A Survey of Venus Shock Crossings Dominated by Kinematic Relaxation
A Survey of Venus Shock Crossings Dominated by Kinematic Relaxation
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DOI：
10.1029/2020ja028256
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发表时间：
2020-05
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影响因子：
--
通讯作者：
S. Pope
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文献类型：
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作者：
S. Pope
无碰撞冲击是已知宇宙中最有效的粒子加速器之一。即使是低马赫数的激波也可能在粒子加热和加速方面起重要作用。理论表明，运动学无碰撞弛豫是准垂直、低马赫数和低β激波中主要的能量再分配机制，即下游非回旋离子群通过无碰撞回旋相混合而热化的过程。然而，在金星、地球和行星际空间利用现场测量对这些冲击进行的观测数量有限。本文介绍了第一次详细的研究结果，使用现场测量，这些冲击的形成的基本参数的效果。在使命期间，金星快车磁场数据确定了行星际日冕物质抛射的磁云阶段期间发生的所有低马赫数激波。从92个确定的激波交叉点中，有38个显示出明显的运动学弛豫证据。结果表明，当当地激波法线与上游磁场夹角大于50°，阿尔文马赫数小于1.4时，金星上的运动弛豫占主导地位。这些激波也可以在一系列太阳天顶角上观察到，这表明金星弓形激波上的任何位置都可能形成这样的结构。金星快车等离子体测量用于验证从磁场估计的参数，并表明重离子的重要性，包括潜在的皮卡O+。
Collisionless shocks are one of the most effective particle accelerators in the known universe. Even low Mach number shocks could have a significant role in particle heating and acceleration. Theory suggests that kinematic collisionless relaxation, the process whereby a downstream nongyroptopic ion population becomes thermalized through collisionless gyrophase mixing, is the dominant energy redistribution mechanism in quasi‐perpendicular, low Mach number, and low β shocks. However, there have only been a limited number of observations of these shocks using in situ measurements at Venus, Earth and in interplanetary space. This paper presents the results of the first detailed study using in situ measurements, of the effect of fundamental parameters on the formation of these shocks. All low Mach number shocks occurring during the magnetic cloud phase of an interplanetary coronal mass ejection are identified in Venus Express magnetic field data over the duration of the mission. From the 92 shock crossings identified, 38 show clear evidence of kinematic relaxation. It is shown that kinematic relaxation is dominant at Venus when the angle between the local shock normal and upstream magnetic field is greater 50° and the Alfvén Mach number is less than 1.4. These shocks are also observed across a range of solar‐zenith‐angles indicating that it is likely that any location on the Venus bow shock could form such a structure. Venus Express plasma measurements are used to verify the parameters estimated from the magnetic field and indicate the importance of heavy ions, including potential pickup O+.