Naturally occurring chorus emissions are a class of electromagnetic waves found in the space environments of the Earth and other magnetized planets. They play an essential role in accelerating high-energy electrons forming the hazardous radiation belt environment. Chorus typically occurs in two distinct frequency bands separated by a gap. The origin of this two-band structure remains a 50-year old question. Here we report, using NASA’s Van Allen Probe measurements, that banded chorus waves are commonly accompanied by two separate anisotropic electron components. Using numerical simulations, we show that the initially excited single-band chorus waves alter the electron distribution immediately via Landau resonance, and suppresses the electron anisotropy at medium energies. This naturally divides the electron anisotropy into a low and a high energy components which excite the upper-band and lower-band chorus waves, respectively. This mechanism may also apply to the generation of chorus waves in other magnetized planetary magnetospheres.
Authors: Jinxing Li1*, Jacob Bortnik1*, Xin An1, Wen Li2, Vassilis Angelopoulos3, Richard M. Thorne1, Christopher T. Russell3, Binbin Ni4,5, Xiaochen Shen2, William S. Kurth6, George B. Hospodarsky6, David P. Hartley6, Herbert O. Funsten7, Harlan E. Spence8and Daniel N. Baker9
1Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California 90095, USA
2Center for Space Physics, Boston University, Boston, Massachusetts, USA.
3Department of Earth, Space and Planetary Sciences, University of California, Los Angeles, California 90095, USA
4Department of Space Physics, School of Electronic Information, Wuhan University, Wuhan 430072, Hubei, China
5CAS Center for Excellence in Comparative Planetology, Anhui, Hefei, China
6Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242-1479, USA
7 Los Alamos National Laboratory, MS-D466, PO Box 1663, Los Alamos, New Mexico 87545, USA
8 Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire 03824-3525, USA
9Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA.