AR_final file_2018-19

closed form expression for the bending angle of light in equatorial plane of Kerr-Sen black hole is derived as a function of impact parameter, spin and charge of the black hole. Results obtained are also com- pared with the corresponding cases of Kerr black hole in general relativity. It is observed that charge parameter behaves qualitatively similar as the spin parameter for photons travelling in direct orbits while behaves differently for photons in retrograde orbits around black hole. As the numerical value of the black hole charge increases, bending angle becomes larger in strong field limit. Further, it is observed that this effect is more pronounced in case of direct orbits in comparison to the retro orbits. For both the direct and retro motion, the bending angle exceeds 2 π , which in turn results in multiple loops and formation of relativistic images. This study has been done in collaboration with Philippe Jetzer. Hemwati Nandan and Uma Papnoi Strong lensing and observables around 5D Myers- Perry black hole spacetime We study the motion of massless test particles in a five dimensional (5D) Myers-Perry black hole spacetime with two spin parameters. The be- haviour of the effective potential in view of different values of black hole parameters is discussed in the equatorial plane. The frequency shift of photons is calculated which is found to depend on the spin parameter of black hole and the observed redshift is discussed accordingly. The deflection angle and the strong deflection limit coefficients are also cal- culated, and their behaviour with the spin param- eters is analysed in detail. It is observed that the behaviour of both deflection angle and strong field coefficient differr from Kerr black hole spacetime in four dimensions (4D) in general relativity (GR), which is mainly due to the presence of two spin parameters in higher dimension. This study has been done in collaboration with Ravi S. Kuniyal, Rashmi Uniyal, and K. D. Purohit. Dibyendu Nandi Prediction of the strength and timing of sunspot cy- cle 25 reveal decadal-scale space environmental con- ditions The Sun’s activity cycle governs the radiation, par- ticle, and magnetic flux in the heliosphere creat- ing hazardous space weather. Decadal-scale vari- ations define space climate and force the Earth’s atmosphere. However, predicting the solar cycle is challenging. Current understanding indicates a short window for prediction best achieved at pre- vious cycle minima. Utilizing magnetic field evolu- tion models for the Sun’s surface and interior, we perform the first century-scale, data-driven simu- lations of solar activity and present a scheme for extending the prediction window to a decade. Our ensemble forecast indicates cycle 25 would be sim- ilar or slightly stronger than the current cycle and peak around 2024. Sunspot cycle 25 may, thus, re- verse the substantial weakening trend in solar ac- tivity, which has led to speculation of an imminent Maunder-like grand minimum and cooling global climate. Our simulations demonstrate fluctuation in the tilt angle distribution of sunspots, which is the dominant mechanism responsible for solar cycle variability.This study has been done in collabora- tion with Prantika Bhowmik. The association of filaments, polarity inversion lines, and coronal hole properties with the sunspot cycle: An analysis of the McIntosh database We study the properties of filaments, PILs, and coronal holes in solar cycles 20, 21, 22, and 23 uti- lizing the McIntosh archive. We detect a prominent cyclic behaviour of filament length, PIL length, and coronal hole area with significant correspon- dence with the solar magnetic cycle. The spatio- temporal evolution of the geometric centers of fil- aments shows a butterfly-like structure and dis- tinguishable poleward migration of long filaments during cycle maxima. We identify this rush to the poles of filaments to be co-temporal with the initiation of polar field reversal as gleaned from Mount Wilson and Wilcox Solar Observatory po- lar field observations, and quantitatively establish their temporal correspondence. We analyze the fil- ament tilt angle distribution to constrain their pos- sible origins. The majority of the filaments exhibit negative and positive tilt angles in the northern and the southern hemispheres, respectively, strongly suggesting that their formation is governed by the overall large-scale magnetic field distribution on the solar photosphere, and not by the small-scale intra- active region magnetic field configurations. We also investigate the hemispheric asymmetry in fil- aments, PILs, and coronal holes. We find that the hemispheric asymmetry in filaments and PILs is ( 203 )