Ring diagram analysis of near-surface flows in the Sun

Ring diagram analysis of solar oscillation power spectra obtained from MDI data is carried out to study the velocity fields in the outer part of the solar convection zone. The three dimensional power spectra are fitted to a model which has a Lorentzian profile in frequency and which includes the advection of the wave front by horizontal flows, to obtain the two components of the sub-surface flows as a function of the horizontal wave number and radial order of the oscillation modes. This information is then inverted using OLA and RLS methods to infer the variation in horizontal flow velocity with depth. The average rotation velocity at different latitudes obtained by this technique agrees reasonably with helioseismic estimates made using frequency splitting data. The shear layer just below the solar surface appears to consist of two parts with the outer part up to a depth of 4 Mm, where the velocity gradient does not show any reversal up to a latitude of 60 degrees. In the deeper part the velocity gradient shows reversal in sign around a latitude of 55 degrees. The zonal flow velocities inferred in the outermost layers appears to be similar to those obtained by other measurements. A meridional flow from equator polewards is found. It has a maximum amplitude of about 30 m/s near the surface and the amplitude is nearly constant in the outer shear layer.Comment: aastex, 12 figures, to appear in Ap.

Seismic investigation of the solar structure using GONG frequencies

Using the recently obtained GONG frequencies, we investigate the properties of the solar interior by constructing solar models with various input physics like opacities, equation of state, nuclear reaction rates etc. The differential asymptotic inversion technique is then used to infer the relative difference in sound speed between the Sun and solar models. Here we apply these results to test equation of state and different formulation for calculating the convective flux.Comment: Latex, 2 pages, 3 figures, To appear in the IAU Symp. # 181: "Sounding solar and stellar interiors", eds. F.X. Schmider & J. Provos

Comments on the four-dimensional effective theory for warped compactification

We derive four-dimensional effective theories for warped compactification of the ten-dimensional IIB supergravity and the eleven-dimensional Horava-Witten model. We show that these effective theories allow a much wider class of solutions than the original higher-dimensional theories. In particular, the effective theories have cosmological solutions in which the size of the internal space decreases with the cosmic expansion in the Einstein frame. This type of compactifying solutions are not allowed in the original higher-dimensional theories. This result indicates that the effective four-dimensional theories should be used with caution, if one regards the higher-dimensional theories more fundamental.Comment: 21 pages, no figure. Minor errors are correcte

Comparison of High-degree Solar Acoustic Frequencies and Asymmetry between Velocity and Intensity Data

Using the local helioseismic technique of ring diagram we analyze the frequencies of high--degree f- and p-modes derived from both velocity and continuum intensity data observed by MDI. Fitting the spectra with asymmetric peak profiles, we find that the asymmetry associated with velocity line profiles is negative for all frequency ranges agreeing with previous observations while the asymmetry of the intensity profiles shows a complex and frequency dependent behavior. We also observe systematic frequency differences between intensity and velocity spectra at the high end of the frequency range, mostly above 4 mHz. We infer that this difference arises from the fitting of the intensity rather than the velocity spectra. We also show that the frequency differences between intensity and velocity do not vary significantly from the disk center to the limb when the spectra are fitted with the asymmetric profile and conclude that only a part of the background is correlated with the intensity oscillations.Comment: Accepted for publication in Astrophysical Journa

Mechanism of enhanced light output in InGaN-based microlight emitting diodes

Micro-light emitting diode (LED) arrays with diameters of 4 to 20 mum have been fabricated and were found to be much more efficient light emitters compared to their broad-area counterparts, with up to five times enhancement in optical power densities. The possible mechanisms responsible for the improvement in performance were investigated. Strain relaxation in the microstructures as measured by Raman spectroscopy was not observed, arguing against theories of an increase in internal quantum efficiency due to a reduction of the piezoelectric field put forward by other groups. Optical microscope images show intense light emission at the periphery of the devices, as a result of light scattering off the etched sidewalls. This increases the extraction efficiency relative to broad area devices and boosts the forward optical output. In addition, spectra of the forward emitted light reveal the presence of resonant cavity modes [whispering gallery (WG) modes in particular] which appear to play a role in enhancing the optical output

The diffusion coefficient of propagating fronts with multiplicative noise

Recent studies have shown that in the presence of noise both fronts propagating into a metastable state and so-called pushed fronts propagating into an unstable state, exhibit diffusive wandering about the average position. In this paper we derive an expression for the effective diffusion coefficient of such fronts, which was motivated before on the basis of a multiple scale ansatz. Our systematic derivation is based on the decomposition of the fluctuating front into a suitably positioned average profile plus fluctuating eigenmodes of the stability operator. While the fluctuations of the front position in this particular decomposition are a Wiener process on all time scales, the fluctuations about the time averaged front profile relax exponentially.Comment: 4 page

The universality class of fluctuating pulled fronts

It has recently been proposed that fluctuating ``pulled'' fronts propagating into an unstable state should not be in the standard KPZ universality class for rough interface growth. We introduce an effective field equation for this class of problems, and show on the basis of it that noisy pulled fronts in {\em d+1} bulk dimensions should be in the universality class of the {\em (d+1)+1}D KPZ equation rather than of the {\em d+1}D KPZ equation. Our scenario ties together a number of heretofore unexplained observations in the literature, and is supported by previous numerical results.Comment: 4 pages, 2 figure

Does the Selection of a Quiet Region Influence the Local Helioseismic Inferences?

We apply the ring-diagram technique to high resolution Dopplergrams in order to estimate the variation in oscillation mode parameters between active and quiet regions. We demonstrate that the difference in mode parameters between two quiet regions can be as large as those between a pair of active and quiet region. This leads us to conclude that the results derived on the basis of a single quiet region could be biased.Comment: 4 pages, Submitted to Conference Proceedings of GONG 2008/ SOHO XX

InGaN nano-ring structures for high-efficiency light emitting diodes

A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlled ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm-1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nm blueshift observed in the cathodoluminescence spectra can be attributed to band filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated