach number of the shock and ..beta.. is the ratio of the thermal to magnetic pressure) depends sensitively on the upstream ..beta.. and electron resistivity, (3) two-dimensional turbulence will become increasingly important as the Mach number is increased, and (4) non-adiabatic bulk electron heating will result when a thermal electron cannot complete a gyro-orbit while transiting the shock. 32 refs., 13 figs
The Dynamic Quasiperpendicular Shock: Cluster Discoveries
The physics of collisionless shocks is a very broad topic which has been
studied for more than five decades. However, there are a number of important
issues which remain unresolved. The energy repartition amongst particle
populations in quasiperpendicular shocks is a multi-scale process related to
the spatial and temporal structure of the electromagnetic fields within the
shock layer. The most important processes take place in the close vicinity of
the major magnetic transition or ramp region. The distribution of
electromagnetic fields in this region determines the characteristics of ion
reflection and thus defines the conditions for ion heating and energy
dissipation for supercritical shocks and also the region where an important
part of electron heating takes place. All of these processes are crucially
dependent upon the characteristic spatial scales of the ramp and foot region
provided that the shock is stationary. The earliest studies of collisionless
shocks identified nonlinearity, dissipation, and dispersion as the processes
that arrest the steepening of the shock transition. Their relative role
determines the scales of electric and magnetic fields, and so control the
characteristics of processes such as of ion reflection, electron heating and
particle acceleration. The purpose of this review is to address a subset of
unresolved problems in collisionless shock physics from experimental point of
view making use multi-point observations onboard Cluster satellites. The
problems we address are determination of scales of fields and of a scale of
electron heating, identification of energy source of precursor wave train, an
estimate of the role of anomalous resistivity in energy dissipation process by
means of measuring short scale wave fields, and direct observation of
reformation process during one single shock front crossing