The mechanical advantage of the magnetosphere: solar-wind-related forces in the magnetosphere-ionosphere-Earth system

摘要

Magnetosphere-ionosphere interactions involve electric currents thatcirculate between the two regions; the associated Lorentz forces,existing in both regions as matched opposite pairs,are generally viewed as the primary mechanism by which linear momentum,derived ultimately from solar wind flow,is transferred from the magnetosphere to the ionosphere, where it is furthertransferred by collisions to the neutral atmosphere. For a given totalamount of current, however, the total force is proportional toℒB and in general,since ℒ2B~ constant by flux conservation,is much larger in the ionosphere than in the magnetosphere(ℒ = effective length, B = magnetic field).The magnetosphere may be describedas possesing a mechanical advantage: the Lorentz force in it is coupledwith a Lorentz force in the ionosphere that has been amplified by a factorgiven approximately by the square root of magnetic field magnitude ratio(~20 to 40 on field lines connected to the outer magnetosphere).The linear momentum transferred to the ionosphere (and thence to theatmosphere) as the result of magnetic stresses applied by the magnetospherecan thus be much larger than the momentum supplied by the solar wind throughtangential stress. The added linear momentum comes from within the Earth,extracted by the Lorentz force on currents that ariseas a consequence of magnetic perturbation fields from the ionosphere(specifically, the shieldingcurrents within the Earth that keep out the time-varying external fields).This implies at once that Fukushima's theorem on the vanishing of ground-levelmagnetic perturbations cannot be fully applicable, a conclusion confirmed byre-examining the assumptions from which the theorem is derived.To balance the inferred Lorentz force within the Earth's interior,there must exist an antisunward mechanical stress there, only a smallpart of which is the acceleration of the entire Earth system by the net forceexerted on it by the solar wind. The solar-wind interaction can thusgive rise to internal forces, significantly larger than the forceexerted by the solar wind itself, between the ionosphere and the neutralatmosphere as well as within the current-carrying regions of the Earth'sinterior.