• A. K. Singh

Articles written in Journal of Astrophysics and Astronomy

• Soft X-ray Focusing Telescope Aboard AstroSat: Design, Characteristics and Performance

The Soft X-ray focusing Telescope (SXT), India’s first X-ray telescope based on the principle of grazing incidence, was launched aboard the AstroSat and made operational on October 26, 2015. X-rays in the energy band of 0.3–8.0 keV are focussed on to a cooled charge coupled device thus providing medium resolution X-ray spectroscopy of cosmic X-ray sources of various types. It is the most sensitive X-ray instrument aboard the AstroSat. In its first year of operation, SXT has been used to observe objects ranging from active stars, compact binaries, supernova remnants, active galactic nuclei and clusters of galaxies in order to study its performance and quantify its characteriztics. Here, we present an overview of its design, mechanical hardware, electronics, data modes, observational constraints, pipeline processing and its in-orbit performance based on preliminary results from its characterization during the performance verification phase.

• Higher-speed coronal mass ejections and their geoeffectiveness

We have attempted to examine the ability of coronal mass ejections to cause geoeffectiveness. To that end, we have investigated total 571 cases of higher-speed (>1000 km/s) coronal mass ejection events observed during the years 1996–2012. On the basis of angular width (W) of observance, events of coronal mass ejection were further classified as front-side or halo coronal mass ejections (W $=$ 360$^{\circ}$); back-side halo coronal mass ejections (W = 360$^{\circ}$); partial halo (120$^{\circ}$ < W < 360$^{\circ}$) and non-halo (W < 120$^{\circ}$). From further analysis, we found that front halo coronal mass ejections were much faster and more geoeffective in comparison of partial halo and non-halo coronal mass ejections. We also inferred that the front-sided halo coronal mass ejections were 67.1% geoeffective while geoeffectiveness of partial halo coronal mass ejections and non-halo coronal mass ejections were found to be 44.2% and 56.6% respectively. During the same period of observation, 43% ofback-sided CMEs showed geoeffectiveness. We have also investigated some events of coronal mass ejections having speed >2500 km/s as a case study. We have concluded that mere speed of coronal mass ejection and their association with solar flares or solar activity were not mere criterion for producing geoeffectiveness but angular width of coronal mass ejections and their originating position also played a key role.

• Effect of geomagnetic storms on VHF scintillations observed at low latitude

A geomagnetic storm affects the dynamics and composition of the ionosphere and also offers an excellent opportunity to study the plasma dynamics. In the present study, we have used the VHF scintillations data recorded at low latitude Indian station Varanasi (Geomag. latitude $=$ 14$^{\circ}$55$'$N, long. $=$ 154$^{\circ}$E) which is radiated at 250 MHz from geostationary satellite UFO-02 during the period 2011–2012 to investigate the effects of geomagnetic storms on VHF scintillation.Various geomagnetic and solar indices such as Dst index, Kpindex, IMF Bz and solar wind velocity (Vx) are used to describe the geomagnetic field variation observed during geomagnetic storm periods. These indices are very helpful to find out the proper investigation and possible interrelation between geomagnetic storms and observed VHF scintillation. The pre-midnight scintillation is sometimes observed when the main phase of geomagnetic storm corresponds to the pre-midnight period. It is observed that for geomagnetic storms for which the recovery phase starts post-midnight, the probability ofoccurrence of irregularities is enhanced during this time and extends to early morning hours.

• Causes responsible for intense and severe storms during the declining phase of Solar Cycle 24

The occurrence of total 113 geomagnetic storms during declining phase of Solar Cycle 24 (2015–2017) subdivided as about 105 moderate storms (${\rm Dst} = −50$ nT to $−$100 nT), 6 intense storms (${\rm Dst} = −100$ nTto $−$200 nT) and 2 severe storms (Dst < $−$200 nT) has been diagnosed on the basis of 5 day active window through the CACTus (Computer aided CME tracking) software. A detailed study has been carried out for the 6intense and 2 severe storms. It is inferred that CMEs are the major source of geomagnetic storms to occur. Out of the 6 intense and 2 severe storms, only 1 has been observed with the origin of CIR. Thus, all analyzed intensegeomagnetic storms are due to coronal mass ejection at the Sun. Most of our results are in good accordance with other reported results.

• Journal of Astrophysics and Astronomy

Current Issue
Volume 40 | Issue 4
August 2019

• Continuous Article Publication

Posted on January 27, 2016

Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

• Editorial Note on Continuous Article Publication

Posted on July 25, 2019