Abstract
Voyager 2 observations have shown that Uranus possesses a well‐developed bipolar magnetotail similar in certain characteristics to that of Earth, in spite of an anomalously large tilt of the planetary magnetic dipole to the rotation axis at Uranus. The intensity of the magnetic field in the tail lobes decreases with increasing distance down the tail from the planet as |xSM|−0.59±0.03. This gradient is similar to that found in the Earth's tail but significantly less steep than that observed in the tails of Jupiter and Saturn. The thickness of the plasma sheet is a minimum (∼10 RU) near the tail center, increasing toward the flanks as at Earth. Pressure balance within the plasma sheet is maintained predominantly by protons and electrons with energies 10 eV to 6 keV. Except in transient events, the contribution of ≥28‐keV protons to pressure balance in the sheet is <5%. This is in contrast to the dominant role played by more energetic plasma ions in the Jovian magnetotail. An average value of β ∼ 7 was found in the plasma sheet at Uranus and ∼0.1 in the lobe plasma. The Uranian magnetic tail was observed to rotate 360° about its longitudinal axis, a result of the approximately sunward pointing planetary rotation axis at the time of encounter. This, together with the large tilt (60°) of the magnetic dipole, results in a small but measurable twist in the tail's magnetic lines of force, with a derived helical pitch of 5.5° ± 3.0°. The Byz(SM) component of the tail field can be modeled as the sum of the twist component, a radial (diverging or converging from the xSM axis) component, and a component parallel to the zSM axis that closes through the neutral sheet and is strongest there. The cross‐tail current density at the neutral sheet is estimated to be ≃3 × 10−11 A m−2. Large temporal variations observed in magnetic fields and plasmas during the Voyager 2 traverse of the magnetotail may have been produced by substorm activity.
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