explanation blue bibcodes open ADS page with paths to full text
Author name code: skartlien
ADS astronomy entries on 2022-09-14
author:"Skartlien, Roar"
---------------------------------------------------------
Title: Numerical simulations of stochastically excited sound waves
in a random medium
Authors: Selwa, M.; Skartlien, R.; Murawski, K.
2004A&A...420.1123S Altcode:
In turbulent acoustic media such as the solar envelope, both wave
sources and the propagation characteristics (background density,
refractive index, dissipation, etc.) are stochastic quantities. By
means of numerical simulation of the Euler equations, we study two
cases in a homogeneous stochastic medium in which the background
density fluctuations and wave sources are 1) correlated and 2)
uncorrelated. We find that in the uncorrelated case, the coherent (or
mean) acoustic field is zero, leaving only an incoherent field. In the
correlated case, the coherent field is nonzero, yielding both coherent
and incoherent fields. We question the use of mean-field dispersion
relations to determine frequency shifts in p-mode and f-mode spectra,
since the coherent field can be non-existent or weak relative to
the incoherent field. We demonstrate the importance of accounting
for a stochastic wave source by showing that the two cases give very
different frequency shifts.
---------------------------------------------------------
Title: Effects in the Solar p-Mode Power Spectrum from Scattering
on a Turbulent Background Flow with Stochastic Wave Sources
Authors: Skartlien, R.
2002ApJ...578..621S Altcode:
This work demonstrates how the scattered wave field, in combination with
stochastic wave sources, influences the p-mode power spectrum. I adopt
a turbulent zone with random fluctuations in sound speed and velocity
(characterized by the respective correlation functions). I present
a general formalism (for plane-parallel conditions) to calculate
the expectation value of the p-mode power spectrum. The power due to
the direct field from the sources dominates, and depends only on the
source correlation function. Smaller, but significant, “corrections”
are due to scattered wave field components. These corrections depend
in general on the correlation functions of the turbulent fluctuations
in the medium. I adopt a simple waveguide for the purpose of clear
demonstration, and show that the corrections generate three important
effects: (1) the line profiles are shifted along the wavenumber
(frequency) axis, with varying effects depending on which “turbulent”
physical quantity we consider; (2) the shapes and widths of the line
profiles are altered; and (3) the “troughs” between the line profiles
are to some extent filled in, contributing to the “solar background
power.” It is demonstrated that turbulent boundary zones can generate
important contributions to the power spectrum.
---------------------------------------------------------
Title: Local Helioseismology as an Inverse Source-Inverse Scattering
Problem
Authors: Skartlien, R.
2002ApJ...565.1348S Altcode:
The inverse source and inverse scattering problems for a general
inhomogeneous medium is investigated within the framework of
helioseismic holography. Holographic images, defined by the method
of Lindsey and Braun, or via the Porter-Bojarski equation, define
a Fredholm integral equation of the first kind in terms of acoustic
sources, scatterers, and absorbers. This integral equation is well
posed in the sense that it can be inverted by standard constrained
inversion methods. The inversion produces an image that does not
distinguish between sources, scatterers, and absorbers. The inversion
is necessarily approximate because of the null space of the kernel that
defines the Fredholm equation. Physically, the null space corresponds
to the nonradiating source contribution, as previously shown by Devaney
and Porter. Numerical experiments based on a solar model show that the
sidelobes in the “imaging point-spread function” are greatly reduced
after inversion, such that the image is sharper than the corresponding
holographic image.
---------------------------------------------------------
Title: Imaging of Acoustic Wave Sources inside the Sun
Authors: Skartlien, R.
