224 research outputs found
Gyrokinetic and kinetic particle-in-cell simulations of guide-field reconnection. I: Macroscopic effects of the electron flows
In this work, we compare gyrokinetic (GK) and fully kinetic Particle-in-Cell
(PIC) simulations of magnetic reconnection in the limit of strong guide field.
In particular, we analyze the limits of applicability of the GK plasma model
compared to a fully kinetic description of force free current sheets for finite
guide fields (). Here we report the first part of an extended comparison,
focusing on the macroscopic effects of the electron flows. For a low beta
plasma (), it is shown that both plasma models develop magnetic
reconnection with similar features in the secondary magnetic islands if a
sufficiently high guide field () is imposed in the kinetic PIC
simulations. Outside of these regions, in the separatrices close to the X
points, the convergence between both plasma descriptions is less restrictive
(). Kinetic PIC simulations using guide fields
reveal secondary magnetic islands with a core magnetic field and less energetic
flows inside of them in comparison to the GK or kinetic PIC runs with stronger
guide fields. We find that these processes are mostly due to an initial shear
flow absent in the GK initialization and negligible in the kinetic PIC high
guide field regime, in addition to fast outflows on the order of the ion
thermal speed that violate the GK ordering. Since secondary magnetic islands
appear after the reconnection peak time, a kinetic PIC/GK comparison is more
accurate in the linear phase of magnetic reconnection. For a high beta plasma
() where reconnection rates and fluctuations levels are reduced,
similar processes happen in the secondary magnetic islands in the fully kinetic
description, but requiring much lower guide fields ().Comment: 18 pages, 13 figures. Revised to match with the published version in
Physics of Plasma
Overview Of Nonlinear Kinetic Instabilities
The saturation of shear Alfven-like waves by alpha particles is presented from the general viewpoint of determining the saturation mechanisms of basic waves in a plasma destabilized by a perturbing source of free energy. The formalism is reviewed and then followed by analyses of isolated mode saturation far from and close to marginal stability. The effect of multiple waves that are isolated or are overlapping is then discussed. The presentation is concluded with a discussion of a non-conventional quasilinear theory that covers both extreme cases as well as the intermediate regime between the extremes.Physic
Electron‐ion Coulomb scattering and the electron Landau damping of Alfvén waves in the solar wind
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95528/1/jgra21228.pd
Hybrid Simulation of Solar-Wind-Like Turbulence
We present 2.5D hybrid simulations of the spectral and thermodynamic evolution of an initial state of magnetic field and plasma variables that in many ways represents solar wind fluctuations. In accordance with Helios near-Sun high-speed stream observations, we start with Alfvnic fluctuations along a mean magnetic field in which the fluctuations in the magnitude of the magnetic field are minimized. Since fluctuations in the radial flow speed are the dominant free energy in the observed fluctuations, we include a field-aligned v(k) with an k(exp 1) spectrum of velocity fluctuations to drive the turbulent evolution. The flow rapidly distorts the Alfvnic fluctuations, yielding spectra (determined by spacecraft-like cuts) transverse to the field that become comparable to the k fluctuations, as in spacecraft observations. The initial near constancy of the magnetic field is lost during the evolution; we show this also takes place observationally. We find some evolution in the anisotropy of the thermal fluctuations, consistent with expectations based on Helios data. We present 2D spectra of the fluctuations, showing the evolution of the power spectrum and cross-helicity. Despite simplifying assumptions, many aspects of simulations and observations agree. The greatly faster evolution in the simulations is at least in part due to the small scales being simulated, but also to the non-equilibrium initial conditions and the relatively low overall Alfvnicity of the initial fluctuations
Electron acceleration and parallel electric fields due to kinetic Alfvén waves in plasma with similar thermal and Alfvén speeds
We investigate electron acceleration due to shear Alfven waves in a collissionless plasma for plasma parameters typical of 4–5RE radial distance from the Earth along auroral field lines. Recent observational work has motivated this study, which explores the plasma regime where the thermal velocity of the electrons is similar to the Alfven speed of the plasma, encouraging Landau resonance for electrons in the wave fields. We use a self-consistent kinetic simulation model to follow the evolution of the electrons as they interact with a short-duration wave pulse, which allows us to determine the parallel electric field of the shear Alfven wave due to both electron inertia and electron pressure effects. The simulation demonstrates that electrons can be accelerated to keV energies in a modest amplitude sub-second period wave. We compare the parallel electric field obtained from the simulation with those provided by fluid approximations
Collisionless Magnetic Reconnection in Space Plasmas
Magnetic reconnection requires the violation of the frozen-in condition which
ties gyrating charged particles to the magnetic field inhibiting diffusion.
Ongoing reconnection has been identified in near-Earth space as being
responsible for the excitation of substorms, magnetic storms, generation of
field aligned currents and their consequences, the wealth of auroral phenomena.
Its theoretical understanding is now on the verge of being completed.
Reconnection takes place in thin current sheets. Analytical concepts proceeded
gradually down to the microscopic scale, the scale of the electron skin depth
or inertial length, recognizing that current layers that thin do preferentially
undergo spontaneous reconnection. Thick current layers start reconnecting when
being forced by plasma inflow to thin. For almost half a century the physical
mechanism of reconnection has remained a mystery. Spacecraft in situ
observations in combination with sophisticated numerical simulations in two and
three dimensions recently clarified the mist, finding that reconnection
produces a specific structure of the current layer inside the electron inertial
(also called electron diffusion) region around the reconnection site, the X
line. Onset of reconnection is attributed to pseudo-viscous contributions of
the electron pressure tensor aided by electron inertia and drag, creating a
complicated structured electron current sheet, electric fields, and an electron
exhaust extended along the current layer. We review the general background
theory and recent developments in numerical simulation on collisionless
reconnection. It is impossible to cover the entire field of reconnection in a
short space-limited review. The presentation necessarily remains cursory,
determined by our taste, preferences, and knowledge. Only a small amount of
observations is included in order to support the few selected numerical
simulations.Comment: Review pape
Thermal disequilibration of ions and electrons by collisionless plasma turbulence
Does overall thermal equilibrium exist between ions and electrons in a weakly
collisional, magnetised, turbulent plasma---and, if not, how is thermal energy
partitioned between ions and electrons? This is a fundamental question in
plasma physics, the answer to which is also crucial for predicting the
properties of far-distant astronomical objects such as accretion discs around
black holes. In the context of discs, this question was posed nearly two
decades ago and has since generated a sizeable literature. Here we provide the
answer for the case in which energy is injected into the plasma via Alfv\'enic
turbulence: collisionless turbulent heating typically acts to disequilibrate
the ion and electron temperatures. Numerical simulations using a hybrid
fluid-gyrokinetic model indicate that the ion-electron heating-rate ratio is an
increasing function of the thermal-to-magnetic energy ratio,
: it ranges from at to at
least for . This energy partition is
approximately insensitive to the ion-to-electron temperature ratio
. Thus, in the absence of other equilibrating
mechanisms, a collisionless plasma system heated via Alfv\'enic turbulence will
tend towards a nonequilibrium state in which one of the species is
significantly hotter than the other, viz., hotter ions at high
, hotter electrons at low . Spectra of
electromagnetic fields and the ion distribution function in 5D phase space
exhibit an interesting new magnetically dominated regime at high and
a tendency for the ion heating to be mediated by nonlinear phase mixing
("entropy cascade") when and by linear phase mixing
(Landau damping) when $\beta_\mathrm{i}\gg1
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