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Astron. Astrophys. 354, 321-327 (2000)

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1. Introduction

In-situ spacecraft measurements show the existence of a velocity shear in the solar wind that streams in the vicinity of weakly magnetized planets and comets (Romanov et al. 1979, Bame et al. 1986, Amata et al. 1991, Lundin et al. 1991, Perez-de-Tejada 1995). Since the drag on the flow due to the pick-up ions is insufficient to explain the observed velocity shear (Perez-de-Tejada 1991) the existence of a "viscous" interaction between the flow and the obstacle has been proposed (Perez-de-Tejada 1995). The microphysics involved in this process is not well understood but some models have been proposed to account for the viscous transfer of momentum through wave particle interactions (Shapiro et al. 1995). In the present study we consider the existence of velocity shears without pin-pointing the source of the inferred anomalous viscosity in collisionless plasmas.

Our aim is to analyze the motion of individual particles and determine the distribution of pick-up ions in the viscous interaction region around weakly magnetized plasma obstacles. The problem was originally addressed by Parker (1958); most recently Perez-de-Tejada and Durand-Manterola (1996 hereafter referred to as PD96) have presented an extension of the work of Parker. Numerical calculations for particular cases of this process have also been presented by Brinca (1984) and Luhmann (1987).

The analytical work of Parker (1958) and PD96 consider particular cases in which the velocity shears the flow direction and the magnetic field are perpendicular to each other. Parker obtains an expression for the path of the assimilated particles as well as an expression for their energy. For the same configuration PD96 also calculate the density profile for the case in which the source of ions is infinite and uniform in x and z directions but is confined in the [FORMULA] region. In the numerical study of Brinca (1984) and Luhmann (1987) particular cases are solved for the interaction of the solar wind with the ionosphere of Venus. We discuss here a more general case, and plausibly more realistic, in which the magnetic field has a component in the direction parallel to the flow and the source is finite in extent.

The paper is organized as follows. In Sect. 2 the basic model used for the calculations is described. The method of solution of the equations of motion is given in detail in Sect. 3, where results are also presented. Sect. 4 deals with the calculation of the ion density profile as a function of the magnetic field direction and source characteristics. In Sect. 5 we discuss the application of our results to the formation of cometary plasma tail rays and finally in Sect. 6 we summarize our results and conclusions.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000