1,593 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
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
Important Habitat for Chimpanzees in Mali
Analysis of botanical data is presented from the standpoint of chimpanzee natural history. The Sudano-Guinean gallery forest type dominated by the tree Gilletiodendron glandulosum appears to be important habitat for chimpanzees due to vegetation structure, presence of permanent surface water, and particularly, abundance of diverse food plants throughout the year. Based on the fecal analysis, observation of feeding remains, observation of sympatric primates, ethnographic research, and literature review, sixty probable chimpanzee food plants have been identified in the Gilletiodendron forest of Mali. Phytogeographical analysis indicates that chimpanzees in Mali's Sudanian climate zone eat mainly Sudano-Guinean plant species. Heavy reliance on Sudano-Guinean vegetation may indicate that modern chimpanzee populations in savanna areas are relicts, and that the species was originally adapted to mesic Guinean forests. There appears to be niche separation based on topography between humans and chimpanzees which breaks down in times of human food shortage, and the potential for competition is high
A Sociological Investigation of Early Gradutes in U.S. High Schools
Traditional high school graduates are typically seen as the standard for “successful” high school graduation because they earned the customary credential of a diploma and did so along a culturally prescribed timeline (i.e., in Spring of the 12th grade). While high school dropouts have long been recognized and researched as clearly deviating from cultural expectations of earning the standard credential and doing so “on time,” they are not the only type of “off time” student to do so. Early graduates, like dropouts, also pursue a non-traditional and off time high school exiting path, but because of a lack of prior research into these types of students, it is not clear how they compare to the traditional “on time” students. In this dissertation, I investigate early graduates in U.S. high schools to generate an initial basis for understanding how these early graduates differ from the normative group of on timer graduates in terms of their demographics, theoretically important considerations and school engagement (including academic and, separately, social engagement dimensions). This investigation also probes into important differences across several conceptualized groups of early graduates and how each of these groups compare to each other and on time graduates. This investigation utilizes several waves of the nationally representative Educational Longitudinal Study (ELS) from the National Center for Education Statistics (NCES). I use a life course theory perspective to inform conceptualizations of these student groups and my analysis of important post-high school life transitions and trajectories patterns among early graduates
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
Acoustic wave propagation in the solar sub-photosphere with localised magnetic field concentration: effect of magnetic tension
Aims: We analyse numerically the propagation and dispersion of acoustic waves in the solar-like sub-photosphere with localised non-uniform magnetic field concentrations, mimicking sunspots with various representative magnetic field configurations.
Methods: Numerical simulations of wave propagation through the solar sub-photosphere with a localised magnetic field concentration are carried out using SAC, which solves the MHD equations for gravitationally stratified plasma. The initial equilibrium density and pressure stratifications are derived from a standard solar model. Acoustic waves are generated by a source located at the height corresponding approximately to the visible surface of the Sun. By means of local helioseismology we analyse the response of vertical velocity at the level corresponding to the visible solar surface to changes induced by magnetic field in the interior.
Results: The results of numerical simulations of acoustic wave propagation and dispersion in the solar sub-photosphere with localised magnetic field concentrations of various types are presented. Time-distance diagrams of the vertical velocity perturbation at the level corresponding to the visible solar surface show that the magnetic field perturbs and scatters acoustic waves and absorbs the acoustic power of the wave packet. For the weakly magnetised case, the effect of magnetic field is mainly thermodynamic, since the magnetic field changes the temperature stratification. However, we observe the signature of slow magnetoacoustic mode, propagating downwards, for the strong magnetic field cases
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
Impact of Locally Suppressed Wave sources on helioseismic travel times
Wave travel-time shifts in the vicinity of sunspots are typically interpreted
as arising predominantly from magnetic fields, flows, and local changes in
sound speed. We show here that the suppression of granulation related wave
sources in a sunspot can also contribute significantly to these travel-time
shifts, and in some cases, an asymmetry between in and outgoing wave travel
times. The tight connection between the physical interpretation of travel times
and source-distribution homogeneity is confirmed. Statistically significant
travel-time shifts are recovered upon numerically simulating wave propagation
in the presence of a localized decrease in source strength. We also demonstrate
that these time shifts are relatively sensitive to the modal damping rates;
thus we are only able to place bounds on the magnitude of this effect. We see a
systematic reduction of 10-15 seconds in -mode mean travel times at short
distances ( Mm) that could be misinterpreted as arising from a
shallow (thickness of 1.5 Mm) increase ( 4%) in the sound speed. At
larger travel distances ( Mm) a 6-13 s difference between the ingoing
and outgoing wave travel times is observed; this could mistakenly be
interpreted as being caused by flows.Comment: Revised version. Submitted to Ap
Solar meridional circulation from twenty-one years of SOHO/MDI and SDO/HMI observations: Helioseismic travel times and forward modeling in the ray approximation
The south-north travel-time differences are measured by applying
time-distance helioseismology to the MDI and HMI medium-degree Dopplergrams
covering May 1996-April 2017. Our data analysis corrects for several sources of
systematic effects: P-angle error, surface magnetic field effects, and
center-to-limb variations. An interpretation of the travel-time measurements is
obtained using a forward-modeling approach in the ray approximation. The
travel-time differences are similar in the southern hemisphere for cycles 23
and 24. However, they differ in the northern hemisphere between cycles 23 and
24. Except for cycle 24's northern hemisphere, the measurements favor a
single-cell meridional circulation model where the poleward flows persist down
to 0.8 , accompanied by local inflows toward the activity belts
in the near-surface layers. Cycle 24's northern hemisphere is anomalous:
travel-time differences are significantly smaller when travel distances are
greater than 20. This asymmetry between northern and southern
hemispheres during cycle 24 was not present in previous measurements (e.g.,
Rajaguru & Antia 2015), which assumed a different P-angle error correction
where south-north travel-time differences are shifted to zero at the equator
for all travel distances. In our measurements, the travel-time differences at
the equator are zero for travel distances less than 30, but they
do not vanish for larger travel distances. This equatorial offset for large
travel distances need not be interpreted as a deep cross-equator flow; it could
be due to the presence of asymmetrical local flows at the surface near the end
points of the acoustic ray paths.Comment: accepted for publication in A&
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