During the ICE fly-by of comet P/Giacobini-Zinner, a depletion is seen in the energetic ion intensities (E > 65 keV for "water group" ions) extending to about 40 min on either side of the time of closest approach (UT 1102 on September 11, 1985). It is demonstrated that the physical size of this hole is a few ion gyroradii (∼1.5 × 104 km). Angular analysis of the ion distributions in this "gyroradius region" can distinguish those ions whose gyromotion is entirely in one comet lobe, from those that cross the current sheet into the other lobe. In the central tail within a few minutes of closest approach, the single-lobe ions exhibit high intensities and little pitch angle dependence whereas the ions crossing the current sheet have lower intensities and are streaming along the magnetic field away from the comet. The derived density gradients are used to determine the orientation of the current sheet at closest approach, as 44° east of north.
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