1,263 research outputs found
Numerical constraints on the model of stochastic excitation of solar-type oscillations
Analyses of a 3D simulation of the upper layers of a solar convective
envelope provide constraints on the physical quantities which enter the
theoretical formulation of a stochastic excitation model of solar p modes, for
instance the convective velocities and the turbulent kinetic energy spectrum.
These constraints are then used to compute the acoustic excitation rate for
solar p modes, P. The resulting values are found ~5 times larger than the
values resulting from a computation in which convective velocities and entropy
fluctuations are obtained with a 1D solar envelope model built with the
time-dependent, nonlocal Gough (1977) extension of the mixing length
formulation for convection (GMLT). This difference is mainly due to the assumed
mean anisotropy properties of the velocity field in the excitation region. The
3D simulation suggests much larger horizontal velocities compared to vertical
ones than in the 1D GMLT solar model. The values of P obtained with the 3D
simulation constraints however are still too small compared with the values
inferred from solar observations. Improvements in the description of the
turbulent kinetic energy spectrum and its depth dependence yield further
increased theoretical values of P which bring them closer to the observations.
It is also found that the source of excitation arising from the advection of
the turbulent fluctuations of entropy by the turbulent movements contributes ~
65-75 % to the excitation and therefore remains dominant over the Reynolds
stress contribution. The derived theoretical values of P obtained with the 3D
simulation constraints remain smaller by a factor ~3 compared with the solar
observations. This shows that the stochastic excitation model still needs to be
improved.Comment: 11 pages, 9 figures, accepted for publication in A&
What Causes P-mode Asymmetry Reversal?
The solar acoustic p-mode line profiles are asymmetric. Velocity spectra have
more power on the low-frequency sides, whereas intensity profiles show the
opposite sense of asymmetry. Numerical simulations of the upper convection zone
have resonant p-modes with the same asymmetries and asymmetry reversal as the
observed modes. The temperature and velocity power spectra at optical depth
have the opposite asymmetry as is observed for the
intensity and velocity spectra. At a fixed geometrical depth, corresponding to
, however, the temperature and velocity spectra have the
same asymmetry. This indicates that the asymmetry reversal is produced by
radiative transfer effects and not by correlated noise.Comment: 16 pages, 10 figures, submitted to Astrophysical Journa
Magnetohydrodynamic turbulence in warped accretion discs
Warped, precessing accretion discs appear in a range of astrophysical
systems, for instance the X-ray binary Her X-1 and in the active nucleus of
NGC4258. In a warped accretion disc there are horizontal pressure gradients
that drive an epicyclic motion. We have studied the interaction of this
epicyclic motion with the magnetohydrodynamic turbulence in numerical
simulations. We find that the turbulent stress acting on the epicyclic motion
is comparable in size to the stress that drives the accretion, however an
important ingredient in the damping of the epicyclic motion is its parametric
decay into inertial waves.Comment: to appear in the proceedings of the 20th Texas Symposium on
Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.
The response of a turbulent accretion disc to an imposed epicyclic shearing motion
We excite an epicyclic motion, whose amplitude depends on the vertical
position, , in a simulation of a turbulent accretion disc. An epicyclic
motion of this kind may be caused by a warping of the disc. By studying how the
epicyclic motion decays we can obtain information about the interaction between
the warp and the disc turbulence. A high amplitude epicyclic motion decays
first by exciting inertial waves through a parametric instability, but its
subsequent exponential damping may be reproduced by a turbulent viscosity. We
estimate the effective viscosity parameter, , pertaining to
such a vertical shear. We also gain new information on the properties of the
disc turbulence in general, and measure the usual viscosity parameter,
, pertaining to a horizontal (Keplerian) shear. We find that,
as is often assumed in theoretical studies, is approximately
equal to and both are much less than unity, for the field
strengths achieved in our local box calculations of turbulence. In view of the
smallness () of and we conclude
that for the timescale for diffusion
or damping of a warp is much shorter than the usual viscous timescale. Finally,
we review the astrophysical implications.Comment: 12 pages, 18 figures, MNRAS accepte
Simulations of Oscillation Modes of the Solar Convection Zone
We use the three-dimensional hydrodynamic code of Stein and Nordlund to
realistically simulate the upper layers of the solar convection zone in order
to study physical characteristics of solar oscillations. Our first result is
that the properties of oscillation modes in the simulation closely match the
observed properties. Recent observations from SOHO/MDI and GONG have confirmed
the asymmetry of solar oscillation line profiles, initially discovered by
Duvall et al. In this paper we compare the line profiles in the power spectra
of the Doppler velocity and continuum intensity oscillations from the SOHO/MDI
observations with the simulation. We also compare the phase differences between
the velocity and intensity data. We have found that the simulated line profiles
are asymmetric and have the same asymmetry reversal between velocity and
intensity as observed. The phase difference between the velocity and intensity
signals is negative at low frequencies and jumps in the vicinity of modes as is
also observed. Thus, our numerical model reproduces the basic observed
properties of solar oscillations, and allows us to study the physical
properties which are not observed.Comment: Accepted for publication in ApJ Letter
