MESSENGER Observations of Extreme Loading and Unloading of Mercury’s Magnetic Tail-猫眼课题宝

DOI：

10.1126/science.1188067

发表时间：

2010-08

期刊：

Science

影响因子：

56.9

通讯作者：

中科院分区：

综合性期刊1区

文献类型：

作者：

J. Slavin;B. Anderson;D. Baker;M. Benna;S. Boardsen;G. Gloeckler;R. Gold;G. Ho;H. Korth;S. Krimigis;R. McNutt;L. Nittler;J. Raines;M. Sarantos;D. Schriver;S. Solomon;R. Starr;P. Trávnícek;T. Zurbuchen

信使号的第三组信息信使号飞船将于2011年3月进入水星轨道，并于2009年9月29日完成了第三次飞越水星。Prockter等人（第668页，7月15日在线发表）展示了在这次飞越中获得的成像数据，显示水星上的火山活动比之前假设的时间要晚得多。火山活动的时间范围，特别是最近一次活动的时间，一直是理解水星全球热演化的缺失因素。Slavin等人（第665页，7月15日在线发表）报告了9月29日水星飞掠时的磁场测量结果，当时水星的磁层与太阳风发生了极强的耦合。这颗行星尾部的磁场在持续2到3分钟的时间内以2到3.5倍的倍数增加然后减少。这些观测结果表明，开放磁通负荷了磁层，随后被亚暴卸载——在亚暴期间，能量在磁尾中迅速释放。在地球上，在加载/卸载周期中，尾部磁场强度的变化要小得多，并且发生在更长的时间尺度上。Vervack等人（第672页，7月15日在线发表）使用信使号上的水星大气和表面成分光谱仪测量了水星的中性和离子外逸层。水星南北两极上镁、钙和钠的高度分布的差异表明，在创造和维持外逸层的过程中，有多种过程在起作用。相对于地球，水星的磁层亚暴更加强烈，发生的时间也更短。在信使号第三次飞越水星的过程中，水星磁尾的磁场在2到3分钟的间隔内增加了2到3.5倍。地球上的磁层亚暴也是由类似的尾载荷驱动的，但其振幅要低约10倍，典型的持续时间为约1小时。在水星观测到的极端尾部负荷表明，亚暴的相对强度一定比地球大得多。尾场增强的持续时间与描述水星磁层等离子体循环的邓吉周期的特征时间之间的对应关系表明，这种循环决定了亚暴的时间尺度。在地球亚暴期间，尾巴卸载的一个关键方面是高能带电粒子的加速，但在信使号飞越期间没有看到加速的迹象。

MESSENGER's Third Set of Messages MESSENGER, the spacecraft en route to insertion into orbit about Mercury in March 2011, completed its third flyby of the planet on 29 September 2009. Prockter et al. (p. 668, published online 15 July) present imaging data acquired during this flyby, showing that volcanism on Mercury has extended to much more recent times than previously assumed. The temporal extent of volcanic activity and, in particular, the timing of most recent activity had been missing ingredients in the understanding of Mercury's global thermal evolution. Slavin et al. (p. 665, published online 15 July) report on magnetic field measurements made during the 29 September flyby, when Mercury's magnetosphere underwent extremely strong coupling with the solar wind. The planet's tail magnetic field increased and then decreased by factors of 2 to 3.5 during periods lasting 2 to 3 minutes. These observations suggest that magnetic open flux loads the magnetosphere, which is subsequently unloaded by substorms—magnetic disturbances during which energy is rapidly released in the magnetotail. At Earth, changes in tail magnetic field intensity during the loading/unloading cycle are much smaller and occur on much longer time scales. Vervack et al. (p. 672, published online 15 July) used the Mercury Atmospheric and Surface Composition Spectrometer onboard MESSENGER to make measurements of Mercury's neutral and ion exospheres. Differences in the altitude profiles of magnesium, calcium, and sodium over the north and south poles of Mercury indicate that multiple processes are at play to create and maintain the exosphere. Relative to Earth, Mercury’s magnetospheric substorms are more intense and occur on shorter time scales. During MESSENGER’s third flyby of Mercury, the magnetic field in the planet’s magnetic tail increased by factors of 2 to 3.5 over intervals of 2 to 3 minutes. Magnetospheric substorms at Earth are powered by similar tail loading, but the amplitude is lower by a factor of ~10 and typical durations are ~1 hour. The extreme tail loading observed at Mercury implies that the relative intensity of substorms must be much larger than at Earth. The correspondence between the duration of tail field enhancements and the characteristic time for the Dungey cycle, which describes plasma circulation through Mercury’s magnetosphere, suggests that such circulation determines the substorm time scale. A key aspect of tail unloading during terrestrial substorms is the acceleration of energetic charged particles, but no acceleration signatures were seen during the MESSENGER flyby.

MESSENGER Observations of Extreme Loading and Unloading of Mercury’s Magnetic Tail

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