Spectral Analysis of Non-Ideal MRI Modes: The effect of Hall diffusion

The effect of magnetic field diffusion on the stability of accretion disks is a problem that has attracted considerable interest of late. In particular, the Hall effect has the potential to bring about remarkable changes in the dynamical behavior of disks that are without parallel. In this paper, we conduct a systematic examination of the linear eigenmodes in a weakly magnetized differentially rotating gas with special focus on Hall diffusion. We first develop a geometrical representation of the eigenmodes and provide a detailed quantitative description of the polarization properties of the oscillatory modes under the combined influence of the Coriolis and Hall effects. We also analyze the effects of magnetic diffusion on the structure of the unstable modes and derive analytical expressions for the kinetic and magnetic stresses and energy densities associated with the non-ideal MRI. Our analysis explicitly demonstrates that, if the dissipative effects are relatively weak, the kinetic stresses and energies make up the dominant contribution to the total stress and energy density when the equilibrium angular momentum and magnetic field vectors are anti-parallel. This is in sharp contrast to what is observed in the case of the ideal or dissipative MRI. We conduct shearing box simulations and find very good agreement with the results derived from linear analysis. As the modes in consideration are also exact solutions of the non-linear equations, the unconventional nature of the kinetic and magnetic stresses may have significant implications for the non-linear evolution in some regions of protoplanetary disks.Comment: 16 pages, 11 figures, accepted by Ap

Analytical Models of Exoplanetary Atmospheres. V. Non-gray Thermal Structure with Coherent Scattering

We apply the picket fence treatment to model the effects brought about by spectral lines on the thermal structure of irradiated atmospheres. The lines may be due to purely absorption processes, purely coherent scattering processes or some combination of absorption and scattering. If the lines arise as a pure absorption process, the surface layers of the atmosphere are cooler whereas this surface cooling is completely absent if the lines are due to pure coherent isotropic scattering. The lines also lead to a warming of the deeper atmosphere. The warming of the deeper layers is, however, independent of the nature of line formation. Accounting for coherent isotropic scattering in the shortwave and longwave continuum results in anti-greenhouse cooling and greenhouse warming on an atmosphere-wide scale. The effects of coherent isotropic scattering in the line and continuum operate in tandem to determine the resulting thermal structure of the irradiated atmosphere.Comment: 8 pages, 5 figures, accepted for publication in Ap

Orbital stability in static axisymmetric fields

We investigate the stability of test-particle equilibrium orbits in axisymmetric, but otherwise arbitrary, gravitational and electromagnetic fields. We extend previous studies of this problem to include a toroidal magnetic field. We find that, even though the toroidal magnetic field does not alter the location of the circular orbits, it enters the problem as a gyroscopic force with the potential to provide gyroscopic stability. This is in essence similar to the situation encountered in the reduced three-body problem where rotation enables stability around the local maxima of the effective potential. Nevertheless, we show that gyroscopic stabilization by a toroidal magnetic field is impossible for axisymmetric force fields in source-free regions because in this case the effective potential does not possess any local maxima. As an example of an axisymmetric force field with sources, we consider the classical problem of a rotating, aligned magnetosphere. By analyzing the dynamics of halo and equatorial particle orbits we conclude that axisymmetric toroidal fields that are antisymmetric about the equator are unable to provide gyroscopic stabilization. On the other hand, a toroidal magnetic field that does not vanish at the equator can provide gyroscopic stabilization for positively charged particles in prograde equatorial orbits.Comment: 11 pages, 3 figures, submitted to Celestial Mechanics and Dynamical Astronom

Evaluation of hypolipidemic effect of Tinospora cordifolia in cholesterol diet induced hyperlipidemia in rats

Background: There is always a need for developing novel drugs with higher efficacy and fewer side effects. Though statins are generally well-tolerated drugs for hyperlipidemia with high efficacy they are not free from adverse effects. Herbal drugs are well known for their cost-effectiveness and minimal side effects. Tinospora cordifolia (T. cordifolia) is one such plant with known hypolipidemic activity and wide availability in India. Hence this study is an attempt to verify and evaluate the extent of efficacy of T. cordifolia as a hypolipidemic agent. The objective of the study is to compare the hypolipidemic activity of aqueous root extract of T. cordifolia with that of Rosuvastatin in cholesterol diet-induced hyperlipidemia in rats.Methods: Hyperlipidemia was induced in male albino rats of wistar strain in the first 30 days of feeding period and continued in the next 30 days of treatment period. Aqueous root extract of T. cordifolia (2.5 and 5g/kg, per oral) was administered as test drug in the treatment period. Rosuvastatin (10 mg/kg,      per oral) was used as the standard drug. Serum lipid profile, atherogenic index and body weights were estimated for all rats on the day before the start of the feeding period and on day 0, 15 and 30 of the treatment period. The results were analyzed statistically using student’s unpaired and paired t-test wherever applicable.Results: Serum lipid levels showed significant reduction (p <0.001) in TC,TG, LDL-C and VLDL-C with significant elevation (p <0.001) of HDL-C in both the rosuvastatin and test groups, but the percentage reduction in lipid levels, percentage elevation of HDL-C and percentage protection from atherosclerosis was higher in rosuvastatin group than in test groups.Conclusions: T.cordifolia has a definite hypolipidemic potential. Although its effectiveness is lesser than rosuvastatin its beneficial role as hypolipidemic agent may be tested in clinical studies

