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Opinion ARTICLE

Front. Phys., 22 July 2016 | https://doi.org/10.3389/fphy.2016.00029

On Electron Adiabaticity in Collisionless Shocks

  • 1Institute for Space Sciences, National Institute for Lasers, Plasma and Radiation Physics, Bucharest, Romania
  • 2Institut für Theoretische Physik, Technische Universität Braunschweig, Braunschweig, Germany

1. Introduction

Collisionless shocks are considered as one of the most efficient phenomena for the electron acceleration in space and astrophysical plasmas, yet the detailed acceleration process and heating mechanism remain unsolved. Various acceleration and heating mechanisms have so far been proposed and discussed to explain the spacecraft measurements in situ at Earth's bow shock, involving the large-scale electromagnetic fields, plasma turbulence, and wave-particle interactions. A key element of the acceleration and heating mechanisms is the spatial scale of the shock transition with respect to the gyro-radius of the electrons [1]. In the simplest picture, since the shock transition occurs on a spatial scale of ion gyro-radius, the electrons behave as magnetized at the shock transition.

The cross-shock potential (electrostatic potential drop across the shock transition) plays two distinct roles in electron dynamics [2]. First, the cross-shock potential reduces a certain amount of the upstream flow kinetic energy (mostly carried by the ions due to their mass); Second, the kinetic energy is used to heat the electrons (DC heating). On the other hand, the electrons can also be scattered (or demagnetized) by high-amplitude, randomly-oscillating electromagnetic fields such as turbulence or strong inhomogeneities (AC heating). Space turbulence can potentially scatter the electrons randomly leaving upstream or downstream of the shock ramp region. For a quasi-perpendicular shock, the electrons become scattered in a non-adiabatic sense perpendicular to the mean magnetic field (Figure 1).

FIGURE 1
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Figure 1. Sketch on adiabatically parallel-accelerated electron (left) and non-adiabatically scattered electron back to the upstream region (right).

In this paper, the adiabaticity in collisionless shocks is addressed to the studies based on particle-in-cell-simulation (PIC). Two important parameters enter a PIC simulation carried out for the study of a collisionless shock: the mass ratio mpme between ions and electrons and the ratio of the electron plasma frequency to gyrofrequency ωpe ∕ Ωce, see e.g., [3]. Due to the computational limitations, the usage of the realistic values for the both parameters is not yet possible. The common way to overcome this problem is to find a compromise for the two parameters that can mostly optimize the purpose of the study. The impact of using low frequency ratio of the electron plasma frequency to gyrofrequency in numerical simulations has been discussed in Krasnoselskikh et al. [4]: the electric field is considerably overestimated than it should be for a comparison with the Earth bow shock. Here, I address the question, “What is the consequence of using a smaller frequency ratio upon the adiabatic motion of the electrons”?

2. Author's Opinion

2.1. Adiabaticity and PIC (Particle-in-Cell) Simulations

Adiabatic electrons are characterized by a constant magnetic moment μ=mv22B and a particle energy (which is a sum of the electrostatic potential and the kinetic energy) E=eΦ+12mv2+12mv2, where m is the electron mass, v|| and v the particle velocity parallel and perpendicular to the local magnetic field with a magnitude of the magnetic field B, respectively, e the elementary charge, and Φ the potential. The potential is electrostatic, and therefore, depends on the frame of Goodrich and Scudder [1]. Convenient choices are the references attached to the shock front such as the shock is in rest. In particular, the normal incident frame (NIF) and the de Hoffmann-Teller frame (HT) are useful in analyzing the electron dynamics in the shock transition layer. The both frames are inertial. On the assumption of a planar geometry and time stationarity, the shock transition reduces to one-dimensional variations along the shock normal direction. NIF sets the incoming flow aligned with the normal direction of the shock. HT minimizes the motional (or convective) electric field by introducing a slide with a speed tangential to the shock.

