Title : Interstellar Polarisation: Grain alignment theories and polarisation study towards high opacity LDN clouds

Abstract

Interstellar polarisation at optical and near- infrared wavelength bands is thought to arise when light from the distant stars passes through the interstellar medium consisting of asymmetric dust grains aligned with the magnetic field. Different theories have been developed to explain the process of grain alignment. They depend upon the sizes of the grain particles, the refractive index of the grains and the wavelength of observations. A study of the wavelength dependence of polarisation gives us insight into these features for the interstellar medium. The talk will mainly focus on the theories dealing with interstellar polarization by light passing through aligned grains. Some results from an ongoing work on Lynds Dark Nebulae (LDN) would also be presented.

Title : Solar flares in the context of filament eruptions

Abstract

Title : Search and characterization of relic radio galaxies in deep fields

Abstract

The typical morphology of a radio galaxy consists of a core, pair of jets, lobes and hot-spots. The phase of radio galaxy evolution after the jets have switched off, is referred as remnant phase. The remnant phase is not well understood due to fact that it is short-lived and only few relic radio galaxies are known. We, therefore, have tried to improve the statistics on radio galaxies that have ceased to be active, or are intermittently active. I shall talk about the search for relic radio galaxies in the VLA-VIMOS VLT Deep Survey (VLA-VVDS) field using deep multi-frequency radio observations.

Title : Modelling the solar and stellar magnetic cycles.

Abstract

The sun and many other low-main sequence stars have active magnetic fields. The solar magnetic field reverses its polarity in about 11 years, and sunspots are the regions of intense magnetic field seen on the solar surface. The number of sunspots varies cyclically with a period of about 11 years which is popularly known as the solar cycle. However, this cycle is not regular; it has variation within a cycle as well as cycle-to-cycle. In this presentation, I will highlight my contribution in explaining these irregular features of the magnetic cycles of the Sun and other stars. Magnetohydrodynamics (MHD) dynamo process is believed to be responsible for such variation. The fluctuations in the so-called Babcock-Leighton process in generating a poloidal field in the solar dynamo, primarily through the scatter in the sunspot tilt, produce variation in the poloidal field and eventually cause irregularities in the solar cycle. I will further highlight our effort in explaining the features in magnetic cycles in other slowly rotating stars.

Title : X-ray studies of GHz-peaked-spectrum (GPS) radio galaxies

Abstract

The GHz-peaked-spectrum (GPS) radio sources, a subclass of radio galaxies, are characterized by a convex radio spectrum peaked around 1 GHz. GPS radio galaxies exhibit compact radio size (~ 10 - 1000 pc) and are believed to represent the early stage of radio galaxy evolution. X-ray studies of GPS sources can help us in understanding the nature of environment around AGN and conditions during early phase of AGN activity. We have carried out X-ray spectral modeling of a sample of GPS sources and found that their X-ray spectra are characterized either by absorbed power law with high column density, or relatively flatter power law with Fe K_alpha emission line of high equivalent width. Hence, we conclude that GPS sources reside in dense circumnuclear environment. In this talk I shall also emphasize the need of hard X-ray spectral studies of GPS sources.

Title : Hydro-meteorological processes in southern India: Insights from stable isotopes in precipitation at Hyderabad

Abstract

Understanding hydro-meteorological processes in southern India is important because it governs the availability and distribution of water in this region. Most of the southern Indian states depends predominantly on precipitation for its water requirement. Hyderabad, located almost at the centre of the Indian peninsula, equidistant from both Arabian Sea and Bay of Bengal, makes it an interesting location to study hydro-meteorological processes such as northward extent of North East monsoon (which is known to bring significant rainfall in the state of Tamil Nadu), effect of locally derived moisture in the rain and seasonal variation in the contribution of various moisture sources. Also, Hyderabad experiences frequent extreme weather conditions such as heat waves, droughts, heavy rainfalls and floods superposed over climatological normal. Understanding of these processes is also important because it has significant political and socio-economic implications especially in the wake of recent water crises in southern India. Stable isotopes of oxygen and hydrogen can be used to trace various hydro-meteorological processes such as vapour source variations, rainout history, vapour recycling, post-precipitation modifications, cloud microphysical processes, etc. Considering this, 182 daily rainfall samples were collected from Hyderabad during 2008-11 as a part of IWIN national programme. The isotopic analysis of these samples in conjunction with ground-based and remotely sensed meteorological parameters will be discussed in this presentation. Also, the importance of isotopes in understanding extreme weather events will be discussed.

Title : Seismic Emission Accompanying Solar Flares

Abstract

Title : Understanding spectral observations of the Sun using numerical simulations

Abstract

Spicules are the jets of cold plasma that are chromospheric in origin. Multiple high-resolution imaging and spectroscopic observations have revealed the ubiquitous presence of spicules, especially the type-II spicules, in the solar corona. Such observations have claimed that the spicules play an important role in providing mass and energy to the solar corona and also help in maintaining its million-degree Kelvin temperature. We numerically simulate the propagation of such cold jet in a gravitationally stratified coronal magnetic structure. The simulation shows the heating of the tip of the spicule to million degree Kelvin due to the shock formation. The analysis of the synthetic spectral lines, constructed from the simulation data, lead us to conclude that probably the spicules provide very less plasma to the solar corona.