Unveiling the true plasma behaviour around comets by spacecraft charging simulations

2019年，彗星拦截器被欧空局选为首个F级任务。这次任务将飞越一颗长周期彗星，并首次使用三个航天器在彗星环境中进行多点测量：A号航天器(欧空局)、B1号探测器(JAXA)和B2号探测器(欧空局)。所有航天器都将携带等离子体仪器，首次允许对彗星等离子体环境进行三维研究。然而，等离子体测量预计会受到航天器潜力的影响，正如其他彗星飞行任务已经观察到的那样(例如，Rosetta任务，Bergman等人，2020年)。在整个飞越过程中，航天器将经过几个具有不同特性的等离子体区域，因此航天器的潜力预计会有所不同。在这项研究中，我们利用粒子在单元(PIC)模拟中估计了彗星拦截器探测器B1在整个彗星掠过过程中的航天器电势，并研究了它对等离子体测量的影响。主要结果如下：(1)在较低的相对掠过速度下，哈雷型彗星内部彗发中电子的有效冷却可以减少航天器的负势。(2)中性碰撞产生的二次电子发射在大多数环境中以较高的相对掠过速度为航天器充电。

In 2019, Comet Interceptor was selected by ESA as the first F-class mission. This mission will make a flyby of a long-period comet and for the first time make multi point measurements in the cometary environment using three spacecraft: Spacecraft A (ESA), Probe B1 (JAXA) and Probe B2 (ESA). All spacecraft will carry plasma instruments, allowing, for the first time, a threedimensional study of the cometary plasma environment. The plasma measurements are, however, expected to be affected by the spacecraft potential, as already observed by other cometary missions (e.g. the Rosetta mission, Bergman et al., 2020). Throughout the flyby, the spacecraft will pass by several plasma regions with different characteristics, and hence the spacecraft potential is expected to vary. In this study, we estimated the spacecraft potential of Probe B1 of Comet Interceptor throughout the cometary flyby using Particle-In-Cell (PIC) simulations and study the influence on the plasma measurements. The main results are as follows:(1) At a low relative flyby velocity, a more effective cooling of the electrons in the inner coma results in less negative spacecraft potentials for a Halley-type comet.(2) Secondary electron emissions from neutral impacts charge the spacecraft positive in most environments at a high relative flyby velocity.