22 research outputs found
Multiphysics simulations of collisionless plasmas
Collisionless plasmas, mostly present in astrophysical and space
environments, often require a kinetic treatment as given by the Vlasov
equation. Unfortunately, the six-dimensional Vlasov equation can only be solved
on very small parts of the considered spatial domain. However, in some cases,
e.g. magnetic reconnection, it is sufficient to solve the Vlasov equation in a
localized domain and solve the remaining domain by appropriate fluid models. In
this paper, we describe a hierarchical treatment of collisionless plasmas in
the following way. On the finest level of description, the Vlasov equation is
solved both for ions and electrons. The next courser description treats
electrons with a 10-moment fluid model incorporating a simplified treatment of
Landau damping. At the boundary between the electron kinetic and fluid region,
the central question is how the fluid moments influence the electron
distribution function. On the next coarser level of description the ions are
treated by an 10-moment fluid model as well. It may turn out that in some
spatial regions far away from the reconnection zone the temperature tensor in
the 10-moment description is nearly isotopic. In this case it is even possible
to switch to a 5-moment description. This change can be done separately for
ions and electrons. To test this multiphysics approach, we apply this full
physics-adaptive simulations to the Geospace Environmental Modeling (GEM)
challenge of magnetic reconnection.Comment: 13 pages, 5 figure
PolyPIC: the Polymorphic-Particle-in-Cell Method for Fluid-Kinetic Coupling
Particle-in-Cell (PIC) methods are widely used computational tools for fluid
and kinetic plasma modeling. While both the fluid and kinetic PIC approaches
have been successfully used to target either kinetic or fluid simulations,
little was done to combine fluid and kinetic particles under the same PIC
framework. This work addresses this issue by proposing a new PIC method,
PolyPIC, that uses polymorphic computational particles. In this numerical
scheme, particles can be either kinetic or fluid, and fluid particles can
become kinetic when necessary, e.g. particles undergoing a strong acceleration.
We design and implement the PolyPIC method, and test it against the Landau
damping of Langmuir and ion acoustic waves, two stream instability and sheath
formation. We unify the fluid and kinetic PIC methods under one common
framework comprising both fluid and kinetic particles, providing a tool for
adaptive fluid-kinetic coupling in plasma simulations.Comment: Submitted to Frontier
Electron Signatures of Reconnection in a Global eVlasiator Simulation
Geospace plasma simulations have progressed toward more realistic descriptions of the solar wind-magnetosphere interaction from magnetohydrodynamic to hybrid ion-kinetic, such as the state-of-the-art Vlasiator model. Despite computational advances, electron scales have been out of reach in a global setting. eVlasiator, a novel Vlasiator submodule, shows for the first time how electromagnetic fields driven by global hybrid-ion kinetics influence electrons, resulting in kinetic signatures. We analyze simulated electron distributions associated with reconnection sites and compare them with Magnetospheric Multiscale (MMS) spacecraft observations. Comparison with MMS shows that key electron features, such as reconnection inflows, heated outflows, flat-top distributions, and bidirectional streaming, are in remarkable agreement. Thus, we show that many reconnection-related features can be reproduced despite strongly truncated electron physics and an ion-scale spatial resolution. Ion-scale dynamics and ion-driven magnetic fields are shown to be significantly responsible for the environment that produces electron dynamics observed by spacecraft in near-Earth plasmas.Peer reviewe
Global MHD simulations of Mercury's magnetosphere with coupled planetary interior: Induction effect of the planetary conducting core on the global interaction
Mercury's comparatively weak intrinsic magnetic field and its close proximity to the Sun lead to a magnetosphere that undergoes more direct spaceâweathering interactions than other planets. A unique aspect of Mercury's interaction system arises from the large ratio of the scale of the planet to the scale of the magnetosphere and the presence of a largeâsize core composed of highly conducting material. Consequently, there is strong feedback between the planetary interior and the magnetosphere, especially under conditions of strong external forcing. Understanding the coupled solar windâmagnetosphereâinterior interaction at Mercury requires not only analysis of observations but also a modeling framework that is both comprehensive and inclusive. We have developed a new global MHD model for Mercury in which the planetary interior is modeled as layers of different electrical conductivities that electromagnetically couple to the surrounding plasma environment. This new modeling capability allows us to characterize the dynamical response of Mercury to timeâvarying external conditions in a selfâconsistent manner. Comparison of our model results with observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft shows that the model provides a reasonably good representation of the global magnetosphere. To demonstrate the capability to model induction effects, we have performed idealized simulations in which Mercury's magnetosphere is impacted by a solar wind pressure enhancement. Our results show that due to the induction effect, Mercury's core exerts strong global influences on the way Mercury responds to changes in the external environment, including modifying the global magnetospheric structure and affecting the extent to which the solar wind directly impacts the surface. The global MHD model presented here represents a crucial step toward establishing a modeling framework that enables selfâconsistent characterization of Mercury's tightly coupled planetary interiorâmagnetosphere system.Key PointsDeveloped global MHD model of Mercury's magnetosphere with coupled interiorThe global model is able to selfâconsistently model induction effect at the coreInduction effect is shown to have significant effects on the global interactionPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112187/1/jgra51879.pd
Multiphysics Simulations of Collisionless Plasmas
