Fine structure of the diamagnetic cavity boundary in comet Halley

We calculated the electric current density in the diamagnetic cavity boundary layer (DCBL) using magnetic field data obtained during the Giotto mission to comet Halley. This current possesses both the component perpendicular to the local magnetic field and the parallel component. The perpendicular current is responsible for the screening of the diamagnetic cavity boundary from the field in the magnetic barrier. This current is supported by the electric field tangential to the boundary. The behavior of the parallel electric current component resembles the Alfvén wings which arise due to the interaction of the magnetized plasma flow with a conducting obstacle. However, the electric current connecting the two wings does not flow through the whole volume of the obstacle. On the contrary, the wings are connected by the perpendicular current in the DCBL. In the inner portion of the DCBL this current diverges producing the parallel current component whose direction is opposite to that in the outer portion of the DCBL. In order to support such a current system at the diamagnetic cavity boundary, the potential drop across DCBL should be as small as 0.5V. Such a small value of the potential drop agrees with the penetration depth of cometary ions from the cavity to the magnetic barrier: it is equal to the ion gyroradius and therefore is not affected by any significant electric field normal to the diamagnetic cavity boundary.