极端ICME冲压压力将水星向阳面Magnetosphere压缩到表面的观测
Observations of Extreme ICME Ram Pressure Compressing Mercury's Dayside Magnetosphere to the Surface
摘要：
众所周知，水星的磁层会受到行星际日冕物质抛射（ICME）内部增强的冲压压力和磁场的影响。在这里，我们报告详细的观测ICME压缩水星的昼侧磁层的表面。2013年11月29日从太阳发射的快速CME于11月30日撞击了第一个环绕水星运行的信使号，随后于12月1日在1 AU附近撞击了STEREO-A。在ICME撞击之后，信使号在太阳风中向内和向北飞向水星表面，直到它到达并通过最接近水星的地方（371公里高度），而没有进入磁层。磁层交叉最终发生在到达地球的夜侧前1分钟，高度为400公里，北纬84 $^\circ$，表明在这个轨道上没有昼侧磁层。此外，信使号此时测得的峰值磁场比ICME前后轨道上测得的值高出40%，这是磁层压缩的结果。使用代理方法在水星和STEREO-A的测量，我们表明，极高的冲压压力与此ICME超过足够高的崩溃水星的弱磁层。因此，ICME等离子体可能与水星表面相互作用，这一点可以通过外逸层中增强的钠离子来证明。水星昼侧磁层的坍缩对M矮星周围的系外行星的可居住性有重要的影响，因为这些事件可能会显着导致这些系统中的行星大气层损失。
英文摘要：
Mercury's magnetosphere is known to be affected by enhanced ram pressures and magnetic fields inside interplanetary coronal mass ejections (ICMEs). Here we report detailed observations of an ICME compressing Mercury's dayside magnetosphere to the surface. A fast CME launched from the Sun on November 29 2013 impacted first MESSENGER, which was orbiting Mercury, on November 30 and later STEREO-A near 1 AU on December 1. Following the ICME impact, MESSENGER remained in the solar wind as the spacecraft traveled inwards and northwards towards Mercury's surface until it reached and passed its closest approach to the planet (at 371 km altitude) without crossing into the magnetosphere. The magnetospheric crossing finally occurred 1 minute before reaching the planet's nightside at 400 km altitude and 84$^\circ$N latitude, indicating the lack of dayside magnetosphere on this orbit. In addition, the peak magnetic field measured by MESSENGER at this time was 40% above the values measured in the orbits just prior to and after the ICME, a consequence of the magnetospheric compression. Using both a proxy method at Mercury and measurements at STEREO-A, we show that the extremely high ram pressure associated with this ICME was more than high enough to collapse Mercury's weak magnetosphere. As a consequence, the ICME plasma likely interacted with Mercury's surface, evidenced by enhanced sodium ions in the exosphere. The collapse of Mercury's dayside magnetosphere has important implications for the habitability of close-in exoplanets around M dwarf stars, as such events may significantly contribute to planetary atmospheric loss in these systems.
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