7,874 research outputs found
Revisiting the solar tachocline: Average properties and temporal variations
The tachocline is believed to be the region where the solar dynamo operates.
With over a solar cycle's worth of data available from the MDI and GONG
instruments, we are in a position to investigate not merely the average
structure of the solar tachocline, but also its time variations. We determine
the properties of the tachocline as a function of time by fitting a
two-dimensional model that takes latitudinal variations of the tachocline
properties into account. We confirm that if we consider central position of the
tachocline, it is prolate. Our results show that the tachocline is thicker at
higher latitudes than the equator, making the overall shape of the tachocline
more complex. Of the tachocline properties examined, the transition of the
rotation rate across the tachocline, and to some extent the position of the
tachocline, show some temporal variations
Effects of granulation on the visibility of solar oscillations
Context. The interaction of solar oscillations with near surface convection
is poorly understood. These interactions are likely the cause of several
problems in helio- and astero-seismology, including the so-called surface
effect and apparently unphysical travel time shifts as a function of center to
limb distance. There is thus a clear need for further theoretical understanding
and observational tests.
Aims. The aim is to determine how the observed modes are affected by the
convection.
Methods. I use HMI velocity and intensity images to construct k-
diagrams showing how the oscillation amplitude and phase depend on the local
granulation intensity.
Results. There is a clear and significant dependence of the observed
properties of the oscillations on the local convection state
Supergranules as Probes of Solar Convection Zone Dynamics
Supergranules are convection cells seen at the Sun's surface as a space
filling pattern of horizontal flows. While typical supergranules have diameters
of about 35 Mm, they exhibit a broad spectrum of sizes from ~10 Mm to ~100 Mm.
Here we show that supergranules of different sizes can be used to probe the
rotation rate in the Sun's outer convection zone. We find that the equatorial
rotation rate as a function of depth as measured by global helioseismology
matches the equatorial rotation as a function of wavelength for the
supergranules. This suggests that supergranules are advected by flows at depths
equal to their wavelengths and thus can be used to probe flows at those depths.
The supergranule rotation profiles show that the surface shear layer, through
which the rotation rate increases inward, extends to depths of ~50 Mm and to
latitudes of at least 70 degrees. Typical supergranules are well observed at
high latitudes and have a range of sizes that extend to greater depths than
those typically available for measuring subsurface flows with local
helioseismology. These characteristics indicate that probing the solar
convection zone dynamics with supergranules can complement the results of
helioseismology.Comment: 6 pages, 3 figures, Accepted for publication in The Astrophysical
Journal Letter
Unusual Trends in Solar P-Mode Frequencies During the Current Extended Minimum
We investigate the behavior of the intermediate-degree mode frequencies of
the sun during the current extended minimum phase to explore the time-varying
conditions in the solar interior. Using contemporaneous helioseismic data from
GONG and MDI, we find that the changes in resonant mode frequencies during the
activity minimum period are significantly greater than the changes in solar
activity as measured by different proxies. We detect a seismic minimum in MDI
p-mode frequency shifts during 2008 July--August but no such signature is seen
in mean shifts computed from GONG frequencies. We also analyze the frequencies
of individual oscillation modes from GONG data as a function of latitude and
observe a signature of the onset of the solar cycle 24 in early 2009. Thus the
intermediate degree modes do not confirm the onset of the cycle 24 during late
2007 as reported from the analysis of the low-degree GOLF frequencies. Further,
both the GONG and MDI frequencies show a surprising anti-correlation between
frequencies and activity proxies during the current minimum, in contrast to the
behavior during the minimum between cycles 22 and 23.Comment: Accepted for publication in ApJ Letter
The Solar Orbiter mission and its prospects for helioseismology
Solar Orbiter is intended to become ESA's next solar mission in heritage of
the successful SOHO project. The scientific objectives of the mission, its
design, and its scientific payload are reviewed. Specific emphasis is given to
the perspectives of Solar Orbiter with respect to helioseismology.Comment: 6 pages, 4 figure
Solar Rotation Rate During the Cycle 24 Minimum in Activity
The minimum of solar cycle 24 is significantly different from most other
minima in terms of its duration as well as its abnormally low levels of
activity. Using available helioseismic data that cover epochs from the minimum
of cycle 23 to now, we study the differences in the nature of the solar
rotation between the minima of cycles 23 and 24. We find that there are
significant differences between the rotation rates during the two minima. There
are differences in the zonal-flow pattern too. We find that the band of fast
rotating region close to the equator bifurcated around 2005 and recombined by
2008. This behavior is different from that during the cycle 23 minimum. By
auto-correlating the zonal-flow pattern with a time shift, we find that in
terms of solar dynamics, solar cycle 23 lasted for a period of 11.7 years,
consistent with the result of Howe et al. (2009). The autocorrelation
coefficient also confirms that the zonal-flow pattern penetrates through the
convection zone.Comment: Accepted for publication in Ap
Capital Taxation, Growth, and Non-renewable Resources
The conventional view within the endogenous growth literature is that interest income taxes impede economic growth and investment subsidies promote economic growth. The present paper lays out a simple framework to see whether this is still true when non-renewable resources enter the ”growth engine” in an essential way. It is not! The framework allows a rich set of determinants of longrun growth, including some fiscal policy measures, but interest income taxes and investment subsidies are not among these. The results not only contrast with the modern literature on taxes and endogenous growth, but also with observations in the literature from the 1970’s on non-renewable resources and taxation - observations which were not based on general equilibrium considerations.non-renewable resources; endogenous growth; greenhouse effect; taxes; subsidies
Size Constraints on Majorana Beamsplitter Interferometer: Majorana Coupling and Surface-Bulk Scattering
Topological insulator surfaces in proximity to superconductors have been
proposed as a way to produce Majorana fermions in condensed matter physics. One
of the simplest proposed experiments with such a system is Majorana
interferometry. Here, we consider two possibly conflicting constraints on the
size of such an interferometer. Coupling of a Majorana mode from the edge (the
arms) of the interferometer to vortices in the centre of the device sets a
lower bound on the size of the device. On the other hand, scattering to the
usually imperfectly insulating bulk sets an upper bound. From estimates of
experimental parameters, we find that typical samples may have no size window
in which the Majorana interferometer can operate, implying that a new
generation of more highly insulating samples must be explored.Comment: 14 pages, 6 figure
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