A comment on the damping of magneto-hydrodynamic waves

It is shown that a revision is necessary in the commonly used expression for the damping scalelength of a magnetohydrodynamic wave in a plasma when the plasma exhibits anisotropic conductivity. The consequence of this revision may be quite drastic in some astrophysical situations.     

Lava flow transition in pāhoehoe-dominated lower pile of deccan traps from Manmad-Chandwad area,western Maharashtra

Morphological evolution of classical pāhoehoe to transitional flow-type in a compound basalt flow occurring in the lower pile of Deccan traps is described here from the southern slope of Mesan Dongar in Manmad-Chandwad area, western part of Maharashtra plateau, India. The transitional flow lobes, largely identified as rubbly pāhoehoe-type, are characterized by a crust of breccias, with thickness ranging from 10 to 20 m. The breccia is generally a welded one and consists of mm to m-size, angular to sub-angular basaltic clasts that vary in vesiculation (vesicle size, shape and population). The crust, at times, is supported by an inner vesicular zone coherently developed above a persistent core that varies in thickness from 10 to 15 m. Base of these lobes is smooth, sparsely vesicular, without any basal breccia.Many of the transitional lobes of Mesan Dongar, however, shared a few common morphological traits with slabby pāhoehoe and this inherent uncertainty in classifying them as rubbly pāhoehoe has been greatly reduced using the semi-quantitative tool of Keszthelyi (2002). Moreover, it is also proposed that this morphological evolution of flow lobes in Mesan Dongar represents a NW-SE trending, southerly propagating and at least four kilometre long flow-front of pāhoehoe lava. Identification of flow fronts is important, even in this ~65 Ma old, dissected Deccan flood basalt province, as these can reveal vital information on flow dynamics, slope and cooling histories of individual flows/lobes.     

EQUILIBRIUM CONDENSATION IN A SOLAR NEBULA

In attempts to reconstruct the environment of condensation of solar system materials, particularly exemplified by certain meteorite components, the relative temperatures of the gas and the solid are of critical importance. The relationships that determine the heat balance in a circumsolar grain-gas system are examined. Fundamental considerations show that regardless of opacity or gas density, the gas will always be at a higher temperature than the solid in such regions of the system where condensation is possible. Implications of the characteristic temperature differential between the gas and the condensing solid are discussed.     

Electric potential on solid spheres in a plasma

We derive the general expression for the potential on a solid sphere immersed in a plasma, showing the dependence of the potential on the radiusa of the sphere ands of the plasma sheath that develops around the sphere. In the limit where the radiusa is much larger than the sheath thicknesss-a we recover from this expression the well-known result for the potential on an infinite wall in contact with a plasma. On the other extreme wheres is much larger thana, we get the result derived by Spitzer (1941) for the potential on spherical grains in the interstellar plasma. Since the surface of the sphere forms a sink for the charged particles, there is a net drift of the plasma towards the surface. The effect of this drift on the potential is examined. Finally, for very small metallic spheres, an effect leading to a revision of the potential is discussed. This effect consists in a lowering of the potential barrier for the electrons due to the image force. The various effects limiting the potential on spheres are discussed.     

Lava channel of Khedrai Dam,northeast of nasik in western Deccan Volcanic province: Detailed morphology and evidences of channel reactivation

The 120 m-wide Khedrai Dam lava channel is one of the widest recorded in the Deccan Volcanic Province. Like many recent ones, this channel also has a pair of linear marginal levees made of breccia and confines the channelfill lava. It is resting over a pahoehoe flow (substrate) of basalt studded with plagioclase megacrysts, also known as the Giant Plagioclase Basalt (GPB). The channel-fill lava, which makes the lining at the edges and basal portion of the channel, is a comb-layered basalt (Cl-B) characterized by alternating layers rich in curved, branching augite crystals and laths of plagioclase — a texture indicative of high order of supercooling and rapid cooling rates. The dominant and centrally filling lava of this channel is a plagioclase-phyric basalt (Pp-B) that has a wide variation in phenocryst number from megacryst-poor to megacryst-rich. Occurrence of centimetre to decimetre size angular fragments of Cl-B within Pp-B fill indicates mechanical dislodgements of channel-lining by flowing lava. This also suggests that the channel was reactivated before it finally lost its hydraulic gradient and solidified.