3,208 research outputs found
A Dynamical Model of Plasma Turbulence in the Solar Wind
A dynamical approach, rather than the usual statistical approach, is taken to
explore the physical mechanisms underlying the nonlinear transfer of energy,
the damping of the turbulent fluctuations, and the development of coherent
structures in kinetic plasma turbulence. It is argued that the linear and
nonlinear dynamics of Alfven waves are responsible, at a very fundamental
level, for some of the key qualitative features of plasma turbulence that
distinguish it from hydrodynamic turbulence, including the anisotropic cascade
of energy and the development of current sheets at small scales. The first
dynamical model of kinetic turbulence in the weakly collisional solar wind
plasma that combines self-consistently the physics of Alfven waves with the
development of small-scale current sheets is presented and its physical
implications are discussed. This model leads to a simplified perspective on the
nature of turbulence in a weakly collisional plasma: the nonlinear interactions
responsible for the turbulent cascade of energy and the formation of current
sheets are essentially fluid in nature, while the collisionless damping of the
turbulent fluctuations and the energy injection by kinetic instabilities are
essentially kinetic in nature.Comment: 29 pages, 2 figures, 196 references, accepted for publication in
Theme Issue on "Dissipation & Heating in Solar Wind Turbulence" in
Philosophical Transactions of the Royal Society
A prescription for the turbulent heating of astrophysical plasmas
The ratio of ion to electron heating due to the dissipation of Alfvenic
turbulence in astrophysical plasmas is calculated based on a cascade model for
turbulence in weakly collisional plasmas. Conditions for validity of this model
are discussed, a prescription for the turbulent heating is presented, and it is
applied to predict turbulent heating in accretion disks and the interstellar
medium.Comment: 5 pages, 4 figures, Accepted to Monthly Notices of Royal Astronomical
Society Letter
A Prospectus on Kinetic Heliophysics
Under the low density and high temperature conditions typical of heliospheric
plasmas, the macroscopic evolution of the heliosphere is strongly affected by
the kinetic plasma physics governing fundamental microphysical mechanisms.
Kinetic turbulence, collision less magnetic reconnection, particle
acceleration, and kinetic instabilities are four poorly understood,
grand-challenge problems that lie at the new frontier of kinetic heliophysics.
The increasing availability of high cadence and high phase-space resolution
measurements of particle velocity distributions by current and upcoming
spacecraft missions and of massively parallel nonlinear kinetic simulations of
weakly collisional heliospheric plasmas provides the opportunity to transform
our understanding of these kinetic mechanisms through the full utilization of
the information contained in the particle velocity distributions. Several major
considerations for future investigations of kinetic heliophysics are examined.
Turbulent dissipation followed by particle heating is highlighted as an
inherently two-step process in weakly collisional plasmas, distinct from the
more familiar case in fluid theory. Concerted efforts must be made to tackle
the big-data challenge of visualizing the high-dimensional (3D-3V) phase space
of kinetic plasma theory through physics-based reductions. Furthermore, the
development of innovative analysis methods that utilize full velocity-space
measurements, such as the field-particle correlation technique, will enable us
to gain deeper insight into these four grand-challenge problems of kinetic
heliophysics. A systems approach to tackle the multi-scale problem of
heliophysics through a rigorous connection between the kinetic physics at
microscales and the self-consistent evolution of the heliosphere at macro
scales will propel the field of kinetic heliophysics into the future.Comment: Ronald C. Davidson Award paper, 13 pages, 1 figure, in press with
Physics of Plasma
The Inherently Three-Dimensional Nature of Magnetized Plasma Turbulence
It is often asserted or implicitly assumed, without justification, that the
results of two-dimensional investigations of plasma turbulence are applicable
to the three-dimensional plasma environments of interest. A projection method
is applied to derive two scalar equations that govern the nonlinear evolution
of the Alfvenic and pseudo-Alfvenic components of ideal incompressible
magnetohydrodynamic (MHD) plasma turbulence. The mathematical form of these
equations makes clear the inherently three-dimensional nature of plasma
turbulence, enabling an analysis of the nonlinear properties of two-dimensional
limits often used to study plasma turbulence. In the anisotropic limit k_perp
>>k_parallel that naturally arises in magnetized plasma systems, the
perpendicular 2D limit retains the dominant nonlinearities that are mediated
