| In magnetospheres of rapidly rotating planets (e.g., Jupiter, Saturn), significant effects of
planetary rotation are possible both within the polar cap (here taken to be the region of the
ionosphere where the magnetic field lines extend into the lobes of the magnetotail) and within
the magnetotail itself. Corotation of the neutral atmosphere at ionospheric altitudes tends to
drag the plasma (by collisions); the resulting plasma motion deforms the magnetic
field, producing currents that flow horizontally in the ionosphere and connect to
vertical, predominantly Birkeland (magnetic-field-aligned) currents — in one direction
over most of the polar cap, and in the opposite direction at its boundaries, closing
somewhere in the outer magnetosphere/magnetotail/magnetosheath. The J Ã B
force in the ionosphere implies a torque both on the neutral atmosphere (tending to
slow down its corotation) and on magnetic flux tubes emanating from the polar
cap. The net result is tranfer of angular momentum from the corotating neutral
atmosphere upward along the magnetic field lines; above the ionosphere, the angular
momentum flux density is essentially the Maxwell stress tensor Ãr. One possible
result is to twist the field lines of the magnetotail; this has been modeled mostly by
analogy to the Parker spiral in the solar wind, which is questionable because the ratio
magnetic/plasma energy density in the magnetotail is drastically different from
that in the solar wind. An aspect that so far does not seem to have received any
attention is that, because magnetic field lines turn from nearly vertical at the polar
cap to nearly solar-wind-aligned in the magnetotail, the angular momentum must
correspondingly turn by ~ 90°; this requires appropriate torques, which can be shown to
imply significant (mainly dawn-dusk) asymmetries in either the configuration of the
magnetotail, the distribution of currents and disturbance fields in the polar cap, or both. |