Modulation of Jupiter's plasma flow, polar currents, and auroral precipitation by solar wind-induced compressions and expansions of the magnetosphere: a simple theoretical model
Modulation of Jupiter's plasma flow, polar currents, and auroral precipitation by solar wind-induced compressions and expansions of the magnetosphere: a simple theoretical model
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太阳风引起的磁层压缩和膨胀对木星等离子体流、极流和极光降水的调节：一个简单的理论模型
DOI：
10.5194/angeo-25-1433-2007
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发表时间：
2007
影响因子：
1.9
通讯作者：
D. Andrews
中科院分区：
文献类型：
--
作者：
S. Cowley;J. Nichols;D. Andrews
抽象的。我们构建了木星磁层中等离子体流、磁层-电离层耦合电流和极光降水的简单模型，并研究它们如何响应太阳风动态压力变化引起的系统压缩和膨胀。主要的简化假设是轴对称，系统建模主要是为了反映白天条件。因此，该模型描述了三个磁层区域，即以磁层顶为界的闭合磁力线上的中磁层和外磁层，以及映射到尾部的开放磁力线区域。计算假设系统最初处于具有特定赤道磁层顶半径的成因等离子体稳定扩散流出的状态，并且磁层顶随后由于太阳风动态压力的变化而快速移入或移出。如果变化足够快（~2-3小时或更短），则等离子体角动量在偏移期间守恒，从而可以根据磁力线的径向位移以及修正的磁层-电离层耦合电流和极光降水来计算修正的等离子体角速度。将这些瞬态的特性与它们在大约 1-2 天的时间间隔内恢复到的稳态的特性进行比较。显示了系统从典型的太阳风稀薄区域的初始膨胀状态快速压缩的结果，说明了适度压缩时发生的总沉淀电子功率的减少，然后在紧急稳定状态下部分恢复。然而，对于主要压缩，典型的太阳风压缩区域的开始，会发生明亮的瞬态，其中在闭合磁力线上感应出超旋转，导致通常的磁层-电离层耦合电流系统意义上的反转。目前的系统反转导致极光电子沉淀加速发生在外磁层区域，而不是像通常那样发生在中磁层，峰值能量通量发生在外磁层和中磁层边界的极地方向。然后，当稳态条件重新出现时，等离子体副自转重新建立，连同闭合场电流系统的通常感觉以及中磁层中更新但减弱的加速电子沉淀。还显示了系统从典型的太阳风压缩区域的初始压缩状态快速膨胀的结果，说明了瞬态中发生的沉淀电子功率的增强，随后随着稳定条件重新出现而部分减少。
Abstract. We construct a simple model of the plasma flow, magnetosphere-ionosphere coupling currents, and auroral precipitation in Jupiter's magnetosphere, and examine how they respond to compressions and expansions of the system induced by changes in solar wind dynamic pressure. The main simplifying assumption is axi-symmetry, the system being modelled principally to reflect dayside conditions. The model thus describes three magnetospheric regions, namely the middle and outer magnetosphere on closed magnetic field lines bounded by the magnetopause, together with a region of open field lines mapping to the tail. The calculations assume that the system is initially in a state of steady diffusive outflow of iogenic plasma with a particular equatorial magnetopause radius, and that the magnetopause then moves rapidly in or out due to a change in the solar wind dynamic pressure. If the change is sufficiently rapid (~2–3 h or less) the plasma angular momentum is conserved during the excursion, allowing the modified plasma angular velocity to be calculated from the radial displacement of the field lines, together with the modified magnetosphere-ionosphere coupling currents and auroral precipitation. The properties of these transient states are compared with those of the steady states to which they revert over intervals of ~1–2 days. Results are shown for rapid compressions of the system from an initially expanded state typical of a solar wind rarefaction region, illustrating the reduction in total precipitating electron power that occurs for modest compressions, followed by partial recovery in the emergent steady state. For major compressions, however, typical of the onset of a solar wind compression region, a brightened transient state occurs in which super-rotation is induced on closed field lines, resulting in a reversal in sense of the usual magnetosphere-ionosphere coupling current system. Current system reversal results in accelerated auroral electron precipitation occurring in the outer magnetosphere region rather than in the middle magnetosphere as is usual, with peak energy fluxes occurring just poleward of the boundary between the outer and middle magnetosphere. Plasma sub-corotation is then re-established as steady-state conditions re-emerge, together with the usual sense of flow of the closed field current system and renewed but weakened accelerated electron precipitation in the middle magnetosphere. Results for rapid expansions of the system from an initially compressed state typical of a solar wind compression region are also shown, illustrating the enhancement in precipitating electron power that occurs in the transient state, followed by partial reduction as steady conditions re-emerge.