8,001 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
Numerical simulations of multiple scattering of the mode by flux tubes
We use numerial simulations to study the absorption and phase shift of
surface-gravity waves caused by groups of magnetic flux tubes. The dependence
of the scattering coefficients with the distance between the tubes and their
positions is analyzed for several cases with two or three flux tubes embedded
in a quiet Sun atmosphere. The results are compared with those obtained
neglecting completely or partially multiple scattering effects. We show that
multiple scattering has a significant impact on the absorption measurements and
tends to reduce the phase shift. We also consider more general cases of
ensembles of randomly distributed flux tubes, and we have evaluated the effects
on the scattering measurements of changing the number of tubes included in the
bundle and the average distance between flux tubes. We find that for the
longest wavelength incoming waves multiple scattering enhances the absorption,
and its efficiency increases with the number of flux tubes and the reduction of
the distance between them.Comment: Accepted for publication in The Astrophysical Journa
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&
Evaluation of the capability of local helioseismology to discern between monolithic and spaghetti sunspot models
The helioseismic properties of the wave scattering generated by monolithic
and spaghetti sunspots are analyzed by means of numerical simulations. In these
computations, an incident f or p1 mode travels through the sunspot model, which
produces absorption and phase shift of the waves. The scattering is studied by
inspecting the wavefield, computing travel-time shifts, and performing
Fourier-Hankel analysis. The comparison between the results obtained for both
sunspot models reveals that the differences in the absorption coefficient can
be detected above noise level. The spaghetti model produces an steep increase
of the phase shift with the degree of the mode at short wavelengths, while
mode-mixing is more efficient for the monolithic model. These results provide a
clue for what to look for in solar observations to discern the constitution of
sunspots between the proposed monolithic and spaghetti models.Comment: Accepted for publication in The Astrophysical Journa
Helioseismic holography of simulated sunspots: magnetic and thermal contributions to travel times
Wave propagation through sunspots involves conversion between waves of
acoustic and magnetic character. In addition, the thermal structure of sunspots
is very different than that of the quiet Sun. As a consequence, the
interpretation of local helioseismic measurements of sunspots has long been a
challenge. With the aim of understanding these measurements, we carry out
numerical simulations of wave propagation through sunspots. Helioseismic
holography measurements made from the resulting simulated wavefields show
qualitative agreement with observations of real sunspots. We use additional
numerical experiments to determine, separately, the influence of the thermal
structure of the sunspot and the direct effect of the sunspot magnetic field.
We use the ray approximation to show that the travel-time shifts in the thermal
(non-magnetic) sunspot model are primarily produced by changes in the wave path
due to the Wilson depression rather than variations in the wave speed. This
shows that inversions for the subsurface structure of sunspots must account for
local changes in the density. In some ranges of horizontal phase speed and
frequency there is agreement (within the noise level in the simulations)
between the travel times measured in the full magnetic sunspot model and the
thermal model. If this conclusion proves to be robust for a wide range of
models, it would suggest a path towards inversions for sunspot structure.Comment: Accepted for publication in The Astrophysical Journa
Validating Forward Modeling and Inversions of Helioseismic Holography Measurements
Here we use synthetic data to explore the performance of forward models and
inverse methods for helioseismic holography. Specifically, this work presents
the first comprehensive test of inverse modeling for flows using
lateral-vantage (deep-focus) holography. We derive sensitivity functions in the
Born approximation. We then use these sensitivity functions in a series of
forward models and inversions of flows from a publicly available
magnetohydrodynamic quiet-Sun simulation. The forward travel times computed
using the kernels generally compare favorably with measurements obtained by
applying holography, in a lateral-vantage configuration, on a 15-hour time
series of artificial Dopplergrams extracted from the simulation. Inversions for
the horizontal flow components are able to reproduce the flows in the upper 3Mm
of the domain, but are compromised by noise at greater depths.Comment: accepted for publication by the Astrophysical
Reconstruction of Solar Subsurfaces by Local Helioseismology
Local helioseismology has opened new frontiers in our quest for understanding
of the internal dynamics and dynamo on the Sun. Local helioseismology
reconstructs subsurface structures and flows by extracting coherent signals of
acoustic waves traveling through the interior and carrying information about
subsurface perturbations and flows, from stochastic oscillations observed on
the surface. The initial analysis of the subsurface flow maps reconstructed
from the 5 years of SDO/HMI data by time-distance helioseismology reveals the
great potential for studying and understanding of the dynamics of the quiet Sun
and active regions, and the evolution with the solar cycle. In particular, our
results show that the emergence and evolution of active regions are accompanied
by multi-scale flow patterns, and that the meridional flows display the
North-South asymmetry closely correlating with the magnetic activity. The
latitudinal variations of the meridional circulation speed, which are probably
related to the large-scale converging flows, are mostly confined in shallow
subsurface layers. Therefore, these variations do not necessarily affect the
magnetic flux transport. The North-South asymmetry is also pronounced in the
variations of the differential rotation ("torsional oscillations"). The
calculations of a proxy of the subsurface kinetic helicity density show that
the helicity does not vary during the solar cycle, and that supergranulation is
a likely source of the near-surface helicity.Comment: 17 pages, 10 figures, in "Cartography of the Sun and the Stars",
Editors: Rozelot, Jean-Pierre, Neiner, Corali
Testing Helioseismic-Holography Inversions for Supergranular Flows Using Synthetic Data
Supergranulation is one of the most visible length scales of solar convection
and has been studied extensively by local helioseismology. We use synthetic
data computed with the Seismic Propagation through Active Regions and
Convection (SPARC) code to test regularized-least squares (RLS) inversions of
helioseismic holography measurements for a supergranulation-like flow. The code
simulates the acoustic wavefield by solving the linearized three-dimensional
Euler equations in Cartesian geometry. We model a single supergranulation cell
with a simple, axisymmetric, mass-conserving flow.
The use of simulated data provides an opportunity for direct evaluation of
the accuracy of measurement and inversion techniques. The RLS technique applied
to helioseismic-holography measurements is generally successful in reproducing
the structure of the horizontal flow field of the model supergranule cell. The
errors are significant in horizontal-flow inversions near the top and bottom of
the computational domain as well as in vertical-flow inversions throughout the
domain. We show that the errors in the vertical velocity are due largely to
cross talk from the horizontal velocity.Comment: 22 pages, 12 figues, accepted for publication in Solar Physic
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