Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userIon acceleration to MeV by the ExB wave mechanism in collisionless shocks
137
0
0.0
(
0
)
Added by
Krzysztof Stasiewicz
Publication date
2021
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
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.
rate research
Read More
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.
The existence and properties of low Mach-number ($M gtrsim 1$) electrostatic collisionless shocks are investigated with a semi-analytical solution for the shock structure. We show that the properties of the shock obtained in the semi-analytical model can be well reproduced in fully kinetic Eulerian Vlasov-Poisson simulations, where the shock is generated by the decay of an initial density discontinuity. Using this semi-analytical model, we study the effect of electron-to-ion temperature ratio and presence of impurities on both the maximum shock potential and Mach number. We find that even a small amount of impurities can influence the shock properties significantly, including the reflected light ion fraction, which can change several orders of magnitude. Electrostatic shocks in heavy ion plasmas reflect most of the hydrogen impurity ions.
We present a study of the acceleration efficiency of suprathermal electrons at collisionless shock waves driven by interplanetary coronal mass ejections (ICMEs), with the data analysis from both the spacecraft observations and test-particle simulations. The observations are from the 3DP/EESA instrument onboard emph{Wind} during the 74 shock events listed in Yang et al. 2019, ApJ, and the test-particle simulations are carried out through 315 cases with different shock parameters. It is shown that a large shock-normal angle, upstream Alfv$acute{text e}$n Mach number, and shock compression ratio would enhance the shock acceleration efficiency. In addition, we develop a theoretical model of the critical shock normal angle for efficient shock acceleration by assuming the shock drift acceleration to be efficient. We also obtain models for the critical values of Mach number and compression ratio with efficient shock acceleration, based on the suggestion of Drury 1983 about the average momentum change of particle crossing of shock. It is shown that the theories have similar trends of the observations and simulations. Therefore, our results suggest that the shock drift acceleration is efficient in the electron acceleration by ICME-driven shocks, which confirms the findings of Yang et al.
The concept of energetic particle reservoirs, essentially based on the assumption of the presence of outer reflecting boundaries/magnetic mirrors or diffusion barriers (deterministic) rather than on the effect of particle diffusive propagation (stochastic) in magnetic turbulence, has been used for decades to describe the space-extended decay phases of energetic particle events within the fields of space physics, solar physics, and plasma physics. Using five-dimensional time-dependent Fokker-Planck transport equation simulations, in this work we demonstrate that the so-called particle reservoirs are naturally explained and quantitatively reproduced by diffusion processes in turbulent magnetic fields, without invoking the hypothesis of reflecting boundaries. Our results strongly suggest that the so-called reservoir (based on deterministic structure) should be renamed flood (based on stochastic diffusion), which symbolizes an authentic shift in thinking and in pragmatic rationale for the studies of energetic particles and relevant plasma phenomena in heliophysics and in astrophysics.
Astrophysical shocks are often collisionless shocks. An open question about collisionless shocks is whether electrons and ions each establish their own post-shock temperature, or whether they quickly equilibrate in the shock region. Here we provide simple relations for the minimal amount of equilibration to expect. The basic assumption is that the enthalpy-flux of the electrons is conserved separately, but that all particle species should undergo the same density jump across the the shock. This assumption results in an analytic treatment of electron-ion equilibration that agrees with observations of collisionless shocks: at low Mach numbers ($<2$) the electrons and ions are close to equilibration, whereas for Mach numbers above $M sim 60$ the electron-ion temperature ratio scales with the particle masses $T_e/T_i = m_e/m_i$. In between these two extremes the electron-ion temperature ratio scales as $T_e/T_i propto 1/M_s^2$. This relation also hold if adiabatic compression of the electrons is taken into account. For magnetised plasmas the compression is governed by the magnetosonic Mach number, whereas the electron-ion temperatures are governed by the sonic Mach number. The derived equations are in agreement with observational data at low Mach numbers, but for supernova remnants the relation requires that the inferred Mach numbers for the observations are over- estimated, perhaps as a result of upstream heating in the cosmic-ray precursor. In addition to predicting a minimal electron/ion temperature ratio, we also heuristically incorporate ion-electron heat exchange at the shock, quantified with a dimensionless parameter ${xi}$. Comparing the model to existing observations in the solar system and supernova remnants suggests that the data are best described by ${xi} sim 5$ percent. (Abridged abstract.)
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