only by the Alfvenic fluctuations but lacks the wave physics associated with
the linear term that is necessary to capture the anisotropic cascade of
turbulent energy. In the in-plane 2D limit, the nonlinear energy transfer is
controlled instead by the pseudo-Alfven waves, with the Alfven waves relegated
to a passive role. In the oblique 2D limit, an unavoidable azimuthal dependence
connecting the wavevector components will likely cause artificial azimuthal
asymmetries in the resulting turbulent dynamics. Therefore, none of these 2D
limits is sufficient to capture fully the rich three-dimensional nonlinear
dynamics critical to the evolution of plasma turbulence.Comment: 18 pages, submitted to Special Issue of Journal of Plasma Physics on
"Present achievements and new frontiers in space plasmas
The Dynamical Generation of Current Sheets in Astrophysical Plasma Turbulence
Turbulence profoundly affects particle transport and plasma heating in many
astrophysical plasma environments, from galaxy clusters to the solar corona and
solar wind to Earth's magnetosphere. Both fluid and kinetic simulations of
plasma turbulence ubiquitously generate coherent structures, in the form of
current sheets, at small scales, and the locations of these current sheets
appear to be associated with enhanced rates of dissipation of the turbulent
energy. Therefore, illuminating the origin and nature of these current sheets
is critical to identifying the dominant physical mechanisms of dissipation, a
primary aim at the forefront of plasma turbulence research. Here we present
evidence from nonlinear gyrokinetic simulations that strong nonlinear
interactions between counterpropagating Alfven waves, or strong Alfven wave
collisions, are a natural mechanism for the generation of current sheets in
plasma turbulence. Furthermore, we conceptually explain this current sheet
development in terms of the nonlinear dynamics of Alfven wave collisions,
showing that these current sheets arise through constructive interference among
the initial Alfven waves and nonlinearly generated modes. The properties of
current sheets generated by a strong Alfven wave collisions are compared to
published observations of current sheets in the Earth's magnetosheath and the
solar wind, and the nature of these current sheets leads to the expectation
that Landau damping of the constituent Alfven waves plays a dominant role in
the damping of turbulently generated current sheets.Comment: 7 pages, 5 figures, accepted to Astrophysical Journal Letter
Current Sheets and Collisionless Damping in Kinetic Plasma Turbulence
We present the first study of the formation and dissipation of current sheets
at electron scales in a wave-driven, weakly collisional, 3D kinetic turbulence
simulation. We investigate the relative importance of dissipation associated
with collisionless damping via resonant wave-particle interactions versus
dissipation in small-scale current sheets in weakly collisional plasma
turbulence. Current sheets form self-consistently from the wave-driven
turbulence, and their filling fraction is well correlated to the electron
heating rate. However, the weakly collisional nature of the simulation
necessarily implies that the current sheets are not significantly dissipated
via Ohmic dissipation. Rather, collisionless damping via the Landau resonance
with the electrons is sufficient to account for the measured heating as a
function of scale in the simulation, without the need for significant Ohmic
dissipation. This finding suggests the possibility that the dissipation of the
current sheets is governed by resonant wave-particle interactions and that the
locations of current sheets correspond spatially to regions of enhanced
heating.Comment: 8 pages, 5 figures, accepted to ApJ
The Alfvenic nature of energy transfer mediation in localized, strongly nonlinear Alfven wavepacket collisions
In space and astrophysical plasmas, violent events or instabilities inject
energy into turbulent motions at large scales. Nonlinear interactions among the
turbulent fluctuations drive a cascade of energy to small perpendicular scales
at which the energy is ultimately converted into plasma heat. Previous work
with the incompressible magnetohydrodynamic (MHD) equations has shown that this
turbulent energy cascade is driven by the nonlinear interaction between
counterpropagating Alfven waves - also known as Alfven wave collisions. Direct
numerical simulations of weakly collisional plasma turbulence enables deeper
insight into the nature of the nonlinear interactions underlying the turbulent
cascade of energy. In this paper, we directly compare four cases: both periodic
and localized Alfven wave collisions in the weakly and strongly nonlinear
limits. Our results reveal that in the more realistic case of localized Alfven
wave collisions (rather than the periodic case), all nonlinearly generated
fluctuations are Alfven waves, which mediates nonlinear energy transfer to
smaller perpendicular scales.Comment: 19 pages, 7 figure
