3,796 research outputs found
Alfven Wave Reflection and Turbulent Heating in the Solar Wind from 1 Solar Radius to 1 AU: an Analytical Treatment
We study the propagation, reflection, and turbulent dissipation of Alfven
waves in coronal holes and the solar wind. We start with the Heinemann-Olbert
equations, which describe non-compressive magnetohydrodynamic fluctuations in
an inhomogeneous medium with a background flow parallel to the background
magnetic field. Following the approach of Dmitruk et al, we model the nonlinear
terms in these equations using a simple phenomenology for the cascade and
dissipation of wave energy, and assume that there is much more energy in waves
propagating away from the Sun than waves propagating towards the Sun. We then
solve the equations analytically for waves with periods of hours and longer to
obtain expressions for the wave amplitudes and turbulent heating rate as a
function of heliocentric distance. We also develop a second approximate model
that includes waves with periods of roughly one minute to one hour, which
undergo less reflection than the longer-period waves, and compare our models to
observations. Our models generalize the phenomenological model of Dmitruk et al
by accounting for the solar wind velocity, so that the turbulent heating rate
can be evaluated from the coronal base out past the Alfven critical point -
that is, throughout the region in which most of the heating and acceleration
occurs. The simple analytical expressions that we obtain can be used to
incorporate Alfven-wave reflection and turbulent heating into fluid models of
the solar wind.Comment: 9 pages, 9 figures, accepted for publication in Ap
Perpendicular Ion Heating by Low-Frequency Alfven-Wave Turbulence in the Solar Wind
We consider ion heating by turbulent Alfven waves (AWs) and kinetic Alfven
waves (KAWs) with perpendicular wavelengths comparable to the ion gyroradius
and frequencies smaller than the ion cyclotron frequency. When the turbulence
amplitude exceeds a certain threshold, an ion's orbit becomes chaotic. The ion
then interacts stochastically with the time-varying electrostatic potential,
and the ion's energy undergoes a random walk. Using phenomenological arguments,
we derive an analytic expression for the rates at which different ion species
are heated, which we test by simulating test particles interacting with a
spectrum of randomly phased AWs and KAWs. We find that the stochastic heating
rate depends sensitively on the quantity epsilon = dv/vperp, where vperp is the
component of the ion velocity perpendicular to the background magnetic field
B0, and dv (dB) is the rms amplitude of the velocity (magnetic-field)
fluctuations at the gyroradius scale. In the case of thermal protons, when
epsilon << eps1, where eps1 is a constant, a proton's magnetic moment is nearly
conserved and stochastic heating is extremely weak. However, when epsilon >
eps1, the proton heating rate exceeds the cascade power that would be present
in strong balanced KAW turbulence with the same value of dv, and
magnetic-moment conservation is violated. For the random-phase waves in our
test-particle simulations, eps1 is approximately 0.2. For protons in low-beta
plasmas, epsilon is approximately dB/B0 divided by the square root of beta, and
epsilon can exceed eps1 even when dB/B0 << eps1. At comparable temperatures,
alpha particles and minor ions have larger values of epsilon than protons and
are heated more efficiently as a result. We discuss the implications of our
results for ion heating in coronal holes and the solar wind.Comment: 14 pages, 5 figures, submitted to Ap
Resonance Broadening and Heating of Charged Particles in Magnetohydrodynamic Turbulence
The heating, acceleration, and pitch-angle scattering of charged particles by
MHD turbulence are important in a wide range of astrophysical environments,
including the solar wind, accreting black holes, and galaxy clusters. We
simulate the interaction of high-gyrofrequency test particles with fully
dynamical simulations of subsonic MHD turbulence, focusing on the parameter
regime with beta ~ 1, where beta is the ratio of gas to magnetic pressure. We
use the simulation results to calibrate analytical expressions for test
particle velocity-space diffusion coefficients and provide simple fits that can
be used in other work.
The test particle velocity diffusion in our simulations is due to a
combination of two processes: interactions between particles and magnetic
compressions in the turbulence (as in linear transit-time damping; TTD) and
what we refer to as Fermi Type-B (FTB) interactions, in which charged particles
moving on field lines may be thought of as beads spiralling around moving
wires. We show that test particle heating rates are consistent with a TTD
resonance which is broadened according to a decorrelation prescription that is
Gaussian in time. TTD dominates the heating for v_s >> v_A (e.g. electrons),
where v_s is the thermal speed of species s and v_A is the Alfven speed, while
FTB dominates for v_s << v_A (e.g. minor ions). Proton heating rates for beta ~
1 are comparable to the turbulent cascade rate. Finally, we show that velocity
diffusion of collisionless, large gyrofrequency particles due to large-scale
MHD turbulence does not produce a power-law distribution function.Comment: 20 pages, 15 figures; accepted by The Astrophysical Journal; added
clarifying appendices, but no major changes to result
Stochastic Weighted Graphs: Flexible Model Specification and Simulation
In most domains of network analysis researchers consider networks that arise
in nature with weighted edges. Such networks are routinely dichotomized in the
interest of using available methods for statistical inference with networks.
