Analysis of the interaction of planetary plasma and dust with the magnetic field

卡西尼号航天器记录的最新等离子体数据表明，土星内磁层中的尘埃和等离子体的相互作用比以前认为的要复杂得多（Wahlund et al., 2009）。这些数据是在土星E环穿越时获得的，E环是一个由亚微米和微米大小的水冰颗粒（尘埃）组成的脆弱的尘埃环（见封面图）。这些颗粒是由活跃的冰卫星土卫二喷出的。来自土卫二的尘埃流由正在进行的NASA/ESA卡西尼-惠更斯土星任务上的宇宙尘埃分析仪（CDA）直接测量（Spahn et al., 2006b; Schmidt et al., 2008）。在轨道上，这些颗粒在行星引力、太阳辐射和土星系统电磁场的影响下旋转。它们还与土星内磁层中的等离子体相互作用。近几十年来，普遍接受的说法是，冕状等离子体给E环颗粒充电（Horányi et al., 1992），它们感受到感应冕状场（Birmingham and Northrop, 1979）和土星磁场的影响，而对等离子体流没有明显的反馈。然而，卡西尼·朗缪尔探测器推断出的等离子体密度（Wahlund et al., 2009）显示出电子和离子密度之间明显的不匹配（后者大多是正电荷，主要是单电荷），当航天器（垂直）穿过E环（图1，上面板）最密集部分附近时，可以看到这一点。同时，测量到的离子漂移速度（图1，下面板）介于开普勒速度（8km/s）和自转速度（25km/s）之间（相对于航天器而言为20km/s）。一个简单的解释是，大量的电子落在土星E环的尘埃颗粒上，因此它们的电荷没有被Langmuir探测器直接记录下来（Wahlund et al., 2009）。反过来，这些颗粒通过库仑阻力减慢了等离子体的流动，这就解释了为什么离子漂移速度比同质化速度慢。如果这一设想是正确的，那么该区域的相关电流将引起土星磁场的偏离，从而对E环尘埃的动力学、土星等离子体的演化以及土星磁层这部分磁场测量的解释产生影响。初步估计表明，大量的小E环颗粒是解释观察到的顺序的电荷不匹配所必需的，小到几十纳米到100纳米之间。该估计采用了陡峭的晶粒尺寸分布（Schmidt等人，2008），以及在E环区域约为-3V的给定电位下适当的与尺寸相关的平衡电荷（Kempf等人，2006）。这很有趣，因为卡西尼等离子体光谱仪（CAPS）的测量结果也暗示，土卫二的羽流中直接产生了大量这种大小的颗粒（Jones et al., 2009）。因此，两个独立的测量结果表明存在一群小的E环颗粒。否则，通过卡西尼CDA的原位测量或使用成像子系统（ISS）或视觉和红外测绘光谱仪（VIMS）获得的图像对E环进行光度测定，很难限制这个颗粒尺寸范围。在该项目中，我们计划扩展现有的模型，以估计E环尘埃的构型到10纳米的粒度，并估计尘埃和等离子体的相互作用，从而推导出等离子体盘中相关的方位电流和土星磁场的so诱导扰动的模型。

Newest plasma data recorded by the Cassini spacecraft suggest that dust and plasma in Saturn s inner magnetosphere interact in a more complex manner than previously thought (Wahlund et al., 2009). The data was obtained during crossings of Saturn s E ring, a tenuous dust ring that is formed by submicron and micron sized grains (dust) of water ice (see cover figure). These grains are ejected by the active ice-moon Enceladus. The dust streams emerging from Enceladus were directly measured by the Cosmic Dust Analyzer (CDA) onboard the ongoing NASA/ESA Cassini-Huygens mission to Saturn (Spahn et al., 2006b; Schmidt et al., 2008). In orbit, the grains revolve under the influence of the planet s gravity, solar radiation, and the electromagnetic field in the Saturn system. They also interact with the plasma in Saturn s inner magnetosphere. The commonly accepted picture in the last decades was that corotational plasma charges the E ring grains (Horányi et al., 1992), which feel the effects of the induced corotational field (Birmingham and Northrop, 1979) and Saturn s magnetic field, while there is no significant feedback on the plasma flow. However, the plasma densities inferred by the Cassini Langmuir probe (Wahlund et al., 2009) show a clear misfit between the densities of electrons and ions (the latter are mostly positive and mainly single charged), which is seen when the spacecraft (vertically) crosses the E ring (Fig. 1, upper panel) near its densest part. Simultaneously, the measured ion drift speeds (Fig. 1, lower panel) are intermediate (20km/s, relative to the spacecraft) between Kepler speed (8km/s) and corotation (25km/s). A simple explanation is that a significant amount of the electrons rests on dust grains in Saturn s E ring, so that their charge is not directly registered by the Langmuir probe (Wahlund et al., 2009). In turn, the grains slow down the plasma flow via coulomb drag, offering an explanation for ion drift speeds slower than corotation. If this scenario is correct then the related currents in this region will induce deviations from the Saturnian magnetic field bearing consequences for the dynamics of the E ring dust, the evolution of Saturnian plasma, and for the interpretation of magnetic field measurements in this part of Saturn s magnetosphere. Preliminary estimates show that a large amount of small E ring grains is necessary to explain a charge misfit of the observed order, down to sizes between tens of nanometers and one hundred nanometers. This estimate employs a steep grain size distribution (Schmidt et al., 2008), together with the appropriate size-dependent equilibrium charges for a given potential in the E ring region of about -3V (Kempf et al., 2006). This is interesting, since also measurements by the Cassini Plasma Spectrometer (CAPS) instrument hint at the abundant production of grains of this size directly in the plume of Enceladus (Jones et al., 2009). Thus, two independent measurements point to an existence of a population of small E ring grains. Otherwise, this range of particle sizes is hard to constrain by in situ measurements by Cassini CDA or by photometry of the E ring, using images obtained by the Imaging Subsystem (ISS) or the Visual and Infrared Mapping Spectrometer (VIMS). In the proposed project we plan to extend existing models to estimate the configuration of E ring dust down to grain sizes of 10 nm, and estimate the interaction of dust and plasma, in order to derive a model for the related azimuthal currents in the plasma disk and for the so induced perturbations of the Saturnian magnetic field.