2001ApJ...554..488S Altcode:
A holographic imaging technique, previously used in underwater
acoustics, is applied to the solar inverse source problem. This
problem consists of forming acoustic images so as to localize impulsive
sources in space and time in the solar interior. The difficulty with
such an imaging method is that the image of a single point source is
spatiotemporally extended and will produce significant overlap between
images of separated sources if the source density is large. I propose
a modification of the existing method in order to make reliable source
images also in the case for large source density. It is suggested
that the method can be used on solar observations to detect relatively
strong and impulsive wave sources inside the sun and to perhaps clarify
unanswered problems regarding excitation of p modes.
---------------------------------------------------------
Title: Excitation of Chromospheric Wave Transients by Collapsing
Granules
Authors: Skartlien, R.; Stein, R. F.; Nordlund, Å.
2000ApJ...541..468S Altcode:
The excitation of acoustic waves is studied using three-dimensional
numerical simulations of the nonmagnetic solar atmosphere and the
upper convection zone. Transient acoustic waves in the atmosphere
are excited at the top of the convective zone (the cooling layer) and
immediately above in the convective overshoot zone, by small granules
that undergo a rapid collapse, in the sense that upflow reverses to
downflow, on a timescale shorter than the atmospheric acoustic cutoff
period (3 minutes). These collapsing granules tend to be located above
downflows at the boundaries of mesogranules where the upward enthalpy
flux is smaller than average. An extended downdraft between larger
cells is formed at the site of the collapse. The waves produced are
long wavelength, gravity modified acoustic waves with periods close to
the 3 minute cutoff period of the solar atmosphere. The oscillation
is initially horizontally localized with a size of about 1 Mm. The
wave amplitude decays in time as energy is transported horizontally and
vertically away from the site of the event. Observed “acoustic events”
and darkening of intergranular lanes could be explained by this purely
hydrodynamical process. Furthermore, the observed “internetwork bright
grains” in the Ca II H and K line cores and associated shock waves
in the chromosphere may also be linked to such wave transients.
---------------------------------------------------------
Title: A Multigroup Method for Radiation with Scattering in
Three-Dimensional Hydrodynamic Simulations
Authors: Skartlien, R.
2000ApJ...536..465S Altcode:
Substantial approximations in the treatment of radiation are still
necessary in three-dimensional simulations in order to avoid extremely
large computational costs. Solar radiation hydrodynamic simulations in
three dimensions have previously assumed local thermodynamic equilibrium
(LTE) an assumption that works well in the deep photosphere. This
work aims at bringing these simulations a step further by including
scattered radiation, with the goal of modeling chromospheres in three
dimensions. We allow for coherent isotropic scattering, which alters
the thermal structure and wave amplitudes in the chromosphere. Group
mean opacity coefficients are used in group mean source functions
that contain approximate scattering terms and exact contributions from
thermal emissivity. The resulting three-dimensional scattering problem
allows for a computationally efficient solution by a new iteration
method. We have compared exact wavelength-integrated monochromatic
solutions with the corresponding approximate solutions for solar
conditions. We find that the total flux divergence obtained from the
groups deviates less than 10% from the exact solution. When using these
groups rather than the full monochromatic solution, the CPU time is
reduced by a factor of about 100 in a test case for solar conditions.
---------------------------------------------------------
Title: p-Mode Intensity-Velocity Phase Differences and Convective
Sources
Authors: Skartlien, R.; Rast, M. P.
2000ApJ...535..464S Altcode:
We study the origin of the solar p-mode intensity-velocity phase
differences at high degree (l>100). Observations show phase
differences that are very different from those derived from linear
theory alone. The theory predicts a smooth variation with frequency,
dependent only on atmospheric parameters, while observations show large
fluctuations across modal frequencies. We support previous suggestions
that fluctuations in the intensity-velocity phase differences and line
asymmetries in the intensity and velocity power spectra are produced by
“contamination” of the p-mode signal with noise correlated with the
excitation sources. It is demonstrated that the qualitative shapes of
the observed phase-difference and power spectra can be realized only if
both temperature (intensity) and velocity (Doppler shift) observations
contain correlated noise. Moreover, the details of the observed spectra
allow only a limited choice of noise parameters and constrain well
the convective process responsible for p-mode excitation. The inferred
correlated noise signals are consistent with the (visible) formation
of convective downflows accompanied by darkening (lowered emergent
intensity) and subsequent acoustic excitation. An upward velocity
pulse follows after the wave excitation, which suggests overshoot of
inflowing material that fills in the evacuated volume in the wake of
the new downflow.
