1,229 research outputs found
Validated helioseismic inversions for 3-D vector flows
According to time-distance helioseismology, information about internal fluid
motions is encoded in the travel times of solar waves. The inverse problem
consists of inferring 3-D vector flows from a set of travel-time measurements.
Here we investigate the potential of time-distance helioseismology to infer 3-D
convective velocities in the near-surface layers of the Sun. We developed a new
Subtractive Optimally Localised Averaging (SOLA) code suitable for pipeline
pseudo-automatic processing. Compared to its predecessor, the code was improved
by accounting for additional constraints in order to get the right answer
within a given noise level. The main aim of this study is to validate results
obtained by our inversion code. We simulate travel-time maps using a snapshot
from a numerical simulation of solar convective flows, realistic Born
travel-time sensitivity kernels, and a realistic model of travel-time noise.
These synthetic travel times are inverted for flows and the results compared
with the known input flow field. Additional constraints are implemented in the
inversion: cross-talk minimization between flow components and spatial
localization of inversion coefficients. Using modes f, p1 through p4, we show
that horizontal convective flow velocities can be inferred without bias, at a
signal-to-noise ratio greater than one in the top 3.5 Mm, provided that
observations span at least four days. The vertical component of velocity (v_z),
if it were to be weak, is more difficult to infer and is seriously affected by
cross-talk from horizontal velocity components. We emphasise that this
cross-talk must be explicitly minimised in order to retrieve v_z in the top 1
Mm. We also show that statistical averaging over many different areas of the
Sun allows for reliably measuring of average properties of all three flow
components in the top 5.5 Mm of the convection zone.Comment: 14 pages main paper, 9 pages electronic supplement, 28 figures.
Accepted for publication in Astronomy & Astrophysic
Time-distance helioseismology: Sensitivity of f-mode travel times to flows
Time-distance helioseismology has shown that f-mode travel times contain
information about horizontal flows in the Sun. The purpose of this study is to
provide a simple interpretation of these travel times. We study the interaction
of surface-gravity waves with horizontal flows in an incompressible,
plane-parallel solar atmosphere. We show that for uniform flows less than
roughly 250 m s, the travel-time shifts are linear in the flow
amplitude. For stronger flows, perturbation theory up to third order is needed
to model waveforms. The case of small-amplitude spatially-varying flows is
treated using the first-order Born approximation. We derive two-dimensional
Fr\'{e}chet kernels that give the sensitivity of travel-time shifts to local
flows. We show that the effect of flows on travel times depends on wave damping
and on the direction from which the observations are made. The main physical
effect is the advection of the waves by the flow rather than the advection of
wave sources or the effect of flows on wave damping. We compare the
two-dimensional sensitivity kernels with simplified three-dimensional kernels
that only account for wave advection and assume a vertical line of sight. We
find that the three-dimensional f-mode kernels approximately separate in the
horizontal and vertical coordinates, with the horizontal variations given by
the simplified two-dimensional kernels. This consistency between quite
different models gives us confidence in the usefulness of these kernels for
interpreting quiet-Sun observations.Comment: 34 pages, accepted to Astrophysical Journa
A procedure for the inversion of f-mode travel times for solar flows
We perform a two-dimensional inversion of f-mode travel times to determine
near-surface solar flows. The inversion is based on optimally localized
averaging of travel times. We use finite-wavelength travel-time sensitivity
functions and a realistic model of the data errors. We find that it is possible
to obtain a spatial resolution of 2 Mm. The error in the resulting flow
estimate ultimately depends on the observation time and the number of travel
distances used in the inversion.Comment: 8 pages, 9 figure
Helioseismology challenges models of solar convection
Convection is the mechanism by which energy is transported through the
outermost 30% of the Sun. Solar turbulent convection is notoriously difficult
to model across the entire convection zone where the density spans many orders
of magnitude. In this issue of PNAS, Hanasoge et al. (2012) employ recent
helioseismic observations to derive stringent empirical constraints on the
amplitude of large-scale convective velocities in the solar interior. They
report an upper limit that is far smaller than predicted by a popular
hydrodynamic numerical simulation.Comment: Printed in the Proceedings of the National Academy of Sciences (2
pages, 1 figure). Available at
http://www.pnas.org/cgi/doi/10.1073/pnas.120887510
Linear Sensitivity of Helioseismic Travel Times to Local Flows
Time-distance helioseismology is a technique for measuring the time for waves
to travel from one point on the solar surface to another. These wave travel
times are affected by advection by subsurface flows. Inferences of plasma flows
based on observed travel times depend critically on the ability to accurately
model the effects of subsurface flows on time-distance measurements. We present
a Born approximation based computation of the sensitivity of time distance
travel times to weak, steady, inhomogeneous subsurface flows. Three sensitivity
functions are obtained, one for each component of the 3D vector flow. We show
that the depth sensitivity of travel times to horizontally uniform flows is
given approximately by the kinetic energy density of the oscillation modes
which contribute to the travel times. For flows with strong depth dependence,
