Giacobini-Zinner magnetotail: ICE magnetic field observations



The ICE magnetic field observations at Comet Giacobini-Zinner have revealed the presence of a well developed magnetotail with a diameter of ∼ 10,000 km at a distance of 7,800 km downstream of the nucleus. The tail is composed of two lobes of opposite magnetic polarity separated by a ∼ 1,500 km thick plasma sheet. A ∼ 250 km diameter region of twisted, high intensity magnetic fields was observed in the plasma sheet and may correspond to a cometary “flux rope.” The magnetotail is bounded by a thin, ∼ 200 km wide magnetopause-like current layer. The field magnitude increases by a factor of 2 between the outer portions of the lobe and the central regions where peak fields of ∼ 60 nT were measured. The magnetic field in the lobes of the near-tail region is highly flared with the field making a mean angle of about 20° with the symmetry axis. Flaring is observed to be least in the high field regions of the central tail and greatest in the outer portions of the lobes where minimum variance analyses on the magnetopause crossings provided flare angles of 20-40°. The cross-tail current layer is quite broad and occupies the full width of the plasma sheet. Minimum variance analysis indicates that it was inclined to the plane of the ecliptic by 43°. This current sheet orientation is consistent with the observed direction of the IMF in the Y-Z plane during the ICE fly-by. It is concluded on the basis of these results that the ICE magnetic field observations downstream of Giacobini-Zinner have confirmed the Alfven field line draping model of type I cometary tails.