The mechanical advantage of the magnetosphere: solar-wind-related forces in the magnetosphere-ionosphere-Earth system
The mechanical advantage of the magnetosphere: solar-wind-related forces in the magnetosphere-ionosphere-Earth system
复制标题
磁层的机械优势：磁层-电离层-地球系统中与太阳风相关的力
DOI：
10.5194/angeo-25-255-2007
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发表时间：
2007
影响因子：
1.9
通讯作者：
V. Vasyliūnas
中科院分区：
文献类型：
--
作者：
V. Vasyliūnas
抽象的。磁层-电离层相互作用涉及在两个区域之间流通的电流;在两个区域中作为匹配的相反对存在的相关洛伦兹力通常被视为主要机制，最终来自太阳风流动的线性动量通过该机制从磁层转移到电离层，在电离层中通过碰撞进一步转移到中性大气。然而，对于一个给定的电流总量，总的力与Δ B成比例，并且一般来说，由于Δ 2B~通量守恒常数，在电离层中比在磁层中大得多（Δ B =有效长度，B =磁场）。磁层可以被描述为具有机械优势：磁层中的洛伦兹力与电离层中的洛伦兹力耦合，电离层中的洛伦兹力被放大了一个因子，该因子大约由磁场幅度比的平方根给出（在连接到外磁层的磁力线上，约为20到40）。因此，由于磁层施加的磁应力而传递到电离层（并从电离层传递到大气层）的线性动量可能比太阳风通过切向应力提供的动量大得多。增加的线性动量来自地球内部，由洛伦兹力作用于电离层磁场扰动产生的电流（特别是地球内部屏蔽电流，阻止随时间变化的外部磁场）。这立刻意味着福岛关于地面磁扰动消失的定理不能完全适用，这一结论是通过重新检查导出该定理的假设而得到证实的。为了平衡地球内部的洛仑兹力，地球内部必须存在一个反太阳的机械应力，其中只有一小部分是整个地球系统被太阳风施加的净力加速度。因此，太阳风的相互作用可以在电离层和中性大气层之间以及地球内部载流区域内产生内力，这种内力比太阳风本身施加的力大得多。
Abstract. Magnetosphere-ionosphere interactions involve electric currents that circulate 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 further transferred by collisions to the neutral atmosphere. For a given total amount of current, however, the total force is proportional to ℒB 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 described as possesing a mechanical advantage: the Lorentz force in it is coupled with a Lorentz force in the ionosphere that has been amplified by a factor given 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 the atmosphere) as the result of magnetic stresses applied by the magnetosphere can thus be much larger than the momentum supplied by the solar wind through tangential stress. The added linear momentum comes from within the Earth, extracted by the Lorentz force on currents that arise as a consequence of magnetic perturbation fields from the ionosphere (specifically, the shielding currents within the Earth that keep out the time-varying external fields). This implies at once that Fukushima's theorem on the vanishing of ground-level magnetic perturbations cannot be fully applicable, a conclusion confirmed by re-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 small part of which is the acceleration of the entire Earth system by the net force exerted on it by the solar wind. The solar-wind interaction can thus give rise to internal forces, significantly larger than the force exerted by the solar wind itself, between the ionosphere and the neutral atmosphere as well as within the current-carrying regions of the Earth's interior.