Bibliography

Generation of Highly Oblique Lower-band Chorus via Nonlinear Three-wave Resonance

Chorus in the inner magnetosphere has been observed frequently at geomagnetically active times, typically exhibiting a two-band structure with a quasi-parallel lower-band and an upper-band with a broad range of wave normal angles. But recent observations by Van Allen Probes confirm another type of lower-band chorus, which has a large wave normal angle close to the resonance cone angle. It has been proposed that these waves could be generated by a low-energy beam-like electron component or by temperature anisotropy of keV electrons in the presence of a low-energy plateau-like electron component. This paper, however, presents an alternative mechanism for generation of this highly oblique lower-band chorus. Through a nonlinear three-wave resonance, a quasi-parallel lower-band chorus wave can interact with a mildly oblique upper-band chorus wave, producing a highly oblique quasi-electrostatic lower-band chorus wave. This theoretical analysis is confirmed by 2D electromagnetic particle-in-cell simulations. Furthermore, as the newly generated waves propagate away from the equator, their wave normal angle can further increase and they are able to scatter low-energy electrons to form a plateau-like structure in the parallel velocity distribution. The three-wave resonance mechanism may also explain the generation of quasi-parallel upper-band chorus which has also been observed in the magnetosphere.

Fu, Xiangrong; Gary, Peter; Reeves, Geoffrey; Winske, Dan; Woodroffe, Jesse;

Published by: Geophysical Research Letters      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017GL074411

oblique whistler; PIC simulation; Ray Tracing; three-wave resonance; Van Allen Probes

Ion Bernstein instability as a possible source for oxygen ion cyclotron harmonic waves

This paper demonstrates that an ion Bernstein instability can be a possible source for recently reported electromagnetic waves with frequencies at or near the singly ionized oxygen ion cyclotron frequency, inline image, and its harmonics. The particle measurements during strong wave activity revealed a relatively high concentration of oxygen ions (\~15\%) whose phase space density exhibits a local peak at energy \~20 keV. Given that the electron plasma-to-cyclotron frequency ratio is inline image, this energy corresponds to the particle speed inline image, where vA is the oxygen Alfv\ en speed. Using the observational key plasma parameters, a simplified ion velocity distribution is constructed, where the local peak in the oxygen ion velocity distribution is represented by an isotropic shell distribution. Kinetic linear dispersion theory then predicts unstable Bernstein modes at or near the harmonics of inline image and at propagation quasi-perpendicular to the background magnetic field, B0. If the cold ions are mostly protons, these unstable modes are characterized by a low compressibility ( inline image), a small phase speed (vph\~0.2vA), a relatively small ratio of the electric field energy to the magnetic field energy (between 10-4 and 10-3), and the Poynting vector directed almost parallel to B0. These linear properties are overall in good agreement with the properties of the observed waves. We demonstrate that superposition of the predicted unstable Bernstein modes at quasi-perpendicular propagation can produce the observed polarization properties, including the minimum variance direction on average almost parallel to B0.

Min, Kyungguk; Denton, Richard; Liu, Kaijun; Gary, Peter; Spence, Harlan;

Published by: Journal of Geophysical Research: Space Physics      Published on: 05/2017

YEAR: 2017     DOI: 10.1002/2017JA023979

O+ Bernstein instability; O+ harmonic waves; O+ ring distribution; Van Allen Probes

Nonlinear subcyclotron resonance as a formationmechanism for gaps in banded chorus

An interesting characteristic of magnetospheric chorus is the presence of a frequency gap at ω≃0.5Ωe, where Ωe is the electron cyclotron angular frequency. Recent chorus observations sometimes show additional gaps near 0.3Ωe and 0.6Ωe. Here we present a novel nonlinear mechanism for the formation of these gaps using Hamiltonian theory and test particle simulations in a homogeneous, magnetized, collisionless plasma. We find that an oblique whistler wave with frequency at a fraction of the electron cyclotron frequency can resonate with electrons, leading to effective energy exchange between the wave and particles.

Fu, Xiangrong; Guo, Zehua; Dong, Chuanfei; Gary, Peter;

Published by: Geophysical Research Letters      Published on: 05/2015

YEAR: 2015     DOI: 10.1002/2015GL064182

banded chorus; nonlinear theory; subcyclotron resonance

Whistler Anisotropy Instabilities as the Source of Banded Chorus: Van Allen Probes Observations and Particle-in-Cell Simulations

Magnetospheric banded chorus is enhanced whistler waves with frequencies ωr < Ωe, where Ωe is the electron cyclotron frequency, and a characteristic spectral gap at ωr ≃ Ωe/2. This paper uses spacecraft observations and two-dimensional particle-in-cell (PIC) 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 HOPE 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 \~ Ωe/2 is a natural consequence of the growth of two whistler modes with different properties.

Fu, Xiangrong; Cowee, Misa; Friedel, Reinhard; Funsten, Herbert; Gary, Peter; Hospodarsky, George; Kletzing, Craig; Kurth, William; Larsen, Brian; Liu, Kaijun; MacDonald, Elizabeth; Min, Kyungguk; Reeves, Geoffrey; Skoug, Ruth; Winske, Dan;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2014

YEAR: 2014     DOI: 10.1002/2014JA020364

Chorus; HOPE; particle-in-cell simulation; Van Allen Probes