Ionization sources in Titan's deep ionosphere
Ionization sources in Titan's deep ionosphere
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DOI：
10.1029/2009ja015100
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发表时间：
2010-07-15
影响因子：
2.8
通讯作者：
Coates, Andrew
中科院分区：
文献类型：
--
作者：
Galand, Marina;Yelle, Roger;Coates, Andrew
我们分析了多仪器的数据集，从四个土卫六遇到的卡西尼号飞船详细调查电离层的形成。该数据集包括对热层和电离层物种以及超热电子的观测。描述太阳能和电子能量沉积的模型被用作卡西尼数据集的组织元素。我们首先比较计算的二次电子产生率与超热电子强度测量推断的速率。然后，我们计算一个有效的电子解离复合系数，采用三种不同的方法卡西尼数据集。我们的发现有三个方面：（1）在深电离层（< 1200 km）阳光照射条件下推导出的有效复合系数与太阳天顶角和飞越无关。在500 K时，其值从1200 km时的6.9 x 10（-7）cm（3）s（-1）到970 km时的5.9 x 10（-6）cm（3）s（-1）。(2)当太阳的贡献足够弱时，一个额外的、次要的电离源的存在就被揭示出来了。这种非太阳源的贡献--高能电子最有可能来自磁层--对于二次电子产生率的贡献变得很明显，仅仅由于太阳照射，二次电子产生率接近或小于约3 x 10（-1）cm（-3）s（-1）。在太阳驱动的电子产生峰值（< 1050公里）以下的太阳终结点附近达到这样的阈值。(3)我们模拟深电离层电子密度的能力非常有限。我们的研究结果强调了需要更多的实验室测量的电子解离复合系数的重离子物种在高电子温度（特别是近500 K）。
We analyze a multi-instrumental data set from four Titan encounters by the Cassini spacecraft to investigate in detail the formation of the ionosphere. The data set includes observations of thermospheric and ionospheric species and suprathermal electrons. A model describing the solar and electron energy deposition is used as an organizing element of the Cassini data set. We first compare the calculated secondary electron production rates with the rates inferred from suprathermal electron intensity measurements. We then calculate an effective electron dissociative recombination coefficient, applying three different approaches to the Cassini data set. Our findings are threefold: (1) The effective recombination coefficient derived under sunlit conditions in the deep ionosphere (< 1200 km) is found to be independent of solar zenith angle and flyby. Its value ranges from 6.9 x 10(-7) cm(3) s(-1) at 1200 km to 5.9 x 10(-6) cm(3) s(-1) at 970 km at 500 K. (2) The presence of an additional, minor source of ionization is revealed when the solar contribution is weak enough. The contribution by this non-solar source-energetic electrons most probably of magnetospheric origin-becomes apparent for secondary electron production rates, due to solar illumination alone, close to or smaller than about 3 x 10(-1) cm(-3) s(-1). Such a threshold is reached near the solar terminator below the main solar-driven electron production peak (< 1050 km). (3) Our ability to model the electron density in the deep ionosphere is very limited. Our findings highlight the need for more laboratory measurements of electron dissociative recombination coefficients for heavy ion species at high electron temperatures (especially near 500 K).