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Nat Commun. 2015 Oct 5;6:8590. doi: 10.1038/ncomms9590.

Wave-driven butterfly distribution of Van Allen belt relativistic electrons.

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School of Physics and Electronic Sciences, Changsha University of Science and Technology, 2nd Section, South Wanjiali Road #960, Yuhua District, Changsha, Hunan 410004, China.
Chinese Academy of Sciences Key Laboratory for Basic Plasma Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100190, China.
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire 03824-3525, USA.
ISR Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
The Aerospace Corporation, Los Angeles, California 90245-4609, USA.


Van Allen radiation belts consist of relativistic electrons trapped by Earth's magnetic field. Trapped electrons often drift azimuthally around Earth and display a butterfly pitch angle distribution of a minimum at 90° further out than geostationary orbit. This is usually attributed to drift shell splitting resulting from day-night asymmetry in Earth's magnetic field. However, direct observation of a butterfly distribution well inside of geostationary orbit and the origin of this phenomenon have not been provided so far. Here we report high-resolution observation that a unusual butterfly pitch angle distribution of relativistic electrons occurred within 5 Earth radii during the 28 June 2013 geomagnetic storm. Simulation results show that combined acceleration by chorus and magnetosonic waves can successfully explain the electron flux evolution both in the energy and butterfly pitch angle distribution. The current provides a great support for the mechanism of wave-driven butterfly distribution of relativistic electrons.

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