Inference of the angular velocity of plasma in the Jovian magnetosphere from the sweepback of magnetic field

Particle observations from the dayside magnetosphere of Jupiter have shown that the plasma subcorotales beyond a radial distance of ≈20 R J . However, no information is available on the azimuthal velocity of plasma on the nightside of Jupiter owing to the unfavorable viewing geometries of the Voyager spacecraft. In this paper, we follow the approach of Vasyliunas (1983) to calculate the torque applied by the magnetic field on the plasma and calculate the resulting plasma angular velocity for various assumed outflow rates. Thus for a given value of the outflow rate we can calculate the sector‐averaged angular velocity of plasma from the observed values of the azimuthal component of the magnetic field ( B ϕ ) in the lobes and the normal component ( B z ) in the plasma sheet. We develop new techniques to determine the normal and azimuthal components of the magnetic field in a plasma sheet coordinate system. From the Voyager 2 data, it appears that the corotation in the postmidnight quadrant of the magnetosphere can be maintained up to a radial distance of ≈50 R J if the outflow rate in that quadrant does not exceed 2.5×10 29 amu/s. For outflow rates exceeding 2.5×10 29 amu/s in that quadrant partial corotation would result. Our conclusions differ from those of Vasyliunas because we have reanalyzed the Voyager data and have improved the estimates of B ϕ and B z . A comparison of results from Voyager 1 and 2 is presented.