Open flux in Saturn's magnetosphere
Open flux in Saturn's magnetosphere
复制标题
土星磁层中的开放通量
DOI：
10.1016/j.icarus.2013.12.004
复制
发表时间：
2014
期刊：
影响因子：
3.2
通讯作者：
Badman S
中科院分区：
文献类型：
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作者：
Badman S
我们通过评估与太阳风相连的“开放”磁通量来研究太阳风和土星磁层之间的相互作用。这是从哈勃太空望远镜拍摄的大量紫外极光图像中推断出来的，使用主极光的极向边界作为电离层开闭场线边界的代理。发现开放通量的量为10-50 GWb，平均为35 GWb。间隔10-60小时的连续观测之间的开放通量的典型变化为-5或+7 GWb。这些变化是昼侧磁层顶的开放通量产生与磁尾闭合之间不平衡的结果。5千兆瓦的典型减少开放通量是一致的原位测量的通量传输后的重连事件。估计的平均，净重联率被认为是典型的几十千伏，与一些极端的例子不平衡磁层顶或尾部重联发生在300千伏。确定的值的范围表明，土星的磁层一般不会达到一个稳定的状态之间的磁通开放在磁层顶和磁通关闭在磁尾。土星磁层中开放的磁通量的百分比与地球上测量的相似（2-11%），但在观测之间闭合的典型百分比明显较低（13%与40-70%相比）。因此，在土星的磁层中，开放通量通常在较小的（少数GWb）事件中是封闭的。这种现象的例外是与太阳风压缩有关的大的、快速的通量闭合事件。虽然通量打开和关闭的速率在长时间尺度上应该是相等的，但它们在较短的时间尺度上（长达数十小时）明显不同。磁层顶和尾部重联率的相对独立性可以归因于将开放场线运输到尾部所需的长加载时间尺度。
We characterise the interaction between the solar wind and Saturn’s magnetosphere by evaluating the amount of ‘open’magnetic flux connected to the solar wind. This is deduced from a large set of Hubble Space Telescope images of the ultraviolet aurora, using the poleward boundary of the main aurora as a proxy for the open-closed field line boundary in the ionosphere. The amount of open flux is found to be 10–50 GWb, with a mean of 35 GWb. The typical change in open flux between consecutive observations separated by 10–60 h is-5 or+ 7 GWb. These changes are a result of imbalance between open flux creation at the dayside magnetopause and its closure in the magnetotail. The 5 GWb typical decrease in open flux is consistent with in situ measurements of the flux transported following a reconnection event. Estimates of average, net reconnection rates are found to be typically a few tens of kV, with some extreme examples of unbalanced magnetopause or tail reconnection occurring at∼ 300 kV. The range of values determined suggest that Saturn’s magnetosphere does not generally achieve a steady state between flux opening at the magnetopause and flux closure in the magnetotail. The percentage of magnetic flux which is open in Saturn’s magnetosphere is similar to that measured at the Earth (2–11%), but the typical percentage that is closed between observations is significantly lower (13% compared to 40–70%). Therefore, open flux is usually closed in smaller (few GWb) events in Saturn’s magnetosphere. The exception to this behaviour is large, rapid flux closure events which are associated with solar wind compressions. While the rates of flux opening and closure should be equal over long timescales, they are evidently different on shorter (up to tens of hours) timescales. The relative independence of the magnetopause and tail reconnection rates can be attributed to the long loading timescales required to transport open field lines into the tail.