Magnetospheric energization by interaction between planetary spin and the solar wind

Abstract

If the solar wind is capable of driving magnetospheric convection, then solar-wind flow past any spinning, magnetized planet with a conducting ionosphere must cause the magnetic field lines in the outer l•aXt of its magnetospheric tail to be twisted into a helix. Such a magnetic field configuration requires magnetically field-aligned (Birkeland) currents in the tail that flow in and near the magnetopa. use• and close by driving Pedersen currents through the planetary ionosphere. The strength of the Birkeland currents (and, by current continuity, the Pedersen Currents) is, to first order, independent of the angle between the planetary-spin vector and the solar-wind velocity vector. Rather, the total current is a function of the magnetic moment of the planet, the radius of the tail, the angular velocity of planetary spin, the conductivity of the ionosphere, and the solar wind speed. For Jupiter, Saturn, Uranus, and perhaps Neptune, the power these currents deliver to the ionosphere is significant with regard to magnetospheric dynamics such as the production of aurora and the generation of low-frequency radio emissions. For Mercury, Venus, Earth, Mars, and probably Pluto, these currents are rela•tively small, although observable effects may be marginally detectable for the case of the Earth's magnetosphere. Jovian magnetotail from plasma wave observations, Geophys.A Faxaday disk dynamo mechanism, Planet. Space $ci., 31, 1187, 1983b. C. F., Magnetospheres of the planets, Space Sci. Rev., 14, 511, 1973. Lyons, L. R., Discrete aurora as the direct result of an inferred high-altitude generating potential distribution, J. Geophys. Res., 86, 1, 1981. Siscoe, G. L., Two magnetic tail models for Uranus, Planet. Space Sci., 19, 483, 1971. Stern, D. P., The electric field and global electrodynamics of the magnetosphere, Rev. Geophys. Space Phys., 17, 626, 1979. Vasyliunas, V. M., Plasma distribution and flow, in Physics of the Jovian Magnetosphere, edited by A. Electron precipitation and related aeronomy of the Jovian thermosphere and ionosphere, J. Geophys. Res., 88, 6143, 1983. Kennel,

Saturn's polar ionospheric flows and their relation to the main auroral oval

“…This problem has been considered by Isbell et al (1984), who, given some assumptions of simplified geometry, showed that in the steady state, the ionospheric plasma sub-corotates with angular velocity…”

“…Connerney et al, 1983;Atreya et al, 1984;Cheng and Waite, 1988). Isbell et al (1984) themselves adopted a conductivity of 10 mho, yielding a value for the dimensionless quantity µ 0 * P V SW ≈6.3 for a solar wind speed of ∼500 km s −1 . Since this value is significantly larger than unity, the angular velocity given by Eq.…”

Field‐aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations

“…(We note that the spin and planetary magnetic axes are co-aligned at Saturn to within~0.1° [Burton et al, 2010].) In the closed field region the field lines are consequently bent out of meridian planes into a "lagging" field configuration, while in the open field region the field becomes twisted within each tail lobe [Isbell et al, 1984]. We note, however, that should some process cause the plasma to rotate faster than the neutral atmosphere, the sense of the current system and magnetic field perturbations would reverse, producing in that case a "leading" field configuration on closed field lines.…”

Comment on “Magnetic field properties of Jupiter's tail at distances from 80 to 7500 Jovian Radii” by M. L. Goldstein, R. P. Lepping, and E. C. Sittler, Jr.

“…We take no issue here either with the Voyager data or with the rather ambitious analysis and interpretation thereof by Goldstein et al [1985], except insofar as their interpretation challenges our previous assumptions [Hill et al, 1983;Isbell et al, 1984] with respect to the proper interpretation of (la) above. We note, however, that their analysis yields a spiral angle that is inconsistent with the arguments we have presented.…”

“…An upper limit on • is obtained in the former case because ionospheric slippage reduces the effective value of fl in (la). The slippage rate depends in a predictable way on the ionospheric conductance [Hill et al, 1983;Isbell et al, 1984], but the value of Jupiter's ionospheric conductance is not well established. The upper limit of 65 ø corresponds to a conductance of 5 mhos, as might be expected in the presence of auroral ionization enhancement [Isbell et al, 1984]; in the absence of auroral enhancement the conductance may be as low as 0.1 mho [Strobel and Atreya, 1983], resulting in a pitch angle • • 10 ø.…”