Rapid formation of “killer” electrons in the magnetosphere as accelerated by ULF waves(Zong, Q.-G., Zhou, X.-Z.)


Energetic electrons in Earth's magnetosphere will cause severe damage to the satellites and spacecraft, they are usually known as 'killer electrons'. Dramatic increase of killer electron flux (i.e., energetic electron storm) is one type of the major space weather hazard events. The acceleration of energetic electrons has long been concern by scientist, but has not yet been fully understood. The acceleration mechanism has become one of the major challenges in space physics and space weather. It is generally believed that the killer electrons are accelerated by VLF waves. However, the acceleration efficiency is too low to explain the observation that killer electrons in the radiation belt are accelerated almost immediately when an interplanetary shock impinges the magnetosphere and the acceleration lasts for several hours.

We have discovered, for the first time, the simultaneous drift resonance of energetic particles with both poloidal mode and toroidal mode ULF waves and proven that poloidal mode ULF waves plays a more important role in the acceleration of inner magnetospheric particles. Further We have seminally proposed the two step mechanism to explain the fast acceleration of killer electrons in Earth's radiation belt due to the impact of interplanetary shocks: (1) Interaction between interplanetary shock and magnetosphere excites poloidal mode ULF waves and compresses the magnetic field which results in a primary adiabatic acceleration; (2) Electrons and poloidal mode ULF waves undergo the drift resonance process which rapidly accelerates electrons of lower energy to killer electrons. This mechanism is also applicable for interaction between interplanetary shock and other planets in the solar system.

The famous DISCOVER magazine (January 2008 issue) has a selection of 100 top science stories of 2007, and our study "How Killer Electrons Form in Space" has been listed as 37. Our study has been selected as scientific achievement highlights by science department of ESA in 2007 and 2009, stating that "These new findings can help us to improve the models predicting the radiation environment in which satellites and astronauts operate." Our study has been selected by the Royal Astronomical Society journal as one of four specific scientific highlights in 10 years of Cluster (the large space mission collaborated by ESA and NASA) observations; has been selected as one of five discoveries by ESA in "Cluster 2000-10: a decade revealing the Sun-Earth connection in 3D". Zong has been rewarded by ESA Outstanding Scientist Award in recognition of his extraordinary scientific contribution to the Cluster mission, especially for the uncovering the fast acceleration mechanism of killer electrons.

Figure-2. Top Story on ESA-Cluster official website: Rapid acceleration of “killer” electrons driven by interplanetary shocks


(1) Zong, Q.-G., Zhou, X.-Z., Wang, Y.-F., Li, X., et al., Energetic electron response to ULF waves induced by interplanetary shocks in the outer radiation belt, JGR, 114, A10204, doi:10.1029/2009JA014393, 2009

(2) Zong, Q.-G., Zhou, X.-Z., Li, X., Song, P., et al., Ultralow frequency modulation of energetic particles in the dayside magnetosphere, GRL, 34, L12105, doi:10.1029/2007GL029915, 2007