Measuring Collisionless Damping in Heliospheric Plasmas using Field-Particle Correlations
An innovative field-particle correlation technique is proposed that uses
single-point measurements of the electromagnetic fields and particle velocity
distribution functions to investigate the net transfer of energy from fields to
particles associated with the collisionless damping of turbulent fluctuations
in weakly collisional plasmas, such as the solar wind. In addition to providing
a direct estimate of the local rate of energy transfer between fields and
particles, it provides vital new information about the distribution of that
energy transfer in velocity space. This velocity-space signature can
potentially be used to identify the dominant collisionless mechanism
responsible for the damping of turbulent fluctuations in the solar wind. The
application of this novel field-particle correlation technique is illustrated
using the simplified case of the Landau damping of Langmuir waves in an
electrostatic 1D-1V Vlasov-Poisson plasma, showing that the procedure both
estimates the local rate of energy transfer from the electrostatic field to the
electrons and indicates the resonant nature of this interaction. Modifications
of the technique to enable single-point spacecraft measurements of fields and
particles to diagnose the collisionless damping of turbulent fluctuations in
the solar wind are discussed, yielding a method with the potential to transform
our ability to maximize the scientific return from current and upcoming
spacecraft missions, such as the Magnetospheric Multiscale (MMS) and Solar
Probe Plus missions.Comment: 6 pages, 4 figures. Accepted for publication in Astrophysical Journal
Letter
Alfven Wave Collisions, The Fundamental Building Block of Plasma Turbulence I: Asymptotic Solution
The nonlinear interaction between counterpropagating Alfven waves is the
physical mechanism underlying the cascade of energy to small scales in
astrophysical plasma turbulence. Beginning with the equations for
incompressible MHD, an asymptotic analytical solution for the nonlinear
evolution of these Alfven wave collisions is derived in the weakly nonlinear
limit. The resulting qualitative picture of nonlinear energy transfer due to
this mechanism involves two steps: first, the primary counterpropagating Alfven
waves interact to generate an inherently nonlinear, purely magnetic secondary
fluctuation with no parallel variation; second, the two primary waves each
interact with this secondary fluctuation to transfer energy secularly to two
tertiary Alfven waves. These tertiary modes are linear Alfven waves with the
same parallel wavenumber as the primary waves, indicating the lack of a
parallel cascade. The amplitude of these tertiary modes increases linearly with
time due to the coherent nature of the resonant four-wave interaction
responsible for the nonlinear energy transfer. The implications of this
analytical solution for turbulence in astrophysical plasmas is discussed. The
solution presented here provides valuable intuition about the nonlinear
interactions underlying magnetized plasma turbulence, in support of an
experimental program to verify in the laboratory the nature of this fundamental
building block of astrophysical plasma turbulence.Comment: 24 pages, 2 figures, accepted to Physics of Plasma
Nonlinear energy transfer and current sheet development in localized Alfven wavepacket collisions in the strong turbulence limit
In space and astrophysical plasmas, turbulence is responsible for
transferring energy from large scales driven by violent events or
instabilities, to smaller scales where turbulent energy is ultimately converted
into plasma heat by dissipative mechanisms. The nonlinear interaction between
counterpropagating Alfven waves, denoted Alfven wave collisions, drives this
turbulent energy cascade, as recognized by early work with incompressible
magnetohydrodynamic (MHD) equations. Recent work employing analytical
calculations and nonlinear gyrokinetic simulations of Alfven wave collisions in
an idealized periodic initial state have demonstrated the key properties that
strong Alfven wave collisions mediate effectively the transfer of energy to
smaller perpendicular scales and self-consistently generate current sheets. For
the more realistic case of the collision between two initially separated Alfven
wavepackets, we use a nonlinear gyrokinetic simulation to show here that these
key properties persist: strong Alfven wavepacket collisions indeed facilitate
the perpendicular cascade of energy and give rise to current sheets.
Furthermore, the evolution shows that nonlinear interactions occur only while
the wavepackets overlap, followed by a clean separation of the wavepackets with
straight uniform magnetic fields and the cessation of nonlinear evolution in
between collisions, even in the gyrokinetic simulation presented here which
resolves dispersive and kinetic effects beyond the reach of the MHD theory.Comment: 19 pages, 7 figure
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