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تسجيل مستخدم جديدCosmic-Ray Momentum Diffusion In Magnetosonic Versus Alfvenic Turbulent Field
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Energetic particle transport in a finite amplitude magnetosonic and Alfvenic turbulence is considered using Monte Carlo particle simulations, which involve an integration of particle equation of motion. We show that in a low-Betha plasma cosmic ray can be the most important damping process for magnetosonic waves. Assuming such conditions we derive the momentum diffusion coefficient for relativistic particles in the presence of anisotropic finite-amplitude turbulent wave field, for flat and Kolmogorov-type turbulence spectra. We confirm the possibility of larger values of a momentum diffusion coefficient occuring due to transit-time damping resonance interaction in the presence of isotropic fast-mode waves in comparison to the Alfven waves of the same amplitude.
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The acceleration of energetic particle transport in high amplitude magnetosonic and Alfvenic turbulence is considered using the method of Monte Carlo particle simulations, involving integration of particle equations of motion. We derive the momentum
diffusion coefficient in the presence of anisotropic turbulent wave in the high-magnetized plasma, for a flat and Kolmogorov-type turbulence spectrum. We confirm the quasilinear result (Shlickeiser & Miller (1997) of enhanced values of the momentum diffusion coefficient due to transit-time resonance interaction in the presence of isotropic fast-mode waves as compared to the case of slab Alfvenic waves of the same amplitude. The relation of the momentum diffusion coefficient to the turbulence amplitude and anisotropy is investigated.
This work has the main objective to provide a detailed investigation of cosmic ray propagation in magnetohydrodynamic turbulent fields generated by forcing the fluid velocity field at large scales. It provides a derivation of the particle mean free p
ath dependences in terms of the turbulence level described by the Alfvenic Mach number and in terms of the particle rigidity. We use an upgrade version of the magnetohydrodynamic code {tt RAMSES} which includes a forcing module and a kinetic module and solve the Lorentz equation for each particle. The simulations are performed using a 3 dimension periodical box in the test-particle and magnetostatic limits. The forcing module is implemented using an Ornstein-Uhlenbeck process. An ensemble average over a large number of particle trajectories is applied to reconstruct the particle mean free paths. We derive the cosmic ray mean free paths in terms of the Alfvenic Mach numbers and particle reduced rigidities in different turbulence forcing geometries. The reduced particle rigidity is $rho=r_L/L$ where $r_L$ is the particle Larmor radius and $L$ is the simulation box length related to the turbulence coherence or injection scale $L_{inj}$ by $L sim 5 L_{inj}$. We have investigated with a special attention compressible and solenoidal forcing geometries. We find that compressible forcing solutions are compatible with the quasi-linear theory or more advanced non-linear theories which predict a rigidity dependence as $rho^{1/2}$ or $rho^{1/3}$. Solenoidal forcing solutions at least at low or moderate Alfvenic numbers are not compatible with the above theoretical expectations and require more refined arguments to be interpreted. It appears especially for Alfvenic Mach numbers close to one that the wandering of field lines controls perpendicular mean free path solutions whatever the forcing geometry.
Properties of two equations describing the evolution of the probability density function (PDF) of the relative dispersion in turbulent flow are compared by investigating their solutions: the Richardson diffusion equation with the drift term and the s
elf-similar telegraph equation derived by Ogasawara and Toh [J. Phys. Soc. Jpn. 75, 083401 (2006)]. The solution of the self-similar telegraph equation vanishes at a finite point, which represents persistent separation of a particle pair, while that of the Richardson equation extends infinitely just after the initial time. Each equation has a similarity solution, which is found to be an asymptotic solution of the initial value problem. The time lag has a dominant effect on the relaxation process into the similarity solution. The approaching time to the similarity solution can be reduced by advancing the time of the similarity solution appropriately. Batchelor scaling, a scaling law relevant to initial separation, is observed only for the telegraph case. For both models, we estimate the Richardson constant, based on their similarity solutions.
We make use of the Parker Solar Probe (PSP) data to explore the nature of solar wind turbulence focusing on the Alfvenic character and power spectra of the fluctuations and their dependence on distance and context (i.e. large scale solar wind propert
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We have performed magnetohydrodynamical simulations to study the amplification of magnetic fields in the precursors of shock waves. Strong magnetic fields are required in the precursors of the strong shocks that occur in supernova remnants. Observati
ons also suggest that magnetic field amplification takes place in the weak shocks that occur in galaxy clusters and that produce so-called radio relics. Here, we extend the study of magnetic field amplification by cosmic-ray driven turbulence to weak shocks. The amplification is driven by turbulence that is produced by the cosmic-ray pressure acting on the density inhomogeneities in the upstream fluid. The clumping that has been inferred from X-ray data for the outskirts of galaxy clusters could provide some of the seed inhomogeneities. Magnetic field power spectra and Faraday maps are produced. Furthermore, we investigate how the synchrotron emission in the shock precursor can be used to verify the existence of this instability and constrain essential plasma parameters.
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