5,718 research outputs found
Damping of Electron Density Structures and Implications for Interstellar Scintillation
The forms of electron density structures in kinetic Alfven wave turbulence
are studied in connection with scintillation. The focus is on small scales cm where the Kinetic Alfv\'en wave (KAW) regime is active in
the interstellar medium. MHD turbulence converts to a KAW cascade, starting at
10 times the ion gyroradius and continuing to smaller scales. These scales are
inferred to dominate scintillation in the theory of Boldyrev et al. From
numerical solutions of a decaying kinetic Alfv\'en wave turbulence model,
structure morphology reveals two types of localized structures, filaments and
sheets, and shows that they arise in different regimes of resistive and
diffusive damping. Minimal resistive damping yields localized current filaments
that form out of Gaussian-distributed initial conditions. When resistive
damping is large relative to diffusive damping, sheet-like structures form. In
the filamentary regime, each filament is associated with a non-localized
magnetic and density structure, circularly symmetric in cross section. Density
and magnetic fields have Gaussian statistics (as inferred from Gaussian-valued
kurtosis) while density gradients are strongly non-Gaussian, more so than
current. This enhancement of non-Gaussian statistics in a derivative field is
expected since gradient operations enhance small-scale fluctuations. The
enhancement of density gradient kurtosis over current kurtosis is not obvious,
yet it suggests that modest fluctuation levels in electron density may yield
large scintillation events during pulsar signal propagation in the interstellar
medium. In the sheet regime the same statistical observations hold, despite the
absence of localized filamentary structures. Probability density functions are
constructed from statistical ensembles in both regimes, showing clear formation
of long, highly non-Gaussian tails
On geometric properties of passive random advection
We study geometric properties of a random Gaussian short-time correlated
velocity field by considering statistics of a passively advected metric tensor.
That describes universal properties of fluctuations of tensor objects frozen
into the fluid and passively advected by it. The problem of one-point
statistics of co- and contravariant tensors is solved exactly, provided the
advected fields do not reach dissipative scales, which would break the symmetry
of the problem. Asymptotic in time duality of the problem is established, which
in the three-dimensional case relates the probabilities of the volume
deformations into "tubes" and into "sheets".Comment: latex, 8 page
Thermodynamic analysis of new cycles for liquid-metal MHD generators
Acceleration devices for liquid metal magnetohydrodynamic generator
Magnetohydrodynamic turbulence mediated by reconnection
Magnetic field fluctuations in MHD turbulence can be viewed as current sheets
that are progressively more anisotropic at smaller scales. As suggested by
Loureiro & Boldyrev (2017) and Mallet et al (2017), below a certain critical
thickness such current sheets become tearing-unstable. We propose
that the tearing instability changes the effective alignment of the magnetic
field lines in such a way as to balance the eddy turnover rate at all scales
smaller than . As a result, turbulent fluctuations become
progressively less anisotropic at smaller scales, with the alignment angle
increasing as , where
is the resistive dissipation scale. Here
is the outer scale of the turbulence, is the corresponding Lundquist
number, and {} is a parameter. The resulting Fourier energy
spectrum is , where is
the wavenumber normal to the local mean magnetic field, and the critical scale
is . The simplest model
corresponds to , in which case the predicted scaling formally agrees
with one of the solutions obtained in (Mallet et al 2017) from a discrete
hierarchical model of abruptly collapsing current sheets, an approach different
and complementary to ours. We also show that the reconnection-mediated interval
is non-universal with respect to the dissipation mechanism. Hyper-resistivity
of the form leads (in the simplest case of )
to the different transition scale
and the energy spectrum , where
is the corresponding hyper-resistive Lundquist number.Comment: submitted for publicatio
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