Particle-In-Cell Simulation

1-D PIC Simulation of generation of banded chorus waves

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 twoband structure remains a 50-year old question. Our study (Li et al, 2019) reports, using NASA’s Van Allen Probe measurements, that banded chorus waves are commonly accompanied by two separate anisotropic electron components. Using particle-in-cell simulations, we show that the initially excited single-band chorus waves alter the electron distribution immediately via Landau resonance, and suppress the electron anisotropy at medium energies. This naturally divides the electron anisotropy into a low and a high energy components which excite the upperband and lower-band chorus waves, respectively. This mechanism may also apply to the generation of chorus waves in other magnetized planetary magnetospheres.

Simulation of chorus wave excitation and the concurrent electron PSD variation. a) The initial electron PSD versus energy at selected pitch angles, showing a cold isotropic population extending to ~100 eV and a warm anisotropic population above ~ 300 eV. b) The wave magnetic spectral intensity for each (ω, k) mode obtained by Fourier transformation of the magnetic field over T = 50–100 τgyro (electron gyro-period), showing a continuous one-band spectral structure, while the excitation of upper-band chorus is faster. c) The electron PSD at T = 150 τgyro, showing a notable parallel acceleration at ~2–3 keV, which is around the Landau resonant energies of the chorus waves. d) As the chorus waves at both bands continues to grow, the wave intensity at 0.5 fce is notably weaker than the two bands, due to the reduce anisotropy at ~2–3 keV. e) At T = 800 τgyro, the electron PSD exhibits two distinct anisotropic components, separated by the electrons that have undergone parallel acceleration by chorus waves. f) The resultant two-band chorus waves with a gap at 0.5 fce. The upper-band chorus grows from the ~1 keV anisotropic component, and the lower-band chorus grows from tens of keV anisotropic component

Reference:

Li, J., Bortnik, J., An, X., Li, W., Angelopoulos, V., et al. (2019), Origin of two-band chorus in the radiation belt of Earth, Nature Communications, 10, 4672, https://doi.org/10.1038/s41467-019-12561-3