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 Widerøe 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.
Schematic showing magnetic reconnection in the tail of Earth’s magnetosphere

Illustration of particle acceleration during the unsteady reconnection
    The achievement has been published in the international famous journal Nature Physics. The European space agency (ESA) had interviewed about it through the telephone, and it’s highlighted on the ESA official web as “Wobbly Magnetic Reconnection Speeds up Electrons”. Meanwhile, The Centre national de la recherche scientifique(CNRS) also made a feature report entitled as“La mission Cluster découvre que les électronsénergétiques sont accélèrés par reconnexion non stationnaire”. Moreover, the Swedish Institute of Space Physics (IRF) and the Institute of Plasma Physics (LPP) of CNRS made a corresponding report.
    Philippe Escoubet, the chief scientist of the ESA’s Cluster satellite program, commented that Huishan Fu and his collaborators finally grasped how exactly magnetic reconnection gives rise to high-energy electrons. This discovery also deepened our understanding of the high-energy electron in the center of Galaxy, the chromosphere of stars and fusion plasma, as the magnetic reconnection is usually unstable in these places. And it will also improve our ability to predict space weather….
    It has obtained a widespread praise among international space community. In the conferences of American Geophysical Society 2013 and the 13th Asia Pacific Wireless, Huishan Fu was invited to make a report on the achievement. At the same time he was also invited to report on Geospace Revised workshop in 2014. In view of the importance of this achievement, he won the Young Scientist Awards (YSA) in the Asia-Pacific Radio Science Conference and the national solar-terrestrial space physics convention respectively.
Details about the research can be found at
Feature focus
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.
Biographical Note

 Huishan Fu is a professor at Space Science Institute, School of Astronautics, Beihang University. His research interests include magnetic reconnection, Earth’s radiation belts, and the particle acceleration process in the solar system.