Drake J F, Swisdak M, Che H, Shay M A
University of Maryland, College Park, Maryland 20742, USA.
Nature. 2006 Oct 5;443(7111):553-6. doi: 10.1038/nature05116.
A long-standing problem in the study of space and astrophysical plasmas is to explain the production of energetic electrons as magnetic fields 'reconnect' and release energy. In the Earth's magnetosphere, electron energies reach hundreds of thousands of electron volts (refs 1-3), whereas the typical electron energies associated with large-scale reconnection-driven flows are just a few electron volts. Recent observations further suggest that these energetic particles are produced in the region where the magnetic field reconnects. In solar flares, upwards of 50 per cent of the energy released can appear as energetic electrons. Here we show that electrons gain kinetic energy by reflecting from the ends of the contracting 'magnetic islands' that form as reconnection proceeds. The mechanism is analogous to the increase of energy of a ball reflecting between two converging walls--the ball gains energy with each bounce. The repetitive interaction of electrons with many islands allows large numbers to be efficiently accelerated to high energy. The back pressure of the energetic electrons throttles reconnection so that the electron energy gain is a large fraction of the released magnetic energy. The resultant energy spectra of electrons take the form of power laws with spectral indices that match the magnetospheric observations.
在空间和天体物理等离子体研究中，一个长期存在的问题是解释当磁场“重新连接”并释放能量时高能电子的产生。在地球磁层中，电子能量可达数十万电子伏特（参考文献1 - 3），而与大规模重联驱动流相关的典型电子能量只有几电子伏特。最近的观测进一步表明，这些高能粒子是在磁场重新连接的区域产生的。在太阳耀斑中，释放能量的50%以上可能以高能电子的形式出现。在这里，我们表明电子通过从重联过程中形成的收缩“磁岛”的端部反射而获得动能。该机制类似于一个球在两个汇聚壁之间反射时能量的增加——球每次反弹都会获得能量。电子与许多磁岛的重复相互作用使得大量电子能够被有效地加速到高能。高能电子的背压会抑制重联，从而使电子能量增益占释放磁能的很大一部分。由此产生的电子能谱呈幂律形式，其谱指数与磁层观测结果相符。
