Equatorial Evolution of the Fast Magnetosonic Mode in the Source Region: Observation‐Simulation Comparison of the Preferential Propagation Direction

Abstract

Recent analysis of an event observed by the Van Allen Probes in the source region outside the plasmapause has shown that fast magnetosonic waves (also referred to as equatorial noise) propagate preferentially in the azimuthal direction, implying that wave amplification should occur during azimuthal propagation. To demonstrate this, we carry out 2-D particle-in-cell simulations of the fast magnetosonic mode at the dipole magnetic equator with the simulation box size, the magnetic field inhomogeneity, and the plasma parameters chosen from the same event recently analyzed. The self-consistently evolving electric and magnetic field fluctuations are characterized by spectral peaks at harmonics of the local proton cyclotron frequency. The azimuthal component of the electric field fluctuations is larger than the radial component, indicating wave propagation mainly along the azimuthal direction. Because the simulation box is within the source region, this also implies wave amplification mainly during azimuthal propagation. The excellent agreement between the wave polarization properties of the present simulations and the recently reported observations is clear evidence that the main wave amplification occurs during azimuthal propagation in the source region.

Equatorial Propagation of the Magnetosonic Mode Across the Plasmapause: 2‐D PIC Simulations

“…In fact, Němec et al () analyzed the electric field polarization ellipsoids of magnetosonic wave events acquired during multiple Cluster perigee passes. Their analysis indicated that azimuthal propagation is statistically dominant where the total plasma number density is low ( cm 3 ), as occurs outside the plasmapause (plasma trough) and that the waves analyzed tended to possess a small radial component in their wave normal directions away from the Earth (consistent with the Min, Boardsen, et al ()'s simulation result). Considering that conditions outside the plasmapause are favored for wave excitation (e.g., Ma, Li, Chen, Thorne, & Angelopoulos, ; Yuan et al, ), this statistical result indicates that azimuthal propagation is indeed optimal for large wave amplification.…”

“…Under these conditions, the wave will remain in resonance for a longer time as it propagates azimuthally, allowing larger wave amplification. This has recently been confirmed by 2‐D PIC simulations of magnetosonic waves propagating perpendicular to the dipole magnetic field in the equatorial plane (i.e., waves with 90° wave normal angle; Min, Boardsen, et al, ). Using the plasma parameters from an event studied by Min, Liu, et al (), the simulations showed that magnetosonic wave amplification is dominant when the waves propagate azimuthally within the source region.…”

“…Boardsen et al () analyzed the wave electric field polarization for two events observed outside the plasmapause and found that the major axis of the polarization ellipsoid (proxy for the wave propagation direction) was within 20–30° of the azimuthal direction. In another study, Boardsen et al () analyzed the same event that Min, Boardsen, et al () simulated and showed that the propagation direction was dominantly in the azimuthal direction at every harmonic peak. The electric field polarization properties were in excellent agreement with those simulated by Min, Boardsen, et al (), which is strong evidence that the wave amplification dominantly occurred with azimuthal propagation (compare Boardsen et al, , Figure 4 with Min, Boardsen, et al , Figure 6).…”

“…In another study, Boardsen et al () analyzed the same event that Min, Boardsen, et al () simulated and showed that the propagation direction was dominantly in the azimuthal direction at every harmonic peak. The electric field polarization properties were in excellent agreement with those simulated by Min, Boardsen, et al (), which is strong evidence that the wave amplification dominantly occurred with azimuthal propagation (compare Boardsen et al, , Figure 4 with Min, Boardsen, et al , Figure 6). The preferential azimuthal propagation of fast magnetosonic waves is not just event specific.…”

Particle‐in‐Cell Simulation of Rising‐Tone Magnetosonic Waves

“…Both satellite observations (Boardsen et al, 1992; Meredith et al, 2008; Perraut et al, 1982) and the linear theory (Chen et al, 2010; Gary et al, 2010; Sun, Gao, Chen, et al, 2016) have linked the generation of magnetosonic waves with ring distribution protons at energies of about 10 keV. Particle‐in‐cell (PIC) simulations in a uniform magnetic field (Liu et al, 2011; Sun, Gao, Lu, et al, 2016; Sun et al, 2017) and a dipole field (Chen et al, 2018; Min et al, 2018) have also demonstrated that the waves can be generated by ring distribution protons. In addition, one event study near or in the source suggests that the wave gain of the magnetosonic mode is primarily in the azimuthal direction (Boardsen et al, 2018).…”