The infrared bands of small silicate spheroids

We have investigated the effects of the size, shape, and alignment of small silicate spheroids on the wavelength positions of their absorption bands near 10 m and 20 m. These particles are believed to be a major constituent of interstellar grains and are reported to have been observed in circumstellar dust shells. It is found that the bands for spheroids can be displaced by 1 m towards longer wavelengths from their positions for corresponding spheres; and that a further, though smaller, displacement can be superimposed upon this by their subsequent alignment.Communications from the Royal Observatory, Edinburgh, No. 178.     

Extinction and scattering by several types of silicate sphere of radius 0.05–1.0μm,for the wavelength range 0.21–50μm

The exact calculation of scattering and absorption by various sub-micron sized silicate spheres is presented here, using accurately determined optical constants in the wavelength range from 50 m to 0.21 m. The extinction features near 10 m and 20 m for various samples are discussed. It is found that the ratio of peak extinction at 20 m to that at 10 m is constant for small particles up tor=0.4 m, but is less for particles of radius 1 m. The ratio of maximum extinction in the ultraviolet to that at 10 m decreases with increasing particle size.     

Intrinsic variations of interstellar extinction laws

The intrinsic ultraviolet colours of early type stars have been derived from the rocket data of J. W. Campbell and A. M. Smith. There exists a considerable spread in these colours. One possible source is the predicted rms variation due to the effect of rotation. It is necessary to recognise that rotation is an important source of uncertainty of the interstellar extinction curves derived from the comparison of pairs of stars.Predicted rms errors due to different sources (rotation, uncertainty of MK spectral type, photometric errors) have been compared with the dispersion of the observed extinction data and it is found that part of the large scatter could be due to intrinsic differences of the properties of interstellar grains.     

Vortex rings in the working surface of radio jets

We study the effects of the presence of vortex rings surrounding a supersonic radio jet inside the cocoon of a radio lobe. We show that both the jet and the shocked ambient medium are pinched. Flow speed inside the cocoon is always close to the sonic value and it stays so by successively passing through several oblique shocks. We also discuss the possibility of the non-linear growth of the instabilities of the contact surface to explain the numerical results in the literature.     

Transition in Late Quaternary Paleoclimate in Schirmacher Region,East Antarctica as Revealed from Lake Sediments

The lacustrine sediments are one of the best sources to provide information on climate change, specially in peri-glacial climatic region. Schirmacher Oasis, located on the Princess Astrid Coast in Queen Maud Land, is one of the few areas in East Antarctica that provides valuable information on paleoclimate of the region with various depositional features formed due to deglaciation process. This Oasis is dotted with more than 100 lakes of proglacial, land-locked and epi-shelf type. The multi-proxy sedimentological data, generated from the sediment cores from land-locked lakes and grab sample from a proglacial lake, lying in the same drainage line in the central part of Schirmacher region has provided better insight into the paleoclimatic evolution of the region. The immature and chemically unaltered lake sediments have shown restricted drainage pattern. Different phases of warmer and cooler intervals are highlighted by the patterns of fluctuations in different sedimentological and statistical parameters. The dominance of glacial signatures is very clear on the lake sediments as revealed by the surface textures of quartz grains. Physical weathering has mainly controlled the overall sediments and the composition of clay fraction. The clay minerals indicate a gradual shift in the weathering regime and therewith in climate from strongly glacial to fluvioglacial specially around 42 ka. This indicates beginning of warming of the area much before the LGM. But the warm period is not strong enough to alter the overall clay chemistry. Proxy records indicate short-period climatic oscillations during late Quaternary.     

Effective recombination coefficient and solar zenith angle effects on low-latitude D-region ionosphere evaluated from VLF signal amplitude and its time delay during X-ray solar flares

Excess solar X-ray radiation during solar flares causes an enhancement of ionization in the ionospheric D-region and hence affects sub-ionospherically propagating VLF signal amplitude and phase. VLF signal amplitude perturbation (ΔA) and amplitude time delay (Δt) (vis-á-vis corresponding X-ray light curve as measured by GOES-15) of NWC/19.8 kHz signal have been computed for solar flares which is detected by us during Jan–Sep 2011. The signal is recorded by SoftPAL facility of IERC/ICSP, Sitapur (22^° 27′N, 87^° 45′E), West Bengal, India. In first part of the work, using the well known LWPC technique, we simulated the flare induced excess lower ionospheric electron density by amplitude perturbation method. Unperturbed D-region electron density is also obtained from simulation and compared with IRI-model results. Using these simulation results and time delay as key parameters, we calculate the effective electron recombination coefficient (α _eff ) at solar flare peak region. Our results match with the same obtained by other established models. In the second part, we dealt with the solar zenith angle effect on D-region during flares. We relate this VLF data with the solar X-ray data. We find that the peak of the VLF amplitude occurs later than the time of the X-ray peak for each flare. We investigate this so-called time delay (Δt). For the C-class flares we find that there is a direct correspondence between Δt of a solar flare and the average solar zenith angle Z over the signal propagation path at flare occurrence time. Now for deeper analysis, we compute the Δt for different local diurnal time slots DT. We find that while the time delay is anti-correlated with the flare peak energy flux ? _max independent of these time slots, the goodness of fit, as measured by reduced-χ ², actually worsens as the day progresses. The variation of the Z dependence of reduced-χ ² seems to follow the variation of standard deviation of Z along the T _x -R _x propagation path. In other words, for the flares having almost constant Z over the path a tighter anti-correlation between Δt and ? _max was observed.     

Onboard performance of the RT-2 detectors

The RT-2 Experiment onboard the CORONAS-PHOTON satellite is designed to study the spectral, temporal, and spatial details of solar hard X-ray flares in the 15–150 keV range. Above this energy (and upto 1000 keV), it also acts as an omni-directional gamma-ray detector with a capability to study gamma-ray bursts (GRB), bright solar flares, and X-ray pulsars. With an ensemble of hard X-ray detectors with different fields of view and coding devices, it also has the capability to investigate the spectrum of Cosmic Diffuse X-ray Background. The performance of the detectors from 2009 February to November is described in this paper. Results obtained on a few GRBs and solar flares are also briefly discussed.