Radiation from relativistic jets
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic
nuclei (AGNs), and Galactic microquasar systems usually have power-law emission
spectra. Recent PIC simulations of relativistic electron-ion
(electron-positron) jets injected into a stationary medium show that particle
acceleration occurs within the downstream jet. In the presence of relativistic
jets, instabilities such as the Buneman instability, other two-streaming
instability, and the Weibel (filamentation) instability create collisionless
shocks, which are responsible for particle (electron, positron, and ion)
acceleration. The simulation results show that the Weibel instability is
responsible for generating and amplifying highly nonuniform, small-scale
magnetic fields. These magnetic fields contribute to the electron's transverse
deflection behind the jet head. The ``jitter'' radiation from deflected
electrons in small-scale magnetic fields has different properties than
synchrotron radiation which is calculated in a uniform magnetic field. This
jitter radiation, a case of diffusive synchrotron radiation, may be important
to understand the complex time evolution and/or spectral structure in gamma-ray
bursts, relativistic jets, and supernova remnants.Comment: 8 pages,3 figures, accepted for the Proceedings of Science of the
Workshop on Blazar Variability across the Electromagnetic Spectrum, April 22
to 25, 200
Evolution of Global Relativistic Jets: Collimations and Expansion with kKHI and the Weibel Instability
One of the key open questions in the study of relativistic jets is their
interaction with the environment. Here, we study the initial evolution of both
electron-proton and electron-positron relativistic jets, focusing on their
lateral interaction with the ambient plasma. We trace the generation and
evolution of the toroidal magnetic field generated by both kinetic
Kelvin-Helmholtz (kKH) and Mushroom instabilities (MI). This magnetic field
collimates the jet. We show that in electron-proton jet, electrons are
perpendicularly accelerated with jet collimation. The magnetic polarity
switches from the clockwise to anti-clockwise in the middle of jet, as the
instabilities weaken. For the electron-positron jet, we find strong mixture of
electron-positron with the ambient plasma, that results in the creation of a
bow shock. Merger of magnetic field current filaments generate density bumps
which initiate a forward shock. The strong mixing between jet and ambient
particles prevents full development of the jet on the studied scale. Our
results therefore provide a direct evidence for both jet collimation and
particle acceleration in the created bow shock. Differences in the magnetic
field structures generated by electron-proton and electron-positron jets may
contribute to observable differences in the polarized properties of emission by
electrons.Comment: 25 pages, 12 figures, ApJ, accepte
3D Radiative Hydrodynamics for Disk Stability Simulations: A Proposed Testing Standard and New Results
Recent three-dimensional radiative hydrodynamics simulations of
protoplanetary disks report disparate disk behaviors, and these differences
involve the importance of convection to disk cooling, the dependence of disk
cooling on metallicity, and the stability of disks against fragmentation and
clump formation. To guarantee trustworthy results, a radiative physics
algorithm must demonstrate the capability to handle both the high and low
optical depth regimes. We develop a test suite that can be used to demonstrate
an algorithm's ability to relax to known analytic flux and temperature
distributions, to follow a contracting slab, and to inhibit or permit
convection appropriately. We then show that the radiative algorithm employed by
Meji\'a (2004) and Boley et al. (2006) and the algorithm employed by Cai et al.
(2006) and Cai et al. (2007, in prep.) pass these tests with reasonable
accuracy. In addition, we discuss a new algorithm that couples flux-limited
diffusion with vertical rays, we apply the test suite, and we discuss the
results of evolving the Boley et al. (2006) disk with this new routine.
Although the outcome is significantly different in detail with the new
algorithm, we obtain the same qualitative answers. Our disk does not cool fast
due to convection, and it is stable to fragmentation. We find an effective
. In addition, transport is dominated by low-order
modes.Comment: Submitted to Ap
Numerical 3D constraints on convective eddy time-correlations : consequences for stochastic excitation of solar p modes
A 3D simulation of the upper part of the solar convective zone is used to
obtain information on the frequency component, chi_k, of the correlation
product of the turbulent velocity field. This component plays an important role
in the stochastic excitation of acoustic oscillations. A time analysis of the
solar simulation shows that a gaussian function does not correctly reproduce
the nu-dependency of chi_k inferred from the 3D simuation in the frequency
range where the acoustic energy injected into the solar p modes is important
(nu ~ 2 - 4 mHz). The nu-dependency of chi_k is fitted with different
analytical functions which can then conveniently be used to compute the
acoustic energy supply rate P injected into the solar radial oscillations. With
constraints from a 3D simulation, adjustment of free parameters to solar data
is no longer necessary and is not performed here. The result is compared with
solar seismic data. Computed values of P obtained with the analytical function
which fits best chi_k are found ~ 2.7 times larger than those obtained with the
gaussian model and reproduce better the solar seismic observations. This
non-gaussian description also leads to a Reynolds stress contribution of the
same order as the one arising from the advection of the turbulent fluctuations
of entropy by the turbulent motions. Some discrepancy between observed and
computed P values still exist at high frequency and possible causes for this
discrepancy are discussed.Comment: 11 pages; 4 figures, accepted for publication in A&
- …