---------------------------------------------------------
Title: Three-Dimensional Modeling of Solar Convection and Atmosphere
Dynamics
Authors: Skartlien, R.
1998PhDT........34S Altcode:
Results from non-magnetic numerical 3D simulations of the solar
atmosphere and the convection zone below are presented. I find that
transient acoustic wave trains in the atmosphere are excited by
smaller granular cells that undergo a rapid collapse in the sense
that upflow is reversed to downflow on a timescale shorter than the
atmospheric acoustic cutoff period. An extended downdraft between
larger cells is formed at the site of the collapse. The main wave
excitation sources are found at the top of the convective layer
(the cooling layer) and immediately above in the convective overshoot
layer. The excitation lasts for 2-3 minutes as the upflow in the granule
reverses to downflow. The following vertical wave components are long
wavelength, gravity modified acoustic waves with periods close to the
3 minute eigenoscillation of the solar atmosphere. These waves shock
at chromospheric layers. The oscillation is initially horizontally
localized with a size of about 1 Mm. The wave amplitude decay in time
as energy is transported horizontally and vertically away from the
site of the event. This is the only convectively generated wave source
I find that shows a clear correlation to large scale atmospheric wave
motions. Observed darkening of intergranular lanes and the associated
photospheric wave motions, the so called “acoustic events” could
be explained by this purely hydrodynamical process. Furthermore,
the observed “internetwork bright grains” in the CaII H and K line
cores and associated shock wave trains in the chromosphere can also be
linked to this wave train. The simulation is an extended Nordlund and
Stein model of solar convection, so as to include the chromosphere. A
large part of the work has been spent on developing a new method for
calculating radiative flux divergence in the simulation, in which I
treat photon scattering in atmospheric layers. The previous NS-model
used the Planck function as source function, while the improved method
includes a scattering term, which introduces a global radiation problem,
requiring an iterative solution at each timestep in the simulation. By
assuming opacity in LTE and coherent isotropic scattering, I calculate
group mean opacity coefficients to be used in a group mean source
function. This source function contains an approximate scattering
term and an exact contribution from thermal emissivity. The resulting
three dimensional scattering problems are solved by iteration using
a new method based on a one-ray approximation in the angle integral
for the mean intensity. The equations to be iterated are tri-diagonal
matrix equations which require a minimum of computer time. I have
compared exact wavelength integrated monochromatic solutions with the
corresponding approximate group mean solutions for solar conditions. I
find that the total flux divergence obtained from groups deviates with
less than 10 % from the exact solution. Flux divergence in individual
groups can deviate with typically 30 % in atmospheric layers. When
using these groups, the CPU time is reduced by a factor of about
100 in a test case for solar conditions. The main difference for the
atmospheric dynamics between LTE and scattering solutions, is that the
wave amplitudes in the chromosphere are larger with scattering. This is
because the radiative damping is smaller than in LTE for a given wave
amplitude. The convection dynamics and therefore the convective wave
sources are almost unaltered, since the radiative cooling that drives
the convection is mainly unchanged. Collapsing granules is therefore
also expected in LTE, but the chromospheric response would be of
smaller amplitude, and the shock train would form at higher altitudes.
---------------------------------------------------------
Title: 3D modeling of solar convection and atmosphere dynamics
Authors: Skartlien, Roar
1998PhDT.......590S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Calcium II phase relations and chromospheric dynamics
Authors: Skartlien, R.; Carlsson, M.; Stein, R. F.
1994chdy.conf...79S Altcode:
No abstract at ADS