Evaluation of hypolipidemic effect of Tinospora cordifolia in cholesterol diet induced hyperlipidemia in rats

Background: There is always a need for developing novel drugs with higher efficacy and fewer side effects. Though statins are generally well-tolerated drugs for hyperlipidemia with high efficacy they are not free from adverse effects. Herbal drugs are well known for their cost-effectiveness and minimal side effects. Tinospora cordifolia (T. cordifolia) is one such plant with known hypolipidemic activity and wide availability in India. Hence this study is an attempt to verify and evaluate the extent of efficacy of T. cordifolia as a hypolipidemic agent. The objective of the study is to compare the hypolipidemic activity of aqueous root extract of T. cordifolia with that of Rosuvastatin in cholesterol diet-induced hyperlipidemia in rats.Methods: Hyperlipidemia was induced in male albino rats of wistar strain in the first 30 days of feeding period and continued in the next 30 days of treatment period. Aqueous root extract of T. cordifolia (2.5 and 5g/kg, per oral) was administered as test drug in the treatment period. Rosuvastatin (10 mg/kg,      per oral) was used as the standard drug. Serum lipid profile, atherogenic index and body weights were estimated for all rats on the day before the start of the feeding period and on day 0, 15 and 30 of the treatment period. The results were analyzed statistically using student’s unpaired and paired t-test wherever applicable.Results: Serum lipid levels showed significant reduction (p <0.001) in TC,TG, LDL-C and VLDL-C with significant elevation (p <0.001) of HDL-C in both the rosuvastatin and test groups, but the percentage reduction in lipid levels, percentage elevation of HDL-C and percentage protection from atherosclerosis was higher in rosuvastatin group than in test groups.Conclusions: T.cordifolia has a definite hypolipidemic potential. Although its effectiveness is lesser than rosuvastatin its beneficial role as hypolipidemic agent may be tested in clinical studies

Boreal summer sub-seasonal variability of the South Asian monsoon in the Met Office GloSea5 initialized coupled model

Boreal summer sub-seasonal variability in the Asian monsoon, otherwise known as the monsoon intra-seasonal oscillation (MISO), is one of the dominant modes of intraseasonal variability in the tropics, with large impacts on total monsoon rainfall and India’s agricultural production. However, our understanding of the mechanisms involved in MISO is incomplete and its simulation in various numerical models is often flawed. In this study, we focus on the objective evaluation of the fidelity of MISO simulation in the Met Office Global Seasonal forecast system version 5 (GloSea5), an initialized coupled model. We analyze a series of nine-member hindcasts from GloSea5 over 1996-2009 during the peak monsoon period (July-August) over the South-Asian monsoon domain focusing on aspects of the time-mean background state and air-sea interaction processes pertinent to MISO. Dominant modes during this period are evident in power spectrum analysis, but propagation and evolution characteristics of the MISO are not realistic. We find that simulated air-sea interactions in the central Indian Ocean are not supportive of MISO initiation in that region, likely a result of the low surface wind variance there. As a consequence, the expected near-quadrature phase relationship between SST and convection is not represented properly over the central equatorial Indian Ocean, and northward propagation from the equator is poorly simulated. This may reinforce the equatorial rainfall mean state bias in GloSea5

Johnson-Kendall-Roberts theory applied to living cells

Johnson-Kendall-Roberts (JKR) theory is an accurate model for strong adhesion energies of soft slightly deformable material. Little is known about the validity of this theory on complex systems such as living cells. We have addressed this problem using a depletion controlled cell adhesion and measured the force necessary to separate the cells with a micropipette technique. We show that the cytoskeleton can provide the cells with a 3D structure that is sufficiently elastic and has a sufficiently low deformability for JKR theory to be valid. When the cytoskeleton is disrupted, JKR theory is no longer applicable