On the flow around the diamagnetic cavity of comet Halley

Abstract

The ion-neutral friction (incluchng mass loachng) happens to be insufficient to support the magnetic baxrier at comet Halley trader conventional parmneters of ion-neutral interaction. It is shown that a centrifugal force resulting front the transverse plasma motion should be added to the momentum balance in order to explain the observed magnetic field structm'e. The radial profile of the transverse plasma velocity is derived fi'om the best fit with magnetic field data. The transverse velocity reaches •8 km s -! at. 10,000 km h'om the nucleus. Tiffs value depends httle on both the ion-neutral momentum exchange rate and the comet production rate, whence one may conclude that it is centrifugal force which dominates in the momentran balas•ce at this distance. 1. INTRODUCTION A ,--4000-km radius diamagnetic cavity surrounding the comet nucleus was discovered during the Giotto flyby near comet Halley [Neubauer et al., 1986]. The magnetic signature of this cavity resembles that of the ionopause of Venus [Russell et al., 1979]. The magnetic barrier exists upstream of the cavity, and the magnetic pressure in the barrier approximately equals the dynamic pressure of the solar wind flow. The magnetic field drops drastically at the cavity boundary (which is •20 km thick) and is a. lnmst absent inside the cavity. However, the plasma signature of tl•c diamagnetic cavity of Venus strongly diffc. rs from that for cmnet Halley. The cold plasma density starts to increase rapidly at the diamagnetic cavity near Venus [Brace et al., 1979] so that this bonndary also represents the ionosphere bound-and Korosmezey, 1986; Huntress et al., 1980] and the neutral gas production rate [Krankowsky et al., 1986] result in a maximum magnetic field strength in the barrier of 25 nT, which is only • 40% of the observed value (see also Haerendel [1987, Table 1]). Second, it is rather surprising that the agreement with the observations happens to be better for less realistic assumptions of plane magnetic field geome-17,045 A.I. Ershkovich, Z.M. Ioffe, and P.L

Fine structure of the diamagnetic cavity boundary in comet Halley

“…The plasma velocity component tangential to the boundary was not measured during the Giotto flyby, but it should be non‐zero in order to fulfil the momentum balance at the boundary [ Israelevich et al , 1993]. The upper limit of the tangential velocity obtained in [ Israelevich et al , 1992] is about 8 km/s. However, the plasma velocity near the DCBL cannot be larger than ≈1 km/s.…”