The gyro-radius of a magnetized electron changes with the magnitude of the magnetic field as ρ ∝ B−1∕2, where ρ denotes the electron gyro-radius. A sharp gradient of the magnetic field can demagnetize the electrons when the gradient scale is smaller than the gyro-radius. The particle becomes unmagnetized and it is perpendicular accelerated by the cross-shock potential.

Naively speaking, since a lower value of the frequency ratio ωpe ∕ Ωce means a higher gyro-frequency, the electrons should be more magnetized. Here I use a condition for the non-adiabatic electrons as [5],

R~NA=mec2eB2|dExdx|1    (1)

where Ex is the electric field along the shock normal in a suitable inertial frame of reference (NIF, HT, or any intermediate). The magnetic field B can be derived by using the centimetre-gram-second system of units (CGS) in terms of the electron plasma- and gyro-frequencies as,

B=mecΩcee=mecΩceωpeme4π=4πneτmec2    (2)

where ne is the upstream electron particle density, c is the speed of light, and the frequency ratio is denoted by,

τ=(ωpeΩce)2    (3)

The electric field E can be estimated by using Faraday's law for an electromagnetic whistler wave of frequency ω and wave number k standing on the shock front (as a precursor) and co-moving with the shock velocity vsh, [4],

E=1cωkB=1cvshB=1cMAVAB=MAmemp4πneτ2mec2    (4)

where, MA is the shock Mach number, and VA is the Alfvén velocity, VA=mempVAe=mempcτ12.

The magnetic field B~sim in the simulation code “em1D” [6], is defined in a dimensionless way by normalizing the field energy in the volume of the Debye cube λD3 (or a Debye sphere without the prefactor 4π∕3) to the rest energy of a single electron mec2,

B~sim2(B2λD3mec2)sim=(B2λD3mec2)obs    (5)

By comparing Equation (2) and Equation (5), the magnetic field obtained from the simulation is related to the observed one as,

Bsim=τobsτsimBobs    (6)

The magnitude of the electric field is normalized in the same fashion as in Equation (5) by replacing the magnetic field B by the electric field E. Then, by using Equation (4),

Esim=τobsτsim(mpme)real(mpme)simEobs    (7)

where (mpme)real is the realistic mass ratio and (mme)sim is the mass ratio used in the simulation. Assuming a spatial periodic structure with the wave number k, one may evaluate as dExdx=kδExkE. The left hand side of Equation (1) can be written as,

R~NAmec2eB2kE=kMAemempmec24πne=kλDMAmemp2τβe    (8)

The normalization relation for the non-adiabaticity condition is obtained as,

(R~NA)sim=τsimτobs(mpme)real(mpme)sim(R~NA)obs    (9)

A typical in-situ value for the electron plasma frequency to the electron gyrofrequency in the solar wind is (ωpe ∕ Ωce)obs ≈ 200, see e.g., [4], while for a common particle-in-cell simulation (using the realistic value for proton-to-electron mass ratio) is (ωpe ∕ Ωce)sim ≈ 10. Equations (6) and (7) show that the electromagnetic field is overestimated in the simulation by a factor of 20 for the magnitude of the magnetic field and by a factor of 400 for the magnitude of the electric field. Similarly, the non-adiabaticity condition in the simulation is underestimated by a factor of 20. Accordingly to Equation (1) and Equation (9), the electron in simulation is 20 times stronger magnetized than it usually has to be in the solar wind plasma.

Let us now briefly discuss the simulation results of Comişel et al. [7]. Here, the Liouville mapping has been applied to the results obtained in a particle-in-cell simulation of a high Mach number quasi-perpendicular shock. The following input parameters have been used: ωpe ∕ Ωce = 8, mpme = 1800, βe = βi = 0.2, MA = 8, and the angle between the upstream magnetic field direction and the normal to the shock is ΘBn = 81°. Due to strong residual electric fields, the cross-shock potential resulted from the Liouville mapping was compared with the integrated electric field in an ad-hoc de Hoffmann-Teller frame where the additional convective field components have been removed. I raise the question why Liouville mapping can provide relatively consistent results with the data from the simulation, although the electrons have to work against the existing motional electric field in the de Hoffmann-Teller frame.

