3,944 research outputs found
Effect of Fractional Kinetic Helicity on Turbulent Magnetic Dynamo Spectra
Magnetic field amplification in astrophysics ultimately requires an
understanding of magnetohydrodynamic turbulence. Kinetic helicity has long been
known to be important for large scale field growth in forced MHD turbulence,
and has been recently demonstrated numerically to be asymptotically consistent
with slow mean field dynamo action in a periodic box. Here we show numerically
that the magnetic spectrum at and below the forcing scale is also strongly
influenced by kinetic helicity. We identify a critical value,
above which the magnetic spectrum develops maxima at wavenumber scale
{\it and} at the forcing scale, For the field peaks only at the
resistive scale. Kinetic helicity may thus be important not only for generating
a large scale field, but also for establishing observed peaks in magnetic
spectra at the forcing scale. The turbulent Galactic disk provides an example
where both large scale ( supernova forcing scale) fields and small scale
( forcing scale, with peak at forcing scale) fields are observed. We
discuss this, and the potential application to the protogalaxy, but also
emphasize the limitations in applying our results to these systems.Comment: version accepted to ApJL, 10 pages, 3 fig
Implications of mean field accretion disc theory for vorticity and magnetic field growth
In addition to the scalar Shakura-Sunyaev turbulent viscosity
transport term used in simple analytic accretion disc modeling, a pseudoscalar
transport term also arises. The essence of this term can be captured even in
simple models for which vertical averaging is interpreted as integration over a
half-thickness and one separately studies each hemisphere. The additional term
highlights a complementarity between mean field magnetic dynamo theory and
accretion disc theory treated as a mean field theory. Such pseudoscalar terms
have been studied, and can lead to large scale magnetic field and vorticity
growth. Here it is shown that vorticity can grow even in the simplest azimuthal
and half-height integrated disc model, for which mean quantities depend only on
radius. The simplest vorticity growth solutions seem to have scales and vortex
survival times consistent those required for facilitating planet formation.
Also it is shown that when the magnetic back-reaction is included to lowest
order, the pseudoscalar driving the magnetic field growth and that driving the
vorticity growth will behave differently with respect to shearing and
non-shearing flows: the former can reverse sign in the two cases, while the
latter will have the same sign.Comment: 17 Pages LaTex, revised versio
Simulations of a Magnetic Fluctuation Driven Large Scale Dynamo and Comparison with a Two-scale Model
Models of large scale (magnetohydrodynamic) dynamos (LSD) which couple large
scale field growth to total magnetic helicity evolution best predict the
saturation of LSDs seen in simulations. For the simplest so called "{\alpha}2"
LSDs in periodic boxes, the electromotive force driving LSD growth depends on
the difference between the time-integrated kinetic and current helicity
associated with fluctuations. When the system is helically kinetically forced
(KF), the growth of the large scale helical field is accompanied by growth of
small scale magnetic (and current) helicity which ultimately quench the LSD.
Here, using both simulations and theory, we study the complementary
magnetically forced(MF) case in which the system is forced with an electric
field that supplies magnetic helicity. For this MF case, the kinetic helicity
becomes the back-reactor that saturates the LSD. Simulations of both MF and KF
cases can be approximately modeled with the same equations of magnetic helicity
evolution, but with complementary initial conditions. A key difference between
KF and MF cases is that the helical large scale field in the MF case grows with
the same sign of injected magnetic helicity, whereas the large and small scale
magnetic helicities grow with opposite sign for the KF case. The MF case can
arise even when the thermal pressure is approximately smaller than the magnetic
pressure, and requires only that helical small scale magnetic fluctuations
dominate helical velocity fluctuations in LSD driving. We suggest that LSDs in
accretion discs and Babcock models of the solar dynamo are actually MF LSDs.Comment: 12 pages, 34 figure
The formation of high-field magnetic white dwarfs from common envelopes
The origin of highly-magnetized white dwarfs has remained a mystery since
their initial discovery. Recent observations indicate that the formation of
high-field magnetic white dwarfs is intimately related to strong binary
interactions during post-main-sequence phases of stellar evolution. If a
low-mass companion, such as a planet, brown dwarf, or low-mass star is engulfed
by a post-main-sequence giant, the hydrodynamic drag in the envelope of the
giant leads to a reduction of the companion's orbit. Sufficiently low-mass
companions in-spiral until they are shredded by the strong gravitational tides
near the white dwarf core. Subsequent formation of a super-Eddington accretion
disk from the disrupted companion inside a common envelope can dramatically
amplify magnetic fields via a dynamo. Here, we show that these disk-generated
fields are sufficiently strong to explain the observed range of magnetic field
strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary
analogue may also contribute to the origin of magnetar fields.Comment: Accepted to Proceedings of the National Academy of Sciences. Under
PNAS embargo until time of publicatio
An architecture for efficient gravitational wave parameter estimation with multimodal linear surrogate models
The recent direct observation of gravitational waves has further emphasized
the desire for fast, low-cost, and accurate methods to infer the parameters of
gravitational wave sources. Due to expense in waveform generation and data
handling, the cost of evaluating the likelihood function limits the
computational performance of these calculations. Building on recently developed
surrogate models and a novel parameter estimation pipeline, we show how to
quickly generate the likelihood function as an analytic, closed-form
expression. Using a straightforward variant of a production-scale parameter
estimation code, we demonstrate our method using surrogate models of
effective-one-body and numerical relativity waveforms. Our study is the first
time these models have been used for parameter estimation and one of the first
ever parameter estimation calculations with multi-modal numerical relativity
waveforms, which include all l <= 4 modes. Our grid-free method enables rapid
parameter estimation for any waveform with a suitable reduced-order model. The
methods described in this paper may also find use in other data analysis
studies, such as vetting coincident events or the computation of the
coalescing-compact-binary detection statistic.Comment: 10 pages, 3 figures, and 1 tabl
Three Dimensional Evolution of a Relativistic Current Sheet : Triggering of Magnetic Reconnection by the Guide Field
The linear and non-linear evolution of a relativistic current sheet of pair
() plasmas is investigated by three-dimensional particle-in-cell
simulations. In a Harris configuration, it is obtained that the magnetic energy
is fast dissipated by the relativistic drift kink instability (RDKI). However,
when a current-aligned magnetic field (the so-called "guide field") is
introduced, the RDKI is stabilized by the magnetic tension force and it
separates into two obliquely-propagating modes, which we call the relativistic
drift-kink-tearing instability (RDKTI). These two waves deform the current
sheet so that they trigger relativistic magnetic reconnection at a crossover
thinning point. Since relativistic reconnection produces a lot of non-thermal
particles, the guide field is of critical importance to study the energetics of
a relativistic current sheet.Comment: 12 pages, 4 figures; fixed typos and added a footnote [24
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