Collisionless plasmas, mostly present in astrophysical and space environments, often require a kinetic treatment as given by the Vlasov equation. Unfortunately, the six-dimensional Vlasov equation can only be solved on very small parts of the considered spatial domain. However, in some cases, e.g. magnetic reconnection, it is sufficient to solve the Vlasov equation in a localized domain and solve the remaining domain by appropriate fluid models. In this paper, we describe a hierarchical treatment of collisionless plasmas in the following way. On the finest level of description, the Vlasov equation is solved both for ions and electrons. The next courser description treats electrons with a 10-moment fluid model incorporating a simplified treatment of Landau damping. At the boundary between the electron kinetic and fluid region, the central question is how the fluid moments influence the electron distribution function. On the next coarser level of description the ions are treated by an 10-moment fluid model as well. It may turn out that in some spatial regions far away from the reconnection zone the temperature tensor in the 10-moment description is nearly isotopic. In this case it is even possible to switch to a 5-moment description. This change can be done separately for ions and electrons. To test this multiphysics approach, we apply this full physics-adaptive simulations to the Geospace Environmental Modeling (GEM) challenge of magnetic reconnection
Spectral Approach to Plasma Kinetic Simulations Based on Hermite Decomposition in the Velocity Space
Spectral (transform) methods for solution of Vlasov-Maxwell system have shown significant promise as numerical methods capable of efficiently treating fluid-kinetic coupling in magnetized plasmas. We discuss SpectralPlasmaSolver (SPS), an implementation of three-dimensional, fully electromagnetic algorithm based on a decomposition of the plasma distribution function in Hermite modes in velocity space and Fourier modes in physical space. A fully-implicit time discretization is adopted for numerical stability and to ensure exact conservation laws for total mass, momentum and energy. The SPS code is parallelized using Message Passing Interface for distributed memory architectures. Application of the method to analysis of kinetic range of scales in plasma turbulence under conditions typical of the solar wind is demonstrated. With only 4 Hermite modes per velocity dimension, the algorithm yields damping rates of kinetic Alfvén waves with accuracy of 50% or better, which is sufficient to obtain a model of kinetic scales capable of reproducing many of the expected statistical properties of turbulent fluctuations. With increasing number of Hermite modes, progressively more accurate values for collisionless damping rates are obtained. Fully nonlinear simulations of decaying turbulence are presented and successfully compared with similar simulations performed using Particle-In-Cell method
Parallel transport modeling of linear divertor simulators with fundamental ion cyclotron heating
The Material Plasma Exposure eXperiment (MPEX) is a steady state linear
device with the goal to perform plasma material interaction (PMI) studies at
future fusion reactor relevant conditions. A prototype of MPEX referred as
Porto-MPEX is designed to carry out research and development related to source,
heating and transport concepts on the planned full MPEX device. The auxiliary
heating schemes in MPEX are based on cyclotron resonance heating with radio
frequency (RF) waves. Ion cyclotron heating (ICH) and electron cyclotron
heating (ECH) in MPEX are used to independently heat the ions and electrons and
provide fusion divertor conditions ranging from sheath-limited to fully
detached divertor regimes at a material target. A Hybrid Particle-In-Cell code-
PICOS++ is developed and applied to understand the plasma parallel transport
during ICH heating in MPEX Proto-MPEX to the target. With this tool, evolution
of the distribution function of MPEX/Proto-MPEX ions is modeled in the presence
of (1) Coulomb collisions, (2) volumetric particle sources and (3) quasi-linear
RF-based ICH. The code is benchmarked against experimental data from Proto-MPEX
and simulation data from B2.5 EIRENE. The experimental observation of
density-drop near the target in Proto-MPEX and MPEX during ICH heating is
demonstrated and explained via physics-based arguments using PICOS++ modeling.
In fact, the density drops at the target during ICH in Proto-MPEX/MPEX to
conserve the flux and to compensate for the increased flow during ICH.
Furthermore, sensitivity scans of various plasma parameters with respect to ICH
power are performed for MPEX to investigate its role on plasma transport and
particle and energy fluxes at the target. Finally, we discuss the pathway to
model ECH in MPEX using the Hybrid PIC formulation herein presented for kinetic
electrons and fluid ions
Generalized, energy-conserving numerical simulations of particles in general relativity. II. Test particles in electromagnetic fields and GRMHD
Direct observations of compact objects, in the form of radiation spectra,
gravitational waves from VIRGO/LIGO, and forthcoming direct imaging, are
currently one of the primary source of information on the physics of plasmas in
extreme astrophysical environments. The modeling of such physical phenomena
requires numerical methods that allow for the simulation of microscopic plasma
dynamics in presence of both strong gravity and electromagnetic fields. In
Bacchini et al. (2018) we presented a detailed study on numerical techniques
for the integration of free geodesic motion. Here we extend the study by
introducing electromagnetic forces in the simulation of charged particles in
curved spacetimes. We extend the Hamiltonian energy-conserving method presented
in Bacchini et al. (2018) to include the Lorentz force and we test its
performance compared to that of standard explicit Runge-Kutta and implicit
midpoint rule schemes against analytic solutions. Then, we show the application
of the numerical schemes to the integration of test particle trajectories in
general relativistic magnetohydrodynamic (GRMHD) simulations, by modifying the
algorithms to handle grid-based electromagnetic fields. We test this approach
by simulating ensembles of charged particles in a static GRMHD configuration
obtained with the Black Hole Accretion Code (BHAC)