太阳风与彗星相互作用

Interactions between solar wind and comets

彗星是太阳系中一类形状不规则的小天体,主要由可挥发物质组成.在彗星接近太阳的过程中,彗核中挥发物被加热释放的中性成分在光致电离、电荷交换和电子碰撞过程后带电,并在太阳风的作用下形成射线状的等离子体彗尾.另一方面,源自彗星的中性成分带电后,将被太阳风携带,称为拾起离子.这些拾起离子多为比H+更重的水族离子(H_(2)O^(+)、O^(+)),因此,在拾起或称质量加载过程中,动量守恒导致太阳风被减速.太阳风等离子体携带的行星际磁场也将在彗核周围堆积.同时,由于太阳风速度方向和行星际磁场方向的夹角,拾起离子的速度分布呈现束流、环状分布.拾起离子与太阳风离子可能通过离子-离子不稳定性激发波动,进而贡献湍流的演化.彗星中性成分和太阳风等离子体在电荷交换过程中还将辐射X射线和极紫外辐射,为通过谱线强度分析确定太阳风的成分和状态提供了可能.研究彗星活动及其与太阳风的相互作用,对于了解彗星物质组成和结构动力学、日球层太阳风的改造演化、太阳系中有机物乃至生命起源,都有重要的意义.本文从原位探测、遥感观测、理论模拟的三个角度回顾了太阳风与彗星相互作用的研究进展,并对未来的研究发展趋势进行展望.在原位探测上,本文介绍了哈雷彗星和彗星67P/Churyumov-Gerasimenko的相关观测,比较了不同彗星活动性下,彗星与太阳风的相互作用过程.在遥感观测上,本文介绍了PSP携带的WISPR白光成像仪和SOHO/LASCO C2对低活动性彗星322P的观测.在理论模拟中,我们介绍了对前述观测的数值模拟验证工作,以及掠日彗星在近日点附近与太阳风相互作用的相关模拟工作.对于日心距离不同、气体活动性不同、探测器容易或不易到达等彗星,我们认为数值模拟、局地观测、地基及空基的遥感观测等多种手段的结合能够更有效率、更加全面地了解彗星与太阳风相互作用过程.我们认为通过相互作用的机理和效应的研究,将有助于诊断日冕及日球层太阳风的状态,认识彗星等太阳系小天体的演化历史和将来命运.

Comets are a class of small,irregularly shaped objects in a solar system consisting mainly of volatile materials.During the approach of a comet to the sun,the neutral component of the volatile matter released by heating in the nucleus becomes charged after a process of photoionization,charge exchange,and electron impact,forming a ray-like plasma comet tail.This charged component results in pick-up ions,which are mostly water group ions(H_(2)O^(+),O^(+))that are heavier than H+.Therefore,the conservation of momentum causes the solar wind to slow down during the pick-up or mass loading.As a result,the interplanetary magnetic field carried by the solar wind plasma accumulates around the comet nucleus.Simultaneously,owing to the angle between the direction of the solar wind velocity and direction of the interplanetary magnetic field,the velocity distribution of the pick-up ions has a ring-like distribution,providing free energy for wave excitation;thus,contributing to the development of turbulence.The neutral cometary component and solar wind plasma also radiate X-rays and extreme ultraviolet radiation during charge exchange,providing the possibility to determine the composition and state of the solar wind using spectral intensity analysis.The study of cometary activity and its interaction with solar wind is essential for understanding the composition and structural dynamics of cometary materials,evolution of solar wind in the heliosphere,and origin of organic matter and life in the solar system.In this study,we reviewed the research progress on solar wind-comet interactions from three perspectives:in situ detection,remote sensing observation,and theoretical simulation,and provided some prospects for future research trends.