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Register a new userHeavy Ion Acceleration by Super-Alfvenic Waves
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A generation mechanism of super Alfv{e}nic (SPA) waves in multi-ion species plasma is proposed and the associated heavy ion acceleration process is discussed. The SPA waves are thought to play important roles in particle acceleration since they have large wave electric field because of their high phase velocity. It is demonstrated by using full particle-in-cell simulation that large amplitude proton cyclotron waves, excited due to proton temperature anisotropy, nonlinearly destabilize SPA waves through parametric decay instability in a three component plasma composed of electrons, protons, and $alpha$ particles. At the same time, $alpha$ cyclotron waves get excited via another decay instability. A pre-accelerated $alpha$ particle resonates simultaneously with the two daughter waves, the SPA waves and the $alpha$ cyclotron waves, and it is further accelerated perpendicular to the ambient magnetic field. The process may work in astrophysical environments where sufficiently large temperature anisotropy of lower mass ions occurs.
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It is shown that ions can be accelerated to MeV energy range in the direction perpendicular to the magnetic field by the ExB mechanism of electrostatic waves. The acceleration occurs in discrete steps of duration being a small fraction the gyroperiod and can explain observations of ion energization to 10 keV at quasi-perpendicular shocks and to 100-1000 keV at quasi-parallel shocks. A general expression is provided for the maximum energy of ions accelerated in shocks of arbitrary configuration. The waves involved in the acceleration are related to three cross-field current-driven instabilities: the lower hybrid drift (LHD) instability induced by the density gradients in shocks and shocklets, followed by the modified two-stream (MTS) and electron cyclotron drift (ECD) instabilities, induced by the ExB drift of electrons in the strong LHD wave electric field. The ExB wave mechanism accelerates heavy ions to energies proportional to the atomic mass number, which is consistent with satellite observations upstream of the bow shock and also with observations of post-shocks in supernovae remnants.
We simulate decaying turbulence in a homogeneous pair plasma using three dimensional electromagnetic particle-in-cell (PIC) method. A uniform background magnetic field permeates the plasma such that the magnetic pressure is three times larger than the thermal pressure and the turbulence is generated by counter-propagating shear Alfven waves. The energy predominately cascades transverse to the background magnetic field, rendering the turbulence anisotropic at smaller scales. We simultaneously move several ion species of varying charge to mass ratios in our simulation and show that the particles of smaller charge to mass ratios are heated and accelerated to non-thermal energies at a faster rate, in accordance with the enhancement of heavy ions and non-thermal tail in their energy spectrum observed in the impulsive solar flares. We further show that the heavy ions are energized mostly in the direction perpendicular to the background magnetic field with a rate consistent with our analytical estimate of the rate of heating due to cyclotron resonance with the Alfven waves of which a large fraction is due to obliquely propagating waves.
We demonstrate the dragging of the magnetic field by the super-Alfvenic shear flows out of the reconnection plane can strongly localize the reconnection x-line in collisionless plasmas, reversing the current direction at the x-line. Reconnection with this new morphology, which is impossible in resistive-magnetohydrodynamic (MHD), is enabled by electron inertia. Surprisingly, the quasi-steady reconnection rate remains of order 0.1 even though the aspect ratio of the local x-line geometry is larger than unity. We explain this by examining the transport of the reconnected magnetic flux and the opening angle made by the upstream magnetic field, concluding that the reconnection rate is still limited by the constraint imposed at the inflow region. This study further suggests the nearly universal fast rate value of order 0.1 cannot be explained by the physics of tearing modes, nor can it be explained by a universal localization mechanism.
Magnetosheath jets are localized fast flows with enhanced dynamic pressure. When they supermagnetosonically compress the ambient magnetosheath plasma, a bow wave or shock can form ahead of them. Such a bow wave was recently observed to accelerate ions and possibly electrons. The ion acceleration process was previously analyzed, but the electron acceleration process remains largely unexplored. Here we use multi-point observations by Time History of Events and Macroscale during Substorms from three events to determine whether and how magnetosheath jet-driven bow waves can accelerate electrons. We show that when suprathermal electrons in the ambient magnetosheath convect towards a bow wave, some electrons are shock-drift accelerated and reflected towards the ambient magnetosheath and others continue moving downstream of the bow wave resulting in bi-directional motion. Our study indicates that magnetosheath jet-driven bow waves can result in additional energization of suprathermal electrons in the magnetosheath. It implies that magnetosheath jets can increase the efficiency of electron acceleration at planetary bow shocks or other similar astrophysical environments.
The magnetization $|Omega_{mathrm e}|/omega_{mathrm{e}}$ is an important parameter in plasma astrophysics, where $Omega_{mathrm e}$ and $omega_{mathrm{e}}$ are the electron gyro-frequency and electron plasma frequency, respectively. It only depends on the mass ratio $m_{mathrm i}/m_{mathrm e}$ and the light-to-Alfven speed ratio $c/v_{mathrm{Ai}}$, where $m_{mathrm i}$ ($m_{mathrm e}$) is the ion (electron) mass, $c$ is the speed of light, and $v_{mathrm{Ai}}$ is the ion Alfven speed. Nonlinear numerical plasma models such as particle-in-cell simulations must often assume unrealistic values for $m_{mathrm i}/m_{mathrm e}$ and for $c/v_{mathrm{Ai}}$. Because linear theory yields exact results for parametric scalings of wave properties at small amplitudes, we use linear theory to investigate the dispersion relations of Alfven/ion-cyclotron and fast-magnetosonic/whistler waves as prime examples for collective plasma behaviour depending on $m_{mathrm i}/m_{mathrm e}$ and $c/v_{mathrm{Ai}}$. We analyse their dependence on $m_{mathrm i}/m_{mathrm e}$ and $c/v_{mathrm{Ai}}$ in quasi-parallel and quasi-perpendicular directions of propagation with respect to the background magnetic field for a plasma with $beta_jsim1$, where $beta_j$ is the ratio of the thermal to magnetic pressure for species $j$. Although their dispersion relations are largely independent of $c/v_{mathrm{Ai}}$ for $c/v_{mathrm{Ai}}gtrsim 10$, the mass ratio $m_{mathrm i}/m_{mathrm e}$ has a strong effect at scales smaller than the ion inertial length. Moreover, we study the impact of relativistic electron effects on the dispersion relations. Based on our results, we recommend aiming for a more realistic value of $m_{mathrm i}/m_{mathrm e}$ than for a more realistic value of $c/v_{mathrm{Ai}}$ in non-relativistic plasma simulations if such a choice is necessary, although $dots$
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