Ion dynamics and the magnetorotational instability in weakly-ionized discs

The magnetorotational instability (MRI) of a weakly ionized, differentially rotating, magnetized plasma disk is investigated in the multi-fluid framework. The disk is threaded by a uniform vertical magnetic field and charge is carried by electrons and ions only. The inclusion of ion dynamics causes significant modification to the conductivity tensor in a weakly ionized disk. The parallel, Pedersen and Hall component of conductivity tensor become time dependent quantities resulting in the AC and DC part of the conductivity. The conductivity may change sign leading to the significant modification of the parameter window in which MRI may operate. The effect of ambipolar and Hall diffusion on the linear growth of the MRI is examined in the presence of time dependent conductivity tensor. We find that the growth rate in ambipolar regime can become somewhat larger than the rotational frequency, especially when the departure from ideal MHD is significant. Further, the instability operates on large scale lengths. This has important implication for the angular momentum transport in the disk.Comment: 13 pages, 12 figure

Some unexplored features of the nonlinear compressive magnetoacoustic Alfvenic waves

The theory of nonlinear magnetoacoustic wave in the past has strictly been focused on purely compressive features of the mode. We show that a complete set of nonlinear equations necessarily includes both compressional and shear components of the magnetic field. These two turn out to be described by exactly the same nonlinear equations, which make the use of such a complete full set of equations far less complicated than expected. Present results should considerably enrich the theory of these waves by opening up new frontiers of investigation and providing some completely new types of nonlinear solutions.Comment: Phys. Scripta, to be publishe

Hall instability of solar flux tubes

The magnetic network which consists of vertical flux tubes located in intergranular lanes is dominated by Hall drift in the photosphere-lower chromosphere region ($\lesssim 1 Mm$). In the internetwork regions, Hall drift dominates above $0.25 Mm$ in the photosphere and below $2.5 Mm$ in the chromosphere. Although Hall drift does not cause any dissipation in the ambient plasma, it can destabilise the flux tubes and magnetic elements in the presence of azimuthal shear flow. The physical mechanism of this instability is quite simple: the shear flow twists the radial magnetic field and generates azimuthal field; torsional oscillations of the azimuthal field in turn generates the radial field completing feedback loop. The maximum growth rate of Hall instability is proportional to the absolute value of the shear gradient and is dependent on the ambient diffusivity. The diffusivity also determines the most unstable wavelength which is smaller for weaker fields. We apply the result of local stability analysis to the network and internetwork magnetic elements and show that the maximum growth rate for kilogauss field occurs around $0.5 Mm$ and decreases with increasing altitude. However, for a $120 G$ field, the maximum growth rate remains almost constant in the entire photosphere-lower chromosphere except in a small region of lower photosphere. For shear flow gradient $\sim 0.01 s^{-1}$, the Hall growth time is 10 minute near the footpoint. Therefore, network fields are likely to be unstable in the photosphere, whereas internetwork fields could be unstable in the entire photosphere-chromosphere. Thus the Hall instability can play an important role in generating low frequency turbulence which can heat the chromosphere.Comment: 8 page, 4 figure

Galactic R Coronae Borealis Stars: The C-2 Swan Bands, The Carbon Problem, And The C-12/C-13 Ratio

Observed spectra of R Coronae Borealis (RCB) and hydrogen-deficient carbon (HdC) stars are analyzed by synthesizing the C-2 Swan bands (1, 0), (0, 0), and (0, 1) using our detailed line list and the Uppsala model atmospheres. The (0, 1) and (0, 0) C-2 bands are used to derive the C-12 abundance, and the (1, 0) (CC)-C-12-C-13 band to determine the C-12/C-13 ratios. The carbon abundance derived from the C-2 Swan bands is about the same for the adopted models constructed with different carbon abundances over the range 8.5 (C/He = 0.1%) to 10.5 (C/He = 10%). Carbon abundances derived from C I lines are about a factor of four lower than the carbon abundance of the adopted model atmosphere over the same C/He interval, as reported by Asplund et al., who dubbed the mismatch between adopted and derived C abundance as the "carbon problem." In principle, the carbon abundances obtained from C-2 Swan bands and that assumed for the model atmosphere can be equated for a particular choice of C/He that varies from star to star. Then, the carbon problem for C-2 bands is eliminated. However, such C/He ratios are in general less than those of the extreme helium stars, the seemingly natural relatives to the RCB and HdC stars. A more likely solution to the C-2 carbon problem may lie in a modification of the model atmosphere's temperature structure. The derived carbon abundances and the C-12/C-13 ratios are discussed in light of the double degenerate and the final flash scenarios.Robert A. Welch Foundation of Houston, TX F-634McDonald Observator

Economic potential of forest resources of Nepal

Nepal's forest resources underpin the livelihoods of rural people in important ways. During the country's 'planned development' over the past 50 years, the government, donors and policy makers have viewed these resources as a key vehicle for ushering in economic growth and for meeting basic needs. They underscore the potential value of forest resources for achieving conservation and socio-economic objectives. To what extent have economic incentives been generated to effectively harness these resources to meet the said objectives is an open question. To address this question, this paper reviews briefly and broadly the economic potential of the country's forest resources in terms of forest goods and services. Estimates of economic potential of timber and non-timber forest products and environmental services have been assessed. A number of recommendations for realizing the potential for achieving development and poverty reduction objectives is provided