the Born approximation can give substantially different results than the ray
approximation.Comment: 6 pages, 6 figure
Surface-effect corrections for oscillation frequencies of evolved stars
Accurate modelling of solar-like oscillators requires that modelled mode
frequencies are corrected for the systematic shift caused by improper modelling
of the near-surface layers, known as the surface effect. ... We investigate how
much additional uncertainty is introduced to stellar model parameters by our
uncertainty about the functional form of the surface effect. At the same time,
we test whether any of the parametrizations is significantly better or worse at
modelling observed subgiants and low-luminosity red giants. We model six stars
observed by Kepler that show clear mixed modes. We fix the input physics of the
stellar models and vary the choice of surface correction ... Models using a
solar-calibrated power law correction consistently fit the observations more
poorly than the other four corrections. Models with the remaining four
corrections generally fit ... about equally well, with the combined surface
correction by Ball & Gizon perhaps being marginally superior. The fits broadly
agree on the model parameters within about the uncertainties, with
discrepancies between the modified Lorentzian and free power law corrections
occasionally exceeding the level. Relative to the best-fitting
values, the total uncertainties on the masses, radii and ages of the stars are
all less than 2, 1 and 6 per cent, respectively. A solar-calibrated power law
... appears unsuitable for use with more evolved solar-like oscillators. Among
the remaining surface corrections, the uncertainty in the model parameters
introduced by the surface effects is about twice as large as the uncertainty in
the individual fits for these six stars. Though the fits are thus somewhat less
certain because of our uncertainty of how to manage the surface effect, these
results also demonstrate that it is feasible to model the individual mode
frequencies of subgiants and low-luminosity red giants. ...Comment: Accepted for publication in Astronomy & Astrophysics. 13 pages, 6
figures, 5 tables. Abstract slightly abridged to meet arXiv's 1920 character
limi
Triple correlations in local helioseismology
A central step in time-distance local helioseismology techniques is to obtain
travel times of packets of wave signals between points or sets of points on the
visible surface. Standard ways of determining group or phase travel times
involve cross-correlating the signal between locations at the solar photosphere
and determining the shift of the envelope of this cross correlation function,
or a zero crossing, using a standard wavelet or a reference wave packet. Here a
novel method is described which makes use of triple correlations, i.e.
cross-correlating signal between three locations. By using an average triple
correlation as reference, differential travel times can be extracted in a
straightforward manner.Comment: 6 pages, 8 figures, submitted to Astronomische Nachrichten (HELAS
workshop proceedings
Sub-Wavelength Resolution Imaging of the Solar Deep Interior
We derive expectations for signatures in the measured travel times of waves
that interact with thermal anomalies and jets. A series of numerical
experiments that involve the dynamic linear evolution of an acoustic wave field
in a solar-like stratified spherical shell in the presence of fully 3D
time-stationary perturbations are performed. The imprints of these interactions
are observed as shifts in wave travel times, which are extracted from these
data through methods of time-distance helioseismology \citep{duvall}. In
situations where at least one of the spatial dimensions of the scatterer was
smaller than a wavelength, oscillatory time shift signals were recovered from
the analyses, pointing directly to a means of resolving sub-wavelength
features. As evidence for this claim, we present analyses of simulations with
spatially localized jets and sound-speed perturbations. We analyze 1 years'
worth solar observations to estimate the noise level associated with the time
differences. Based on theoretical estimates, Fresnel zone time shifts
associated with the (possible) sharp rotation gradient at the base of the
convection zone are of the order 0.01 - 0.1 s, well below the noise level that
could be reached with the currently available amount of data ( s
with 10 yrs of data).Comment: Accepted, ApJ; 17 pages, 12 figure
Local helioseismology and the active Sun
The goal of local helioseismology is to elicit three-dimensional information
about the sub-surface (or far-side) structure and dynamics of the Sun from
observations of the helioseismic wave field at the surface. The physical
quantities of interest include flows, sound-speed deviations and magnetic
fields. However, strong surface magnetic fields induce large perturbations to
the waves making inversions difficult to interpret. The purpose of this paper
is to outline the methods of analysis used in local helioseismology, review
discoveries associated with the magnetic Sun made using local helioseismology
from the past three years, and highlight the efforts towards imaging the
interior in the presence of strong magnetic fields.Comment: 6 pages, 4th HELAS International Conference, Lanzarote, Spain, 1-5
February 201
Convectively stabilised background solar models for local helioseismology
In local helioseismology numerical simulations of wave propagation are useful
to model the interaction of solar waves with perturbations to a background
solar model. However, the solution to the equations of motions include
convective modes that can swamp the waves we are interested in. For this
reason, we choose to first stabilise the background solar model against
convection by altering the vertical pressure gradient. Here we compare the
eigenmodes of our convectively stabilised model with a standard solar model
(Model S) and find a good agreement.Comment: 3 pages, 3 figures, HELAS NA3, The Acoustic Solar Cycle, Birmingham,
6-8 January 200
- …