A unidimensional model of comet ionosphere structure

Abstract

We have considered the one‐dimensional continuity and momentum equations for the plasma created in the expanding coma of a comet near the Sun. For an inverse square dependence of neutral density we have solved the continuity equation in the presence of radial plasma flow. For distances large compared to the size of the nucleus, our distribution falls off as r−1 with a velocity dependent coefficient which reduces to the photochemical steady state case in the limit of vanishing radial plasma outflow. This density distribution is used in the treatment of the momentum equation from which an analytic expression for the magnetic field configuration is derived in the presence of outflow, photoionization, dissociative recombination, plasma fluid pressure, and friction between the ions and neutrals. The magnetic field thus derived can be shown to have a maximum value which we fix to be attained at the point corresponding to the point shown by the Giotto observations and find that for a Halley type comet there will be a region sunward of the nucleus from which the magnetic field is excluded, primarily by the effect of ion neutral collisions, which is consistent with the Giotto observations. We have performed the calculations for both Halley and Giacobini‐Zinner type comets. In the field‐free region we find that the dominant terms in the momentum equation balancing the magnetic pressure gradient are the ion neutral friction and the net mass loading momentum gain. For large distances the field reduces to a constant field. The scale size of field diffusion as a result of finite conductivity, brought about by electron neutral collisions, is found to be vanishingly small.