Energy partitioning in the Io plasma torus
Energy partitioning in the Io plasma torus
复制标题
Io 等离子体环中的能量分配
DOI：
10.1029/ja090ia10p09469
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发表时间：
1985
影响因子：
--
通讯作者：
D. Strobel
中科院分区：
文献类型：
--
作者：
Robert A. Smith;D. Strobel
用S+、S++、S++、O+和O++分布函数的一组耦合拟线性方程描述了热环面等离子体态的局部齐次稳态模型。这些方程包含离子-离子和离子-电子碰撞的模型Fokker-Planck算符、与物种和能量无关的损失率τ−1、S、S+、S++、O和O+的碰撞电离、S+++的重组和11个电荷交换反应。唯一不受模型中包含的物理过程控制的自由参数是离子约束或停留寿命τ以及硫、nS和氧、nO的中性密度。同样，如果采用nS、nO和电子密度ne作为参数，则通过施加电荷中性来确定τ。在新生成离子的吸收机制是主要能量来源，环面处于离子对电子的碰撞能量损失和离子-电子碰撞激发离子的辐射紫外损失平衡的约束下，计算了所有离子的密度、平均能量和分布函数以及电子的温度作为输入参数的函数（假设为麦克斯韦函数）。结果表明：(1)主要物种的离子速度分布明显非麦克斯韦分布，高能尾巴延伸到拾取能量；当环面极紫外光度小于0.2 eV cm−3 S−1时，O+和/或S+离子速度分布函数存在准热核。磁芯“温度”最高为Ti ~ 100 eV；总分布的平均能量一般小于拾取能量的0.5。对于环面的典型参数，离子速度分布不驱动离子损失锥不稳定性。(2)旅行者1号与环面相遇时的极紫外光光度为~ 0.15 eV cm−3 S−1，比Shemansky和Smith（1981）报道的低~ 1.8倍。这种EUV强度的向下修正与Holberg等人（1982）在更长的波长（912-1050 A）采用的校准调整一致。(3)在旅行者1号遭遇期间，环面离子密度的近似平均为S+: <350, S++: 420, S++: 10-20；O+: 660, O++: 40-80 cm−33,Te ~ 4.8 eV， τ ~ 60天，ne ~ 2000 cm−3。中性环面平均密度为nS ~ 6和nO ~ 30 cm−3，与SO2源一致。(4) Brown et al. （1983b）对O++浓度的上限只适用于硫驱动的火山在木卫一表面发生短暂大喷发的短时间内（McEwen and Soderblom, 1983），中性环体的质量优先被硫加载。由此产生的等离子体环面包含S+和S++密度为~ 600，O+ ~ 120, O++ < 10和S++ < 40 cm−3。增加的质量负荷将τ降低至~ 1周。
The plasma state of the hot torus is studied with a local, homogeneous, steady state model described by a set of coupled quasi-linear equations for the distribution functions of S+, S++, S+++, O+, and O++. The equations contain model Fokker-Planck operators for ion-ion and ion-electron collisions, a species- and energy-independent loss rate τ−1, impact ionization of S, S+, S++, O, and O+, recombination of S+++, and 11 charge exchange reactions. The only free parameters, which are not governed by physical processes contained in the model, are the ion confinement or residence lifetime τ and the neutral densities of sulfur, nS, and oxygen, nO. Equivalently, if nS, nO, and the electron density ne are adopted as parameters, then τ is determined by imposing charge neutrality. Under the constraints that the pickup mechanism for newly created ions is the dominant energy source and that the torus is in equilibrium between collisional energy loss from ions to electrons and radiative UV loss from ions excited by ion-electron collisions, the densities, average energies, and distribution functions of all ion species and the temperature of the electrons, which are assumed to be Maxwellian, are calculated as functions of the input parameters. Among the results are the following: (1) The ion velocity distributions are significantly non-Maxwellian for the major species, with high-energy tails extending to the pickup energy. A quasi-thermal core exists for the O+ and/or S+ ion velocity distribution functions only if the EUV luminosity of the torus is less than 0.2 eV cm−3 s−1. The core “temperature” is at most Ti ∼ 100 eV; the average energy of the total distribution is generally less than 0.5 of the pickup energy. The ion velocity distributions do not drive the ion loss cone instability for parameters generally typical of the torus. (2) The EUV luminosity of the torus during the Voyager 1 encounter was ∼0.15 eV cm−3 S−1, a factor of ∼1.8 less than that reported by Shemansky and Smith (1981). This downward revision in the EUV intensities is in agreement with calibration adjustments adopted by Holberg et al. (1982) at longer wavelengths (912–1050 A). (3) During the Voyager 1 encounter the approximate average torus ion densities were S+: <350, S++: 420, S+++: 10–20; O+: 660, O++: 40–80 cm−33 with Te ∼ 4.8 eV, τ ∼ 60 days, and ne ∼ 2000 cm−3. The average neutral torus densities were nS ∼ 6 and nO ∼ 30 cm−3 and consistent with an SO2 source. (4) The upper limit obtained by Brown et al. (1983b) on O++ concentrations is only applicable to short duration periods when transient, large eruptions of sulfur-driven volcanoes occur on the surface of Io (McEwen and Soderblom, 1983) and mass loading of the neutral torus is preferentially by sulfur. The resultant plasma torus contains S+ and S++ densities of ∼600, O+ ∼120, O++ < 10, and S+++ < 40 cm−3. The increased mass loading reduces τ to ∼1 week.