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Register a new userA stochastic model for non-relativistic particle acceleration
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Added by
Giuseppe Pallocchia
Publication date
2016
fields
Physics
and research's language is
English
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A stochastic model is proposed for the acceleration of non-relativistic particles yielding to energy spectra with a shape of a Weibulltextquoteright s function. Such particle distribution is found as the stationary solution of a diffusion-loss equation in the framework of a second order Fermitextquoteright s mechanism producing anomalous diffusion for particle velocity. The present model is supported by in situ observations of energetic particle enhancements at interplanetary shocks, as here illustrated by means of an event seen by STEREO B instruments in the heliosphere. Results indicate that the second order Fermitextquoteright s mechanism provides a viable explanation for the acceleration of energetic particles at collisioness shock waves.
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Based on Magnetospheric Multiscale (MMS) observations from the Earths bow shock, we have identified two plasma heating processes that operate at quasi-perpendicular shocks. Ions are subject to stochastic heating in a process controlled by the heating function $chi_j = m_j q_j^{-1} B^{-2}mathrm{div}(mathbf{E}_perp)$ for particles with mass $m_j$ and charge $q_j$ in the electric and magnetic fields $mathbf{E}$ and $mathbf{B}$. Test particle simulations are employed to identify the parameter ranges for bulk heating and stochastic acceleration of particles in the tail of the distribution function. The simulation results are used to show that ion heating and acceleration in the studied bow shock crossings is accomplished by waves at frequencies (1-10)$f_{cp}$ (proton gyrofrequency) for the bulk heating, and $f>10f_{cp}$ for the tail acceleration. When electrons are not in the stochastic heating regime, $|chi_e|<1$, they undergo a quasi-adiabatic heating process characterized by the isotropic temperature relation $T/B=(T_0/B_0)(B_0/B)^{1/3}$. This is obtained when the energy gain from the conservation of the magnetic moment is redistributed to the parallel energy component through the scattering by waves. The results reported in this paper may also be applicable to particle heating and acceleration at astrophysical shocks.
The Fermi LAT discovery that classical novae produce >100 MeV gamma-rays establishes that shocks and relativistic particle acceleration are key features of these events. These shocks are likely to be radiative due to the high densities of the nova ejecta at early times coincident with the gamma-ray emission. Thermal X-rays radiated behind the shock are absorbed by neutral gas and reprocessed into optical emission, similar to Type IIn (interacting) supernovae. Gamma-rays are produced by collisions between relativistic protons with the nova ejecta (hadronic scenario) or Inverse Compton/bremsstrahlung emission from relativistic electrons (leptonic scenario), where in both scenarios the efficiency for converting relativistic particle energy into LAT gamma-rays is at most a few tens of per cent. The ratio of gamma-ray and optical luminosities, L_gam/L_opt, thus sets a lower limit on the fraction of the shock power used to accelerate relativistic particles, e_nth. The measured values of L_gam/L_opt for two classical novae, V1324 Sco and V339 Del, constrains e_nth > 1e-2 and > 1e-3, respectively. Inverse Compton models for the gamma-ray emission are disfavored given the low electron acceleration efficiency, e_nth ~ 1e-4-1e-3, inferred from observations of Galactic cosmic rays and particle-in-cell (PIC) numerical simulations. A fraction > 100(0.01/e_nth) and > 10(0.01/e_nth) per cent of the optical luminosity is powered by shocks in V1324 Sco and V339 Del, respectively. Such high fractions challenge standard models that instead attribute all nova optical emission to the direct outwards transport of thermal energy released near the white dwarf surface.
Several types of foreshock transients upstream of Earths bow shock possessing a tenuous, hot core have been observed and simulated. Because of the low dynamic pressure in their cores, these phenomena can significantly disturb the bow shock and the magnetosphere-ionosphere system. Recent observations have also demonstrated that foreshock transients can accelerate particles which, when transported earthward, can affect space weather. Understanding the potential of foreshock transients to accelerate particles can help us understand shock acceleration at Earth and at other planetary and astrophysical systems. To further investigate foreshock transients potential for acceleration we conduct a statistical study of ion and electron energization in the core of foreshock transients. We find that electron energies typically increase there, evidently due to an internal acceleration process, whereas, as expected, ion energies most often decrease to support transient formation and expansion. Nevertheless, ion energy enhancements can be seen in some events suggesting an internal ion acceleration process as well. Formation conditions of foreshock transients are related to weak solar wind magnetic field strength and fast solar wind speed. Ion and electron energization are also positively correlated with solar wind speed.
Powerful stellar winds and supernova explosions with intense energy release in the form of strong shock waves can convert a sizeable part of the kinetic energy release into energetic particles. The starforming regions are argued as a favorable site of energetic particle acceleration and could be efficient sources of nonthermal emission. We present here a non-linear time-dependent model of particle acceleration in the vicinity of two closely approaching fast magnetohydrodynamic (MHD) shocks. Such MHD flows are expected to occur in rich young stellar cluster where a supernova is exploding in the vicinity of a strong stellar wind of a nearby massive star. We find that the spectrum of the high energy particles accelerated at the stage of two closely approaching shocks can be harder than that formed at a forward shock of an isolated supernova remnant. The presented method can be applied to model particle acceleration in a variety of systems with colliding MHD flows.
We use particle-in-magnetohydrodynamics-cells to model particle acceleration and magnetic field amplification in a high Mach, parallel shock in three dimensions and compare the result to 2-D models. This allows us to determine whether 2-D simulations can be relied upon to yield accurate results in terms of particle acceleration, magnetic field amplification and the growth rate of instabilities. Our simulations show that the behaviour of the gas and the evolution of the instabilities are qualitatively similar for both the 2-D and 3-D models, with only minor quantitative differences that relate primarily to the growth speed of the instabilities. The main difference between 2-D and 3-D models can be found in the spectral energy distributions (SEDs) of the non-thermal particles. The 2-D simulations prove to be more efficient, accelerating a larger fraction of the particles and achieving higher velocities. We conclude that, while 2-D models are sufficient to investigate the instabilities in the gas, their results have to be treated with some caution when predicting the expected SED of a given shock.
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