The generalized exponential random graph model (GERGM) is a recently proposed
method used to simulate and model the edges of a weighted graph. The GERGM
specifies a joint distribution for an exponential family of graphs with
continuous-valued edge weights. However, current estimation algorithms for the
GERGM only allow inference on a restricted family of model specifications. To
address this issue, we develop a Metropolis--Hastings method that can be used
to estimate any GERGM specification, thereby significantly extending the family
of weighted graphs that can be modeled with the GERGM. We show that new
flexible model specifications are capable of avoiding likelihood degeneracy and
efficiently capturing network structure in applications where such models were
not previously available. We demonstrate the utility of this new class of
GERGMs through application to two real network data sets, and we further assess
the effectiveness of our proposed methodology by simulating non-degenerate
model specifications from the well-studied two-stars model. A working R version
of the GERGM code is available in the supplement and will be incorporated in
the gergm CRAN package.Comment: 33 pages, 6 figures. To appear in Social Network
Constraining Low-Frequency Alfvenic Turbulence in the Solar Wind Using Density Fluctuation Measurements
One proposed mechanism for heating the solar wind, from close to the sun to
beyond 10 AU, invokes low-frequency, oblique, Alfven-wave turbulence. Because
small-scale oblique Alfven waves (kinetic Alfven waves) are compressive, the
measured density fluctuations in the solar wind place an upper limit on the
amplitude of kinetic Alfven waves and hence an upper limit on the rate at which
the solar wind can be heated by low-frequency, Alfvenic turbulence. We evaluate
this upper limit for both coronal holes at 5 solar radii and in the near-Earth
solar wind. At both radii, the upper limit we find is consistent with models in
which the solar wind is heated by low-frequency Alfvenic turbulence. At 1 AU,
the upper limit on the turbulent heating rate derived from the measured density
fluctuations is within a factor of 2 of the measured solar wind heating rate.
Thus if low-frequency Alfvenic turbulence contributes to heating the near-Earth
solar wind, kinetic Alfven waves must be one of the dominant sources of solar
wind density fluctuations at frequencies of order 1 Hz. We also present a
simple argument for why density fluctuation measurements do appear to rule out
models in which the solar wind is heated by non-turbulent high-frequency waves
``sweeping'' through the ion-cyclotron resonance, but are compatible with
heating by low-frequency Alfvenic turbulence.Comment: 8 pages, 3 figures, submitted to Ap
Dominant next-to-leading order QCD corrections to Higgs plus three jet production in vector-boson fusion
We present the calculation of the dominant next to leading order QCD
corrections to Higgs boson production in association with three jets via vector
boson fusion in the form of a NLO parton-level Monte Carlo program. QCD
corrections to integrated cross sections are modest, while the shapes of some
kinematical distributions change appreciably at NLO. Scale uncertainties are
shown to be reduced at NLO for the total cross section and for distributions.
We consider a central jet veto at the LHC and analyze the veto probability for
typical vector boson fusion cuts. Scale uncertainties of the veto probability
are sufficiently small at NLO for precise Higgs coupling measurements at the
LHC.Comment: 40 pages, 17 figures, 2 tables, published versio
A Stellar Rotation Census of B Stars: from ZAMS to TAMS
Two recent observing campaigns provide us with moderate dispersion spectra of
more than 230 cluster and 370 field B stars. Combining them and the spectra of
the B stars from our previous investigations (430 cluster and 100
field B stars) yields a large, homogeneous sample for studying the rotational
properties of B stars. We derive the projected rotational velocity ,
effective temperature, gravity, mass, and critical rotation speed for each star. We find that the average is significantly lower
among field stars because they are systematically more evolved and spun down
than their cluster counterparts. The rotational distribution functions of
for the least evolved B stars show that lower mass B
stars are born with a larger proportion of rapid rotators than higher mass B
stars. However, the upper limit of that may separate
normal B stars from emission line Be stars (where rotation promotes mass loss
into a circumstellar disk) is smaller among the higher mass B stars. We compare
the evolutionary trends of rotation (measured according to the polar gravity of
the star) with recent models that treat internal mixing. The spin-down rates
observed in the high mass subset () agree with predictions, but
the rates are larger for the low mass group (). The faster spin
down in the low mass B stars matches well with the predictions based on
conservation of angular momentum in individual spherical shells. Our results
suggest the fastest rotators (that probably correspond to the emission line Be
stars) are probably formed by evolutionary spin up (for the more massive stars)
and by mass transfer in binaries (for the full range of B star masses).Comment: 44 pages, 10 figures, accepted for publication in Ap
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