807 research outputs found
Time-Distance Imaging of Solar Far-Side Active Regions
It is of great importance to monitor large solar active regions in the
far-side of the Sun for space weather forecast, in particular, to predict their
appearance before they rotate into our view from the solar east limb. Local
helioseismology techniques, including helioseismic holography and
time-distance, have successfully imaged solar far-side active regions. In this
Letter, we further explore the possibility of imaging and improving the image
quality of solar far-side active regions by use of time-distance
helioseismology. In addition to the previously used scheme with four acoustic
signal skips, a five-skip scheme is also included in this newly developed
technique. The combination of both four- and five-skip far-side images
significantly enhances the signal-to-noise ratio in the far-side images, and
reduces spurious signals. The accuracy of the far-side active region imaging is
also assessed using one whole year solar observation.Comment: 13 pages, 5 figures, accepted by ApJ Letter
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
Solar radius measurements
Preliminary results of measurements made during 1979-1980 are discussed. Variability in the radius measurements of 0.4 pi is found, of unknown origin
Seismic Constraints on Interior Solar Convection
We constrain the velocity spectral distribution of global-scale solar
convective cells at depth using techniques of local helioseismology. We
calibrate the sensitivity of helioseismic waves to large-scale convective cells
in the interior by analyzing simulations of waves propagating through a
velocity snapshot of global solar convection via methods of time-distance
helioseismology. Applying identical analysis techniques to observations of the
Sun, we are able to bound from above the magnitudes of solar convective cells
as a function of spatial convective scale. We find that convection at a depth
of with spatial extent , where is the
spherical harmonic degree, comprise weak flow systems, on the order of 15 m/s
or less. Convective features deeper than are more difficult
to image due to the rapidly decreasing sensitivity of helioseismic waves.Comment: accepted, ApJ Letters, 5 figures, 10 pages (in this version
Additional Evidence Supporting a Model of Shallow, High-Speed Supergranulation
Recently, Duvall and Hanasoge ({\it Solar Phys.} {\bf 287}, 71-83, 2013)
found that large distance separation travel-time differences from a
center to an annulus implied a model of the average
supergranular cell that has a peak upflow of at a depth of
and a corresponding peak outward horizontal flow of
at a depth of . In the present work, this effect
is further studied by measuring and modeling center-to-quadrant travel-time
differences , which roughly agree with this model.
Simulations are analyzed that show that such a model flow would lead to the
expected travel-time differences. As a check for possible systematic errors,
the center-to-annulus travel-time differences are found
not to vary with heliocentric angle. A consistency check finds an increase of
with the temporal frequency by a factor of two,
which is not predicted by the ray theory
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
Observing and modeling the poloidal and toroidal fields of the solar dynamo
Context. The solar dynamo consists of a process that converts poloidal field
to toroidal field followed by a process which creates new poloidal field from
the toroidal field.
Aims. Our aim is to observe the poloidal and toroidal fields relevant to the
global solar dynamo and see if their evolution is captured by a
Babcock-Leighton dynamo.
Methods. We use synoptic maps of the surface radial field from the KPNSO/VT
and SOLIS observatories to construct the poloidal field as a function of time
and latitude, and Wilcox Solar Observatory and SOHO/MDI full disk images to
infer the longitudinally averaged surface azimuthal field. We show that the
latter is consistent with an estimate of that due to flux emergence and
therefore closely related to the subsurface toroidal field.
Results. We present maps of the poloidal and toroidal magnetic field of the
global solar dynamo. The longitude-averaged azimuthal field observed at the
surface results from flux emergence. At high latitudes this component follows
the radial component of the polar fields with a short time lag (1-3 years). The
lag increases at lower latitudes. The observed evolution of the poloidal and
toroidal magnetic fields is described by the (updated) Babcock-Leighton dynamo
model.Comment: A&
Probing sunspots with two-skip time-distance helioseismology
Previous helioseismology of sunspots has been sensitive to both the
structural and magnetic aspects of sunspot structure. We aim to develop a
technique that is insensitive to the magnetic component so the two aspects can
be more readily separated. We study waves reflected almost vertically from the
underside of a sunspot. Time-distance helioseismology was used to measure
travel times for the waves. Ray theory and a detailed sunspot model were used
to calculate travel times for comparison. It is shown that these large distance
waves are insensitive to the magnetic field in the sunspot. The largest travel
time differences for any solar phenomena are observed. With sufficient modeling
effort, these should lead to better understanding of sunspot structure
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