595 research outputs found
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
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SLiM: a code for the simulation of wave propagation through an inhomogeneous, magnetised solar atmosphere
In this paper we describe the semi-spectral linear MHD (SLiM) code which we
have written to follow the interaction of linear waves through an inhomogeneous
three-dimensional solar atmosphere. The background model allows almost
arbitrary perturbations of density, temperature, sound speed as well as
magnetic and velocity fields. We give details of several of the tests we have
used to check the code. The code will be useful in understanding the
helioseismic signatures of various solar features, including sunspots.Comment: 6 pages, 7 figure
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
SLiM: a code for the simulation of wave propagation through an inhomogeneous, magnetised solar atmosphere
In this paper we describe the semi-spectral linear MHD (SLiM) code which we
have written to follow the interaction of linear waves through an inhomogeneous
three-dimensional solar atmosphere. The background model allows almost
arbitrary perturbations of density, temperature, sound speed as well as
magnetic and velocity fields. We give details of several of the tests we have
used to check the code. The code will be useful in understanding the
helioseismic signatures of various solar features, including sunspots.Comment: 6 pages, 7 figure
Helioseismology of Sunspots: Confronting Observations with Three-Dimensional MHD Simulations of Wave Propagation
The propagation of solar waves through the sunspot of AR 9787 is observed
using temporal cross-correlations of SOHO/MDI Dopplergrams. We then use
three-dimensional MHD numerical simulations to compute the propagation of wave
packets through self-similar magneto-hydrostatic sunspot models. The
simulations are set up in such a way as to allow a comparison with observed
cross-covariances (except in the immediate vicinity of the sunspot). We find
that the simulation and the f-mode observations are in good agreement when the
model sunspot has a peak field strength of 3 kG at the photosphere, less so for
lower field strengths. Constraining the sunspot model with helioseismology is
only possible because the direct effect of the magnetic field on the waves has
been fully taken into account. Our work shows that the full-waveform modeling
of sunspots is feasible.Comment: 21 pages, Accepted in Solar Physic
F-mode sensitivity kernels for flows
We compute f-mode sensitivity kernels for flows. Using a two-dimensional
model, the scattered wavefield is calculated in the first Born approximation.
We test the correctness of the kernels by comparing an exact solution (constant
flow), a solution linearized in the flow, and the total integral of the kernel.
In practice, the linear approximation is acceptable for flows as large as about
400 m/s.Comment: 4 pages, 3 figures. Proceedings of SOHO18/GONG 2006/HELAS I. Beyond
the Spherical Sun: A new era of helio- and asteroseismology. Sheffield,
England. August, 200
Spatially resolved vertical vorticity in solar supergranulation using helioseismology and local correlation tracking
Flow vorticity is a fundamental property of turbulent convection in rotating
systems. Solar supergranules exhibit a preferred sense of rotation, which
depends on the hemisphere. This is due to the Coriolis force acting on the
diverging horizontal flows. We aim to spatially resolve the vertical flow
vorticity of the average supergranule at different latitudes, both for outflow
and inflow regions. To measure the vertical vorticity, we use two independent
techniques: time-distance helioseismology (TD) and local correlation tracking
of granules in intensity images (LCT) using data from the Helioseismic and
Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Both maps
are corrected for center-to-limb systematic errors. We find that 8-h TD and LCT
maps of vertical vorticity are highly correlated at large spatial scales.
Associated with the average supergranule outflow, we find tangential (vortical)
flows that reach about 10 m/s in the clockwise direction at 40{\deg} latitude.
In average inflow regions, the tangential flow reaches the same magnitude, but
in the anti-clockwise direction. These tangential velocities are much smaller
than the radial (diverging) flow component (300 m/s for the average outflow and
200 m/s for the average inflow). The results for TD and LCT as measured from
HMI are in excellent agreement for latitudes between 60{\deg} and 60{\deg}.
From HMI LCT, we measure the vorticity peak of the average supergranule to have
a full width at half maximum of about 13 Mm for outflows and 8 Mm for inflows.
This is larger than the spatial resolution of the LCT measurements (about 3
Mm). On the other hand, the vorticity peak in outflows is about half the value
measured at inflows (e.g. 4/(10^6 s) clockwise compared to 8/(10^6 s)
anti-clockwise at 40{\deg} latitude). Results from MDI/SOHO obtained in 2010
are biased compared to the HMI/SDO results for the same period.Comment: 12 pages, 13 figures (plus appendix), accepted for publication in A&
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