The Origin of Helicity in Solar Active Regions

We present calculations of helicity based on our solar dynamo model and show that the results are consistent with observational data.Comment: To appear in the Proceedings of IAU Symposium 22

Dimension Driven Accelerating Universe

The current acceleration of the universe leads us to investigate higher dimensional gravity theory, which is able to explain acceleration from a theoretical view point without the need of introducing dark energy by hand. We argue that the terms containing higher dimensional metric coefficients produce an extra negative pressure that apparently drives an acceleration of the 3D space, tempting us to suggest that the accelerating universe seems to act as a window to the existence of extra spatial dimensions. Interesting to point out that in this case our cosmology apparently mimics the well known quintessence scenario fuelled by a generalised Chaplygin-type of fluid where a smooth transition from a dust dominated model to a de Sitter like one takes place. Correspondence to models generated by a tachyonic form of matter is also briefly discussed

Helicity of solar active regions from a Dynamo Model

We calculate helicities of solar active regions based on the idea that poloidal flux lines get wrapped around a toroidal flux tube rising through the convection zone, thereby giving rise to the helicity. Rough estimates based on this idea compare favorably with the observed magnitude of helicity. We use our solar dynamo model based on the Babcock-Leighton a-effect to study how helicity varies with latitude and time. At the time of solar maximum, our theoretical model gives negative helicity in the northern hemisphere and positive helicity in the south, in accordance with observed hemispheric trends. However, we find that during a short interval at the beginning of a cycle, helicities tend to be opposite of the preferred hemispheric trends

Plasma spraying of an indigenous yttria stabilized zirconia powder prepared by the sol-gel technique

An indigenous sol-gel derived yttria-partially stabilized zirconia (Y-PSZ) powder has been characterized and its suitability for plasma spraying applications evaluated. The powder, determined to have about 5.1% yttria content, predominantly consisted of spherical particles with an average equivalent particle diameter close to 25 mum. Furthermore, it was found that the powder did not contain any particles > 50 mum, which is considered the ideal upper size limit for spray-grade ceramic powders in order to ensure complete melting during spraying. The sol-gel produced powder exhibited good flow characteristics and the plasma sprayed coatings developed using this powder were also found to have excellent thermal shock resistance. The corresponding results obtained using an imported Y-PSZ powder are also presented for the purpose of comparison

A Comparative Analysis of Machine-learning Models for Solar Flare Forecasting : Identifying High-performing Active Region Flare Indicators

Solar flares create adverse space weather impacting space- and Earth-based technologies. However, the difficulty of forecasting flares, and by extension severe space weather, is accentuated by the lack of any unique flare trigger or a single physical pathway. Studies indicate that multiple physical properties contribute to active region flare potential, compounding the challenge. Recent developments in machine learning (ML) have enabled analysis of higher-dimensional data leading to increasingly better flare forecasting techniques. However, consensus on high-performing flare predictors remains elusive. In the most comprehensive study to date, we conduct a comparative analysis of four popular ML techniques (k nearest neighbors, logistic regression, random forest classifier, and support vector machine) by training these on magnetic parameters obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory for the entirety of solar cycle 24. We demonstrate that the logistic regression and support vector machine algorithms perform extremely well in forecasting active region flaring potential. The logistic regression algorithm returns the highest true skill score of 0.967 +/- 0.018, possibly the highest classification performance achieved with any strictly parametric study. From a comparative assessment, we establish that magnetic properties like total current helicity, total vertical current density, total unsigned flux, R_VALUE, and total absolute twist are the top-performing flare indicators. We also introduce and analyze two new performance metrics, namely, severe and clear space weather indicators. Our analysis constrains the most successful ML algorithms and identifies physical parameters that contribute most to active region flare productivity.Peer reviewe

Preparation of ZrO2-CaO and ZrO2-MgO fibres by alkoxide Sol-Gel processing

Extensive studies were made on the spinnability of Zr(O-nC3H7)4-derived sols for the preparation of ZrO2-CaO and ZrO2-MgO fibres. An improved preparative route was proposed in which acetylacetone controlled the hydrolyzability of the alkoxide and 2-methoxyethanol served as a slow-drying solvent. The advantages of the method of sol preparation over the known methods were highlighted. Crystallization behaviour of the gel fibres was studied mostly up to 1300-degrees-C for ZrO2-CaO fibres (up to 20 %mol CaO) and up to 1000-degrees-C for ZrO2-MgO fibres (up to 12 %mol MgO). ZrO2-CaO fibres (7-11% mol CaO) were Found to yield only the tetragonal and cubic polymorphs of ZrO2 after heating at 1300-degrees-C/4h and furnace cooling. ZrO2-MgO fibres, on the other hand, showed exsolution of MgO above 800-degrees-C and development of monoclinic ZrO2

Preparation of pure and stabilizer (MgO,CeO2)-added ZrO2 powders and their crystallization behaviour

Amorphous precursor materials for pure and MgO and CeO" containing Zr02 were prepared via alkoxide hydrolysis, gel formation and solution spray drying techniques. The crystallization behaviour of such precursor powders was studied by calcining them at 700°, 1000° and noocc. It was shown that the initial product(s) of crystallization, as well as the conversion product(s) at higher temperatures could depend significantly not only on the overall mode of precursor preparation, but also on subtle differences within the same preparative technique

Ambient drying and cracking phenomena in monolithic zirconia gels

Preparation of monolithic zirconia gels from alkoxides has been described for a series of compositions in the system zirconium propoxide-2-propanol-acetylacetone-water. When large volumes of the sols were cast in glass containers, an unusual crack pattern was observed in all cases; in each sample, the crack divided the gel into a top part with a central root-like protrusion and a bottom part surrounding the protrusion. The bottom part characteristically contained more volatiles than the top. The relatively high viscosity of the alcohol and the adhesion of the gel to the glass container wall were suggested to be the major factors guiding the crack pattern