A simple quantitative model of plasma flows and currents in Saturn's polar ionosphere
A simple quantitative model of plasma flows and currents in Saturn's polar ionosphere
复制标题
土星极地电离层中等离子体流和电流的简单定量模型
DOI：
--
复制
发表时间：
2004
期刊：
影响因子：
--
通讯作者：
J. O'Rourke
中科院分区：
文献类型：
--
作者：
S. Cowley;E. Bunce;J. O'Rourke
[1] 我们提出了土星极地电离层中发生的等离子体流和电流的简单说明性轴对称模型，这是由于内部磁层等离子体过程和太阳风相互作用造成的。该模型的特征基于先前的物理讨论，由封闭场线上的 Voyager 等离子体观测和开放场线上的遥感红外多普勒观测定量指导。随着纬度的增加，所代表的流动特征包括一个约25°共纬度的极向区域，其中由于中央磁层内部源的等离子体产生，角速度从刚性共转连续下降到约60%的刚性共转，这是一个较高但仍副共转角速度的窄带，映射到外部封闭场中的邓吉循环回流和瓦西柳纳斯循环流动区域 磁层，最后是极冠开放磁力线上的低角速度区域，大约 30% 的刚性共转。我们证明这些流动需要场对准电流的四区域模式。随着纬度的增加，这些区域由闭合场线上的向上和向下电流区域组成，峰值为每平方米几十纳安（对于 1 mho 的有效电离层 Pedersen 电导率）、穿过 100 nA m−2 量级的开闭场线边界的向上场对准电流的窄环，以及 10 nA m−2 量级的开放场线上的分布式向下电流。在向上的电流中，只有在开闭磁力线边界处的电流具有足够的强度，需要磁层电子的显着加速，导致总沉淀电子功率~0.03-0.06 TW，以及几十千雷的极光紫外线发射。后者的发射发生在大约13°余纬度的数百公里纬度环中，因此我们将其与土星的主要极光椭圆形联系起来。我们还估计了边界赤道附近的闭合场线上未加速电子沉淀的类似功率，导致分布式“漫射”紫外线发射只有千瑞利的一小部分。然而，到目前为止，极地高层大气最重要的能量输入是由于电离层佩德森电流的焦耳加热，我们估计，在约20°纬度的极地方向，通常为每平方米几毫瓦。在每个半球的开放和封闭场线上，总焦耳功率估计为~2.5 TW（电导率为~1 mho且中性大气没有滑移），因此代表土星热层的非常重要的能量输入，比全球平均太阳输入大一个数量级以上。因此，焦耳加热可能对解释为什么土星热层被观测到很热（约 400-600 K）做出了重大贡献，而仅根据太阳加热预计的热层温度低于约 200 K。
[1] We propose a simple illustrative axisymmetric model of the plasma flows and currents that occur in Saturn's polar ionosphere which are due to both internal magnetospheric plasma processes and the solar wind interaction. The features of the model are based on previous physical discussion, guided quantitatively by both Voyager plasma observations on closed field lines and remote-sensing IR Doppler observations on open field lines. With increasing latitude the flow features represented include a region poleward of ∼25° colatitude where the angular velocities decrease continuously from rigid corotation to ∼60% of rigid corotation due to plasma production from internal sources in the central magnetosphere, a narrow band of higher but still subcorotating angular velocities mapping to Dungey cycle return flow and Vasyliunas cycle flow regions in the outer closed field magnetosphere, and, finally, a region of low angular velocities, ∼30% of rigid corotation, on open field lines in the polar cap. We show that these flows require a four-region pattern of field-aligned currents. With increasing latitude, these consist of regions of upward and downward current on closed field lines peaking at a few tens of nanoamperes per square meter (for an effective ionospheric Pedersen conductivity of 1 mho), a narrow ring of upward field-aligned current across the open-closed field line boundary of order 100 nA m−2, and distributed downward currents on open field lines of order 10 nA m−2. Of the upward currents, only that at the open-closed field line boundary is of sufficient intensity to require significant acceleration of magnetospheric electrons, resulting in total precipitating electron powers of ∼0.03–0.06 TW, together with auroral UV emissions of a few tens of kilorayleighs. The latter emissions occur in a ring of a few hundred kilometers latitudinal width at ∼13° colatitude, which we thus associate with Saturn's main auroral oval. We also estimate similar powers in unaccelerated electron precipitation on closed field lines equatorward of the boundary, leading to distributed “diffuse” UV emissions of a fraction of a kilorayleigh. However, by far the most important energy input to the polar upper atmosphere is due to Joule heating by the ionospheric Pedersen currents, which we estimate as typically several milliwatts per square meter poleward of ∼20° colatitude. The overall Joule powers are estimated to be ∼2.5 TW on both open and closed field lines in each hemisphere (for a conductivity of ∼1 mho and no slippage of the neutral atmosphere), thus representing a very significant energy input to Saturn's thermosphere, more than an order of magnitude larger than the globally averaged solar input. Joule heating is thus likely to make a significant contribution to an explanation of why Saturn's thermosphere is observed to be hot, ∼400–600 K, compared with less than ∼200 K expected on the basis of solar heating alone.