Both heliophysics and planetary physics seek to understand the complex nature of the solar wind’s interaction with solar system obstacles like Earth’s magnetosphere, the ionospheres of Venus and Mars, and comets. Studies with this objective are frequently conducted with the help of single or multipoint in situ electromagnetic field and particle observations, guided by the predictions of both local and global numerical simulations, and placed in context by observations from far and extreme ultraviolet (FUV, EUV), hard X-ray, and energetic neutral atom imagers (ENA). Each proposed interaction mechanism (e.g., steady or transient magnetic reconnection, local or global magnetic reconnection, ion pick-up, or the Kelvin-Helmholtz instability) generates diagnostic plasma density structures. The significance of each mechanism to the overall interaction (as measured in terms of atmospheric/ionospheric loss at comets, Venus, and Mars or global magnetospheric/ionospheric convection at Earth) remains to be determined but can be evaluated on the basis of how often the density signatures that it generates are observed as a function of solar wind conditions. This paper reviews efforts to image the diagnostic plasma density structures in the soft (low energy, 0.1–2.0 keV) X-rays produced when high charge state solar wind ions exchange electrons with the exospheric neutrals surrounding solar system obstacles.The introduction notes that theory, local, and global simulations predict the characteristics of plasma boundaries such the bow shock and magnetopause (including location, density gradient, and motion) and regions such as the magnetosheath (including density and width) as a function of location, solar wind conditions, and the particular mechanism operating. In situ measurements confirm the existence of time- and spatial-dependent plasma density structures like the bow shock, magnetosheath, and magnetopause/ionopause at Venus, Mars, comets, and the Earth. However, in situ measurements rarely suffice to determine the global extent of these density structures or their global variation as a function of solar wind conditions, except in the form of empirical studies based on observations from many different times and solar wind conditions. Remote sensing observations provide global information about auroral ovals (FUV and hard X-ray), the terrestrial plasmasphere (EUV), and the terrestrial ring current (ENA). ENA instruments with low energy thresholds (∼1keV$\sim1~\mbox{keV}$) have recently been used to obtain important information concerning the magnetosheaths of Venus, Mars, and the Earth. Recent technological developments make these magnetosheaths valuable potential targets for high-cadence wide-field-of-view soft X-ray imagers.Section 2 describes proposed dayside interaction mechanisms, including reconnection, the Kelvin-Helmholtz instability, and other processes in greater detail with an emphasis on the plasma density structures that they generate. It focuses upon the questions that remain as yet unanswered, such as the significance of each proposed interaction mode, which can be determined from its occurrence pattern as a function of location and solar wind conditions. Section 3 outlines the physics underlying the charge exchange generation of soft X-rays. Section 4 lists the background sources (helium focusing cone, planetary, and cosmic) of soft X-rays from which the charge exchange emissions generated by solar wind exchange must be distinguished. With the help of simulations employing state-of-the-art magnetohydrodynamic models for the solar wind-magnetosphere interaction, models for Earth’s exosphere, and knowledge concerning these background emissions, Sect. 5 demonstrates that boundaries and regions such as the bow shock, magnetosheath, magnetopause, and cusps can readily be identified in images of charge exchange emissions. Section 6 reviews observations by (generally narrow) field of view (FOV) astrophysical telescopes that confirm the presence of these emissions at the intensities predicted by the simulations. Section 7 describes the design of a notional wide FOV “lobster-eye” telescope capable of imaging the global interactions and shows how it might be used to extract information concerning the global interaction of the solar wind with solar system obstacles. The conclusion outlines prospects for missions employing such wide FOV imagers.

中文翻译：

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太阳风电荷与中性物质交换产生的软 X 射线中的等离子体密度结构成像

太阳物理学和行星物理学都试图了解太阳风与太阳系障碍物（如地球磁层、金星和火星的电离层以及彗星）相互作用的复杂性质。具有此目标的研究经常在单点或多点原位电磁场和粒子观测的帮助下进行，以局部和全球数值模拟的预测为指导，并结合远紫外和极紫外（FUV、EUV）的观测结果、硬 X 射线和高能中性原子成像仪 (ENA)。每个提议的相互作用机制（例如，稳定或瞬态磁重联、局部或全局磁重联、离子拾取或开尔文-亥姆霍兹不稳定性）产生诊断等离子体密度结构。每种机制对整体相互作用的重要性（以彗星、金星和火星的大气/电离层损失或地球的全球磁层/电离层对流来衡量）仍有待确定，但可以根据它产生的密度特征被观察为太阳风条件的函数。本文回顾了在高电荷态太阳风离子与太阳系障碍物周围的外层中性粒子交换电子时产生的软（低能量，0.1-2.0 keV）X 射线中对诊断等离子体密度结构进行成像的努力。介绍指出理论、局部和全局模拟预测等离子体边界的特征，例如弓形激波和磁层顶（包括位置、密度梯度、和运动）和诸如磁鞘（包括密度和宽度）之类的区域，作为位置、太阳风条件和特定运行机制的函数。原位测量证实了与时间和空间相关的等离子体密度结构的存在，如金星、火星、彗星和地球的弓形激波、磁鞘和磁层顶/电离层顶。然而，原位测量很少足以确定这些密度结构的全球范围或其作为太阳风条件的函数的全球变化，除非以基于许多不同时间和太阳风条件的观测的经验研究的形式。遥感观测提供了关于极光椭圆（FUV 和硬 X 射线）、地球等离子体层 (EUV) 和地球环流 (ENA) 的全球信息。具有低能量阈值 (∼1keV$\sim1~\mbox{keV}$) 的 ENA 仪器最近已被用于获取有关金星、火星和地球磁鞘的重要信息。最近的技术发展使这些磁鞘成为高频率宽视场软 X 射线成像仪的有价值的潜在目标。第 2 节描述了提议的日侧相互作用机制，包括重新连接、开尔文-亥姆霍兹不稳定性和其他更详细的过程强调它们产生的等离子体密度结构。它侧重于尚未得到解答的问题，例如每个提议的交互模式的重要性，这可以从作为位置和太阳风条件的函数的发生模式确定。第 3 节概述了软 X 射线电荷交换产生的基本物理原理。第 4 节列出了软 X 射线的背景源（氦聚焦锥、行星和宇宙），必须从中区分太阳风交换产生的电荷交换发射。借助采用最先进的太阳风-磁层相互作用磁流体动力学模型、地球外逸层模型以及有关这些背景排放的知识的模拟，第 1 节。图 5 表明，可以在电荷交换发射的图像中轻松识别边界和区域，例如弓形激波、磁鞘、磁层顶和尖峰。第 6 节回顾了（通常是窄的）视场 (FOV) 天体物理望远镜的观察结果，这些望远镜确认了这些发射在模拟预测的强度下的存在。第 7 节描述了能够对全球相互作用进行成像的概念性宽视场“龙虾眼”望远镜的设计，并展示了如何使用它来提取有关太阳风与太阳系障碍物的全球相互作用的信息。结论概述了使用这种宽 FOV 成像器的任务的前景。