75 research outputs found
Spherical shock in the presence of an external magnetic field
We investigate spherical collisionless shocks in the presence of an external magnetic field. Spherical collisionless shocks are common resultant of interactions between a expanding plasma and a surrounding plasma, such as the solar wind, stellar winds, and supernova remnants. Anisotropies often observed in shock propagations and their emissions, and it is widely believed a magnetic field plays a major role. Since the local observations of magnetic fields in astrophysical plasmas are not accessible, laboratory experiments provide unique capability to investigate such phenomena. We model the spherical shocks in the universe by irradiating a solid spherical target surrounded by a plasma in the presence of a magnetic field. We present preliminary results obtained by shadowgraphy
Relativistic particle acceleration in developing Alfv\'{e}n turbulence
A new particle acceleration process in a developing Alfv\'{e}n turbulence in
the course of successive parametric instabilities of a relativistic pair plasma
is investigated by utilyzing one-dimensional electromagnetic full particle
code. Coherent wave-particle interactions result in efficient particle
acceleration leading to a power-law like energy distribution function. In the
simulation high energy particles having large relativistic masses are
preferentially accelerated as the turbulence spectrum evolves in time. Main
acceleration mechanism is simultaneous relativistic resonance between a
particle and two different waves. An analytical expression of maximum
attainable energy in such wave-particle interactions is derived.Comment: 15 pages, 9 figures, 1 tabl
Nonthermal Electron Acceleration at Collisionless Quasi-perpendicular Shocks
Shock waves propagating in collisionless heliospheric and astrophysical
plasmas have been studied extensively over the decades. One prime motivation is
to understand the nonthermal particle acceleration at shocks. Although the
theory of diffusive shock acceleration (DSA) has long been the standard for
cosmic-ray acceleration at shocks, plasma physical understanding of particle
acceleration remains elusive. In this review, we discuss nonthermal electron
acceleration mechanisms at quasi-perpendicular shocks, for which substantial
progress has been made in recent years. The discussion presented in this review
is restricted to the following three specific topics. The first is stochastic
shock drift acceleration (SSDA), which is a relatively new mechanism for
electron injection into DSA. The basic mechanism, related in-situ observations
and kinetic simulations results, and how it is connected with DSA will be
discussed. Second, we discuss shock surfing acceleration (SSA) at very high
Mach number shocks relevant to young supernova remnants (SNRs). While the
original proposal under the one-dimensional assumption is unrealistic, SSA has
now been proven efficient by a fully three-dimensional kinetic simulation.
Finally, we discuss the current understanding of the magnetized
Weibel-dominated shock. Spontaneous magnetic reconnection of self-generated
current sheets within the shock structure is an interesting consequence of
Weibel-generated strong magnetic turbulence. We argue that high Mach number
shocks with both Alfven and sound Mach numbers exceeding 20-40 will likely
behave as a Weibel-dominated shock. Despite a number of interesting recent
findings, the relative roles of SSDA, SSA, and magnetic reconnection for
electron acceleration at collisionless shocks and how the dominant particle
acceleration mechanisms change depending on shock parameters remain to be
answered.Comment: To appear in Reviews of Modern Plasma Physics as an invited revie
Relativistic Electron Shock Drift Acceleration in Low Mach Number Galaxy Cluster Shocks
An extreme case of electron shock drift acceleration in low Mach number
collisionless shocks is investigated as a plausible mechanism of initial
acceleration of relativistic electrons in large-scale shocks in galaxy clusters
where upstream plasma temperature is of the order of 10 keV and a degree of
magnetization is not too small. One-dimensional electromagnetic full particle
simulations reveal that, even though a shock is rather moderate, a part of
thermal incoming electrons are accelerated and reflected through relativistic
shock drift acceleration and form a local nonthermal population just upstream
of the shock. The accelerated electrons can self-generate local coherent waves
and further be back-scattered toward the shock by those waves. This may be a
scenario for the first stage of the electron shock acceleration occurring at
the large-scale shocks in galaxy clusters such as CIZA J2242.8+5301 which has
well defined radio relics.Comment: 26 pages, 10 figures, accepted for publication in Ap
Absolute and convective instabilities of parallel propagating circularly polarized Alfvén waves: numerical results
Context.The stability of parallel propagating circularly polarized Alfvén waves (pump waves) has been studied for more than four decades with the use of normal mode analysis. It is well known that the normal mode analysis does not answer the question if a pump wave looks stable or unstable in a particular reference frame. To answer this question it is necessary to find out if the instability is absolute or convective in this reference frame.
Aims.We extend our previous study of absolute and convective instabilities of pump waves with small amplitude to pump waves with arbitrary amplitude.
Methods.To study the absolute and convective instabilities of pump waves with arbitrary amplitude we numerically implement Brigg's method.
Results.We show that the wave is absolutely unstable in a reference frame moving with the velocity U with respect to the rest plasma if U satisfies the inequality Ul Ur) we study the signalling problem. We show that spatially amplifying waves exist only when the signalling frequency is in two symmetric frequency bands, and calculate the dependences of the boundaries of these bands on U for different values of a . We also obtain the dependences of the maximum spatial amplification rate on U for different values of a . The implication of these results on the interpretation of observational data from space missions is discussed. In particular, it is shown that circularly polarized Alfvén waves propagating in the solar wind are convectively unstable in a reference frame of any realistic spacecraft
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