不稳定磁重联加速高能电子
2015-01-16
  Energetic particles pervade the universe; they can injure astronauts and destroy spacecraft, hence provide a dangerous environment for human space activity. How these particles are accelerated is poorly understood. Traditionally, magnetic reconnection—the process leading to the change of magnetic field topology—is proposed to explain the acceleration of high-energy particles. Whether this proposition is true and how it works in detail is still unknown.
   A team of scientists, led by Huishan Fu, has discovered a missing piece in the puzzle of where high-energy particles in Earth's magnetosphere come from. They found that magnetic reconnection can accelerate electrons to very high energies - as long as reconnection happens at a variable pace rather than steadily. The result will improve predictions of space weather, studies of fusion plasma, and the understanding of astrophysical environments affected by magnetic fields.
  Betatron acceleration, in which a time-varying magnetic field is responsible for increasing the energy of the electrons, was originally described by Norwegian physicist Rolf Wider?e in 1928. This mechanism was also applied to artificial particle accelerators by American physicist Donald Kerst, who constructed one of the first machines to produce high-energy electrons on the ground.
  Fermi acceleration, which was first suggested by Italian-American physicist Enrico Fermi in 1949, is a mechanism whereby particles are energized by repeatedly bouncing off the turbulent magnetic field, which results in a net and substantial gain of energy.
  Betatron acceleration leads to the enhancement of electron fluxes in the direction perpendicular to the magnetic field, while Fermi acceleration leads to the enhancement of electron fluxes in the field-aligned direction.
  The erratic rate of reconnection can be caused by temporal variability in the speed of the inflowing plasma that triggers the process, or by turbulence that develops directly in the magnetic reconnection site.
  The discovery is based on data gathered by one of the spacecraft (C1) of ESA's Cluster mission. Cluster is a constellation of four spacecraft flying in formation around Earth. It is the first space mission able to study, in three dimensions, the natural physical processes occurring within and in the near vicinity of the Earth's magnetosphere.
  Using the Cluster measurements of a reconnection event on 23 August 2006, Huishan Fu et al. revealed that the electron acceleration by unsteady reconnection is quasi-adiabatic: betatron and Fermi acceleration in outflow jets are two processes contributing to electron energization. The localized betatron acceleration in the outflow is responsible for at least half of the energy gain for the peak observed fluxes. Betatron acceleration occurs primarily during the rising speed portion of the plasma jets, while Fermi acceleration dominates during the falling speed portion.


  这一成果发表在国际知名期刊Nature Physics上。欧洲空间局(ESA)于2013年7月12日对该成果进行了电话采访,并在ESA 科学技术网的首页上做了题为“Wobbly Magnetic Reconnection Speeds up Electrons” 的专题报道。同期,法国国家科学研究院(CNRS)对该项成果也做了题为 “La mission Cluster découvre que les électronsénergétiques sont accélèrés par reconnexion non stationnaire”的专题报道。瑞典空间物理研究所(IRF)以及法国科学院等离子体物理研究所(LPP)对这一研究成果也做出了相应的报道。

  
  欧洲空间局Cluster卫星计划首席科学家Philippe Escoubet评价说:“符等人的呀就工作终于解释了磁场重联是如何产生高能电子的(finally grasped how exactly magnetic reconnection gives rise to high-energy electrons)。这一发现对了解银河系中心的高能电子,太阳大气中的高能电子,以及核聚变中的高能电子提供了很好的帮助,因为在这些地方,磁场重联通常都是不稳定的。这一发现改进了我们对空间天气的预测能力(it will also improve our ability to predict space weather…).
  该成果获得了国际同行的广泛好评。在2013年美国地球物理学年会和第13届亚太无线电大会上,符慧山对该项成果做了大会特邀报告。同时应邀在2014年“ Geospace Revised”研讨会上对该成果做大会特邀报告。鉴于该成果的重要性,符慧山获得了亚太无线电大会“青年科学家”奖,以及全日地空间物理学年会“青年优秀论文奖”。

相关链接
http://sci.esa.int/cluster/52077-wobbly-magnetic-reconnection-speeds-up-electrons/ http://www.insu.cnrs.fr/node/4448 http://www.lpp.fr/In-the-headlines-at-INSU-CNRS-and
相关文章
Fu, H. S., Y. V. Khotyaintsev, A. Vaivads, A. Retinò, and M. André (2013), Energetic electron acceleration by unsteady magnetic reconnection, Nature Physics, 9, 426–430, doi:10. 1038/nphys2664.

 
作者简介
符慧山,男,1983年生,北京航空航天大学青年千人岗聘教授,受聘于宇航学院空间科学研究所和国际交叉科学研究院,研究方向为地球空间磁场重联和高能粒子加速。