Romanelli N, DiBraccio G A
Department of Astronomy, University of Maryland, College Park, MD, USA.
Planetary Magnetospheres Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA.
Nat Commun. 2021 Nov 19;12(1):6748. doi: 10.1038/s41467-021-26344-2.
Studies of Mercury's foreshock have analyzed in detail the properties of ultra-low frequency waves. However, an open question remains in regards to understanding favorable conditions for these planetary foreshocks waves. Here, we report that 0.05-0.41 Hz quasi-monochromatic waves are mostly present under quasi-radial and relatively low intensity Interplanetary Magnetic Field, based on 17 Mercury years of MESSENGER Magnetometer data. These conditions are consistent with larger foreshock size and reflection of solar wind protons, their most likely source. Consequently, we find that the wave occurrence rate increases with Mercury's heliocentric distance. Detection of these waves throughout Mercury's highly eccentric orbit suggests the conditions for backstreaming protons are potentially present for all of Mercury's heliocentric distances, despite the relatively low solar wind Alfvén Mach number regime. These results are relevant for planetary magnetospheres throughout the solar system, and the magnetospheres of exoplanets, and provide knowledge of particle acceleration mechanisms occurring inside foreshocks.
对水星前震的研究已经详细分析了超低频波的特性。然而，在理解这些行星前震波的有利条件方面，仍然存在一个悬而未决的问题。在此，基于信使号磁力计17个水星年的数据，我们报告称，0.05 - 0.41赫兹的准单色波大多出现在准径向且强度相对较低的行星际磁场条件下。这些条件与更大的前震规模以及太阳风质子（它们最可能的来源）的反射相一致。因此，我们发现波的发生率随着水星到太阳的距离增加而升高。在水星高度偏心的整个轨道上对这些波的探测表明，尽管太阳风阿尔文马赫数相对较低，但在水星到太阳的所有距离上，都可能存在质子回流的条件。这些结果对于整个太阳系的行星磁层以及系外行星的磁层都具有重要意义，并提供了有关前震内部粒子加速机制的知识。