Here, I argue that the low pitch angle electrons could cross the shock bound on the magnetic field line experiencing only the electrostatic component of the total electric field. Due to the simulation setup (low ωpe ∕ Ωce ratio or high gyrofrequency Ωce), these electrons should have small gyroradii and they consequently lose their adiabaticity. However, as long as the condition for breaking the adiabaticity is not fulfilled, the particles remain adiabatic and get parallel energy from the electrostatic field.

The energy gain for an adiabatic electron moving along the magnetic field line (or at low pitch angles) can be evaluated in a one-dimensional treatment of the two-fluid model of plasma as,

Δϵefluideb··Pene(n·b)dx            =ene(Pexxx+bybxPexyx+bzbxPexzx)dx    (10)

where the electric field in the de Hoffmann-Teller frame is approximated by the ambipolar term of the electron momentum equation and by an additional motional electric field component, Pe is the electron pressure tensor, b is the magnetic field unit vector, and n is the unit vector normal to the shock surface. The first term in the above integral represents the electron energy gain associated with the fluid estimate of the cross shock potential in the de Hoffmann-Teller frame, see e.g., [8]. The next two terms are the energy gain contribution from the electron stress (the non-diagonal terms of the electron pressure tensor) and has the role of electron heating at local electron kinetic scales. A good match between Δϵefluid and the energy gain provided by the Liouville mapping should support the proposed scenario. For highly energetic particles, the electromagnetic effect does not affect the energy gain irrespectively of the pitch angles, which should confirm the expectation that such highly energetic particles cross the shock layer so quickly and do not feel the variations of the electromagnetic field.

3. Summary and Future Perspective

I have analyzed the consequences of using low ratio values of the electron plasma frequency to gyrofrequency in particle-in-cell simulations. The electric field of electromagnetic origin tends to be overestimated due to the numerical setup of the electron Alfvén speed that is not representative for a typical solar wind condition and due to the overestimated magnetic field. The impact on the electron adiabaticity was discussed in the terms of the non-adiabaticity condition derived in Gedalin et al. [5]. The answer on the question addressed in the introduction is that the ratio between the electron plasma frequency to gyrofrequency can influence and significantly change the dynamics of the electron acceleration and heating in a quasi-perpendicular collisionless shock. By using small values of the frequency ratio τ, the electrons strengthen their adiabaticity and could have a less physical behavior than in observation. Further studies on the electron adiabaticity by using the Liouville mapping and particle-in-cell-simulations should consider more realistic values for the plasma input parameters.

Author Contributions

The author confirms being the sole contributor of this work and approved it for publication.

Funding

Financial supports by CNCS-UEFIS-CDI, PN–II–RU–TE–2014–4–2420, Studiu multi-scala al turbulentei in plasme astrofizice, of the Romanian Ministry for Scientific Research and Innovation and Collaborative Research Center 963, Astrophysical Flow, Instabilities, and Turbulence of the German Science Foundation are acknowledged.

Conflict of Interest Statement

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

HC is grateful for discussions with Manfred Scholer, Octav Marghitu, and Yasuhito Narita for their advices, critiques, and suggestions during the preparation of this manuscript. The authors gratefully acknowledge the computing time granted by the John von Neumann Institute for Computing (NIC) and provided on the supercomputer JURECA at Juelich Supercomputing Centre (JSC).

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Keywords: adiabaticity, electrons, collisionless shocks

Citation: Comişel H (2016) On Electron Adiabaticity in Collisionless Shocks. Front. Phys. 4:29. doi: 10.3389/fphy.2016.00029

Received: 18 February 2016; Accepted: 04 July 2016;
Published: 22 July 2016.

Edited by:

Michael Gedalin, Ben-Gurion University of the Negev, Israel

Reviewed by:

Simon Walker, The University of Sheffield, UK

Copyright © 2016 Comişel. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Horia Comişel, comisel@spacescience.ro