The Giacobini‐Zinner magnetotail: Tail configuration and current sheet

Abstract

The high‐resolution plasma electron and magnetic field data sets from the ICE tail traversal of comet Giacobini‐Zinner have been combined to make a detailed study of the draped Giacobini‐Zinner magnetotail in general, and its field‐reversing current sheet in particular. The geometry of the magnetotail at the time of the ICE crossing is determined and is shown to be consistent with a circular tail cross section rotated 10.5° in the normal sense of aberration and 9.9° above the ecliptic plane, bisected by a cross‐tail current sheet which is rotated 43° out of the ecliptic about the solar wind velocity vector. MHD continuity, momentum, and energy equations are combined with the plasma and field observations to determine unmeasured plasma properties at ICE and upstream at the average point along each streamline where the cometary ions are picked up. The ion temperature, beta, and flow speed at ICE range from 1–1.5 × 106 K, 1–4, and −20 to −30 km s−1, respectively, in the draped lobes to ∼1.2 × 105 K, up to ∼40, and ∼−20 km s−1 in the current sheet. Upstream at the average pickup locations, the flow velocity, ion temperature, density, and ion source rates range from ∼−75 km s−1, ∼4 × 106 K, ∼20 cm−3, and ∼1.5 cm−3 s−1 in the regions upstream from the lobes to ∼ −12 km s−1, ∼1 × 105 K, 200–600 cm−3, and ∼3.6 cm−3 s−1 in the prime mass‐loading region upstream from the current sheet. Gradients in the plasma properties at the edges of this region are quite strong, and the diameter of the region is ∼1500 km, in good agreement with the expected size scale of the cometary ionopause; implications of our inferred plasma properties in this region are examined. The derived and measured flow velocity and measured plasma density at ICE are combined to calculate the transport rate of ions past the plane of the ICE encounter, and we calculate that ∼2.5–5 × 1028 ions per second (neutrals per second in the steady state) are produced by the comet with only ∼1% of these traveling down the magnetotail.