Bibhas R. De

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.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.

On the mechanism of formation of loop prominences

A mechanism is suggested for the formation of loop-type prominences in solar-active regions following flare events. The mechanism is based on the already existing idea of compression of a coronal plasma element resulting in enhanced radiation and consequent cooling of the element. A model is suggested for such a compression based on the concept of a contracting, force-free filamentary structure. If the current in a filament increases with time, then there is a radial contraction of the filament. Since the coronal plasma is frozen into the magnetic field lines of the filament, a contraction of the filament causes a compression of the filamentary plasma. This model of compression is shown to be in approximate qualitative and quantitative agreement with observations.

Special Issue Dedicated to Professor Hannes Alfvén on the Occasion of his 80th Birthday, 30 May 1988

Ten years ago, Astrophysics and Space Science dedicated its May issue to Professor Hannes Alfven on the occasion of his seventieth birthday. Time has now come to honour him on his eightieth birthday by publishing this special issue of Astrophysics and Space Science.

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.