Characteristics of Saturn's FUV aurora observed with the Space Telescope Imaging Spectrograph
Characteristics of Saturn's FUV aurora observed with the Space Telescope Imaging Spectrograph
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太空望远镜成像摄谱仪观测到的土星FUV极光特征
DOI：
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发表时间：
2004
期刊：
影响因子：
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通讯作者：
J. Trauger
中科院分区：
文献类型：
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作者：
J. Gérard;D. Grodent;J. Gustin;A. Saglam;J. Clarke;J. Trauger
[1]我们分析了1997年10月至2001年1月期间使用哈勃太空望远镜成像光谱仪（STIS）获得的一组15张FUV图像，提供了土星南极光椭圆的良好视图。结果表明，极光的形态和亮度分布是动态的，在小时或更短的时间尺度上变化，昼侧主椭圆位于70° ~ 80°之间，上午比下午亮而薄。下午部分的特点是在高纬度地区更分散的排放。弱发射也观察到向极的主椭圆到极点。一个增强的极光降水点，暂时被确定为日侧尖点的光学特征，有时会在中午部门的主椭圆的极向观察到，特别是在早晨弧没有完全发展的时期。偶尔可以观察到主椭圆的螺旋结构，在同一扇区的两个纬度上有弧形。主椭圆的亮度范围从低于1 kR的H2发射的STIS阈值到1075 kR。总的电子沉淀功率在20到140 GW之间变化，与地球上活跃的极光相当，但比木星上的极光小两个数量级。从1997年到2000-2001年的观测中，太阳的沉淀功率有增加的趋势，这可能与太阳活动的增加有关。在1200-1700 A范围内，极光中午区的6个光谱主要由莱曼-α谱线和H2 Werner和莱曼谱带的辐射组成。它们与电子激发的H2发射的合成模型的比较表明，甲烷柱在1400 A以下存在弱吸收，范围在7 × 1015和2 × 1016 cm−2之间。根据旅行者号的掩星测量，使用低纬度大气模型，估计初级极光电子的相应能量为12 ± 3 keV。主椭圆形亮度和特征电子能量与土星极光的最新模型基本一致，该模型将主椭圆形与狭窄的向上场向电流系统共存，该系统与开闭场线边界附近偏离等离子体共转相关。明亮的早晨弧的纬度略低于模型预测的等离子体流速的基础上测量的旅行者在中磁层。
[1] We analyze a set of 15 FUV images obtained between October 1997 and January 2001 with the Hubble Space Telescope Imaging Spectrograph (STIS), providing a good view of Saturn's south auroral oval. It is found that the morphology and brightness distribution of the aurora are dynamical with variations occurring on time scales of hours or less. The dayside main oval lies between 70° and 80° and is generally brighter and thinner in the morning than in the afternoon sector. The afternoon sector is characterized by more diffuse emission at higher latitudes. Weak emission is also observed poleward of the main oval up to the pole. A spot of enhanced auroral precipitation, tentatively identified as the optical signature of the dayside cusp, is sometimes observed poleward of the main oval in the noon sector, especially during periods when the morning arc is not fully developed. A spiral structure of the main oval with arcs at two latitudes in the same sector is occasionally observed. The brightness of the main oval ranges from below the STIS threshold of 1 kR of H2 emission up to ∼75 kR. The total electron precipitated power varies between 20 and 140 GW, that is, comparable to the Earth's active aurora but about two orders of magnitude less than on Jupiter. An increasing trend of the precipitated power between the 1997 and the 2000–2001 observations may be related to the rising solar activity. Six spectra of the aurora in the noon sector covering the 1200–1700 A range are dominated by emissions of the Lyman-α line and H2 Werner and Lyman bands. Their comparison with a synthetic model of electron excited H2 emissions indicates the presence of a weak absorption below 1400 A by a column of methane ranging between 7 × 1015 and 2 × 1016 cm−2. The corresponding energy of the primary auroral electrons is estimated 12 ± 3 keV, using a low-latitude model atmosphere based on Voyager occultation measurements. The main oval brightness and the characteristic electron energy are generally consistent with recent models of Saturn's aurora, which colocate the main oval with the narrow upward field-aligned current system associated with departure from plasma corotation near the open-closed field line boundary. The latitude of the bright morning arc is somewhat lower than model predictions based on the plasma flow velocity measured by Voyager in the middle magnetosphere.