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Whistler anisotropy instabilities as the source of banded chorus: Van Allen Probes observations and particle-in-cell simulations

Author

Fu, Xiangrong
Cowee, Misa
Friedel, Reinhard
Funsten, Herbert
Gary, S
Hospodarsky, George
Kletzing, Craig
Kurth, William
Larsen, Brian
Liu, Kaijun
MacDonald, Elizabeth
Min, Kyungguk
Reeves, Geoffrey
Skoug, Ruth
Winske, Dan
0000-0003-3117-4030
0000-0002-7985-8098
0000-0002-6817-1039
0000-0002-5228-0281
0000-0003-4515-0208
0000-0001-5882-1328
0000-0003-3117-4030
0000-0002-5471-6202
0000-0001-5882-1328
0000-0002-4305-6624
0000-0001-9200-9878
0000-0002-4136-3348
0000-0003-1991-2643

Abstract

Magnetospheric banded chorus is enhanced whistler waves with frequencies (r)<(e), where (e) is the electron cyclotron frequency, and a characteristic spectral gap at (r)similar or equal to(e)/2. This paper uses spacecraft observations and two-dimensional particle-in-cell simulations in a magnetized, homogeneous, collisionless plasma to test the hypothesis that banded chorus is due to local linear growth of two branches of the whistler anisotropy instability excited by two distinct, anisotropic electron components of significantly different temperatures. The electron densities and temperatures are derived from Helium, Oxygen, Proton, and Electron instrument measurements on the Van Allen Probes A satellite during a banded chorus event on 1 November 2012. The observations are consistent with a three-component electron model consisting of a cold (a few tens of eV) population, a warm (a few hundred eV) anisotropic population, and a hot (a few keV) anisotropic population. The simulations use plasma and field parameters as measured from the satellite during this event except for two numbers: the anisotropies of the warm and the hot electron components are enhanced over the measured values in order to obtain relatively rapid instability growth. The simulations show that the warm component drives the quasi-electrostatic upper band chorus and that the hot component drives the electromagnetic lower band chorus; the gap at approximate to(e)/2 is a natural consequence of the growth of two whistler modes with different properties.