Dust modification of the plasma conductivity in the mesosphere

Relative transverse drift (with respect to the ambient magnetic field) between the weakly magnetized electrons and the unmagnetized ions at the lower altitude (80 km) and between the weakly magnetized ions and unmagnetized dust at the higher altitude (90 km) gives rise to the finite Hall conductivity in the Earth's mesosphere. If, on the other hand, the number of free electrons is sparse in the mesosphere and most of the negative charge resides on the weakly magnetized, fine, nanometre sized dust powder and positive charge on the more massive, micron sized, unmagnetized dust, the sign of the Hall conductivity due to their relative transverse drift will be opposite to the previous case. Thus the sign of the Hall effect not only depends on the direction of the local magnetic field but also on the nature of the charge carrier in the partially ionized dusty medium. As the Hall and the Ohm diffusion are comparable below 80 km, the low frequency long wavelength waves will be damped at this altitude with the damping rate typically of the order of few minutes. Therefore, the ultra--low frequency magnetohydrodynamic waves can not originate below 80 km in the mesosphere. However, above 80 km since Hall effect dominates Ohm diffusion the mesosphere can host the ultra--low frequency waves which can propagate across the ionosphere with little or, no damping.Comment: 21 pages, 3 figures; to appear in the Journal of Atmospheric and Solar-Terrestrial Physic

Evidence for two spin-glass transitions with magnetoelastic and magnetoelectric couplings in the multiferroic (Bi1−x_{1-x}Bax_x)(Fe1−x_{1-x}Tix_x)O3_3 system

For disordered Heisenberg systems with small single ion anisotropy, two spin glass transitions below the long range ordered phase transition temperature has been predicted theoretically for compositions close to the percolation threshold. Experimental verification of these predictions is still controversial for conventional spin glasses. We show that multiferroic spin glass systems can provide a unique platform for verifying these theoretical predictions via a study of change in magnetoelastic and magnetoelectric couplings, obtained from an analysis of diffraction data, at the spin glass transition temperatures. Results of macroscopic and microscopic (x-ray and neutron scattering) measurements are presented on disordered BiFeO3, a canonical Heisenberg system with small single ion anisotropy, which reveal appearance of two spin glass phases SG1 and SG2 in coexistence with the LRO phase below the A-T and G-T lines. It is shown that the temperature dependence of the integrated intensity of the antiferromagnetic peak shows dips with respect to the Brillouin function behaviour around the SG1 and SG2 transition temperatures. The ferroelectric polarisation changes significantly at the two spin glass transition temperatures. These results, obtained using microscopic techniques, clearly demonstrate that the SG1 and SG2 transitions occur on the same magnetic sublattice and are intrinsic to the system. We also construct a phase diagram showing all the magnetic phases in BF-xBT system. While our results on the two spin glass transitions support the theoretical predictions, it also raises several open questions which need to be addressed by revisiting the existing theories of spin glass transitions by taking into account the effect of magnetoelastic and magnetoelectric couplings as well as electromagnons.Comment: 59 pages 21 figure

GRB000301C with peculiar afterglow emission

The CCD magnitudes in Johnson V and Cousins R and I photometric passbands are determined for GRB 000301C afterglow starting ~ 1.5 day after the gamma-ray burst. In fact we provide the earliest optical observations for this burst. Light curves of the afterglow emissions in U, B, V, R, I, J and K' passbands are obtained by combining the present measurements with the published data. Flux decay shows a very uncommon variation relative to other well observed GRBs. Overall, there is a steepening of the optical and near-infrared flux decay caused by a geometric and sideways expanding jet. This is superimposed by a short term variability especially during early time (Delta t < 8 days). The cause of variability is not well understood, though it has occurred simultaneously with similar amplitude in all the filters. We derive the early and late time flux decay constants using jet model. The late time flux decay is the steepest amongst the GRB OTs observed so far with alpha ~ 3. Steepening in the flux decay seems to have started simultaneously around Delta t ~ 7.6 day in all passbands. The value of spectral index in the optical-near IR region is ~ -1.0. Redshift determination with z=2.0335 indicates cosmological origin of the GRB having a luminosity distance of 16.6 Gpc. Thus it becomes the second farthest amongst the GRBs with known distances. An indirect estimate of the fluence > 20 keV indicates, if isotropic,> =10^53 ergs of release of energy. The enormous amount of released energy will be reduced, if the radiation is beamed which is the case for this event. Using a jet break time of 7.6 days, we infer a jet opening angle of ~ 0.15 radian. This means the energy released is reduced by a factor of ~ 90 relative to the isotropic value.Comment: LaTeX file, 11 pages including 4 figures, uses psfig.sty, Bull. Astron. Society of India(accepted, Sept, 2000 issue

An efficient high-order Nystr\"om scheme for acoustic scattering by inhomogeneous penetrable media with discontinuous material interface

This text proposes a fast, rapidly convergent Nystr\"{o}m method for the solution of the Lippmann-Schwinger integral equation that mathematically models the scattering of time-harmonic acoustic waves by inhomogeneous obstacles, while allowing the material properties to jump across the interface. The method works with overlapping coordinate charts as a description of the given scatterer. In particular, it employs "partitions of unity" to simplify the implementation of high-order quadratures along with suitable changes of parametric variables to analytically resolve the singularities present in the integral operator to achieve desired accuracies in approximations. To deal with the discontinuous material interface in a high-order manner, a specialized quadrature is used in the boundary region. The approach further utilizes an FFT based strategy that uses equivalent source approximations to accelerate the evaluation of large number of interactions that arise in the approximation of the volumetric integral operator and thus achieves a reduced computational complexity of $O(N \log N)$ for an $N$-point discretization. A detailed discussion on the solution methodology along with a variety of numerical experiments to exemplify its performance in terms of both speed and accuracy are presented in this paper

Modified limb lead system: Its effects on wave amplitudes and axis in surface ECG

Modified limb lead (MLL) electrocardiogram (ECG) system may be used during rest or exercise ECG test, or in atrial activity enhancement. Due to modifications to limb electrode placement, changes are likely to happen in ECG wave amplitudes and frontal plane axis, which may alter the clinical limits of normality and ECG diagnostic criteria (1). There are also several other alterna- tive lead systems that are placed on the human torso to record and study the electrical activity of the atria (2–4). A few recent studies have proposed modification to the standard 12-lead ECG system of placing the limb electrodes closer to the atria in order to enhance atrial ECG components (5–7)