标题： 卡西尼飞船在大结局期间与土星电离层相互作用的粒子模拟 显示英文标题

标题： Particle-In-Cell Simulations of the Cassini Spacecraft's Interaction with Saturn's Ionosphere during the Grand Finale

摘要： 卡西尼号任务末期“大结局”阶段观察到一个令人惊讶且出乎意料的现象：飞船在土星电离层中被充电至正电势。在此过程中，电离层等离子体中的自由电子耗尽，而带负电的离子和尘埃积累，占到了总负电荷密度的95%以上。 为了进一步理解飞船与等离子体的相互作用，我们对一个模拟的卡西尼号飞船模型进行了三维粒子-in-cell（PIC）研究，该模型沉浸在类似于土星电离层的等离子体环境中。 模拟结果显示了复杂的相互作用特征，例如电子翼以及包含飞船尺度涡旋的高度结构化的尾迹。 结果表明，大量负离子浓度与较大的负离子质量比至正离子质量比结合，能够驱动飞船达到观测到的正电势。尽管存在高程度的电子耗尽，但发现电子特性与磁场方向一起作为控制飞船电势的重要因素，磁场方向诱导了一个横跨卡西尼不对称结构体的电势梯度。 本研究揭示了卡西尼号在“大结局”阶段经历的整体飞船相互作用，以及这种相互作用如何受到意外出现的负离子和尘埃群体的影响。 显示英文摘要

摘要： A surprising and unexpected phenomenon observed during Cassini's Grand Finale was the spacecraft charging to positive potentials in Saturn's ionosphere. Here, the ionospheric plasma was depleted of free electrons with negatively charged ions and dust accumulating up to over 95 % of the negative charge density. To further understand the spacecraft-plasma interaction, we perform a three dimensional Particle-In-Cell study of a model Cassini spacecraft immersed in plasma representative of Saturn's ionosphere. The simulations reveal complex interaction features such as electron wings and a highly structured wake containing spacecraft-scale vortices. The results show how a large negative ion concentration combined with a large negative to positive ion mass ratio is able to drive the spacecraft to the observed positive potentials. Despite the high electron depletions, the electron properties are found as a significant controlling factor for the spacecraft potential together with the magnetic field orientation which induces a potential gradient directed across Cassini's asymmetric body. This study reveals the global spacecraft interaction experienced by Cassini during the Grand Finale and how this is influenced by the unexpected negative ion and dust populations.