Proton Cyclotron Waves and Pickup Ion Ring Distribution Instabilities Upstream of Mars
Kun Cheng ,Kaijun Liu ,Ameneh Mousavi ,Misa Cowee ,Xianming Zheng ,Jingyi Zhou ,et al
Proton cyclotron waves (PCWs) upstream of Mars are thought to be associated with the instabilities of pickup ions. The instabilities of pickup ions of ring distributions have larger maximum growth rates compared with beam distributions. However, observations revealed a notably-reduced occurrence rate of PCWs for pickup ions of ring distributions. Linear instability analysis and a corresponding two-dimensional particle-in-cell (PIC) simulation are performed to investigate the instabilities of pickup ion ring distributions upstream of Mars. Linear instability analysis indicates that a pickup ion ring distribution is unstable to the ion cyclotron, ion Bernstein, and mirror instabilities. The corresponding PIC simulation confirms the linear analysis results and further demonstrates that the pickup ions are scattered toward an isotropic shell distribution by the waves excited. Interestingly, the saturation energy of the waves is much lower than that driven by a corresponding pickup ion beam distribution, and mirror waves eventually dominate the system.
Fig. 1. Linear growth rate γ/Ωp as a function of k││λp and θ. Shown from left to right are (a) the ion-cyclotron instability and the first-order IB instability, (b) the mirror instability, and (c) the higher-order IB instability. The black dotted lines in panels (a, c) represent the wave polarization （P=0） and frequency, respectively.
Fig. 2. (a) The evolution of the x-component (red line), y-component (blue line), z-component (green line), and total (black line) wave magnetic field energy densities. (b–d) The wave number power spectra of the wave magnetic component By at three different simulation times as labeled. The color bar represents the power spectra on a base-10 logarithmic scale. The red dashed, red solid, and white solid lines in panel (b) are the linear wave growth rate contour lines of γ = 0.3 Ωp for the IC, IB, and mirror instabilities, respectively, derived from Figure 1.
Fig. 3. Ion velocity distributions in a logarithmic scale at tΩp = 0, 10, 20 and 40 in the simulation..
Fig. 4. Magnetic wave power spectra in a base-10 logarithmic scale (a, b) and the corresponding wave ellipticity (c, d) obtained from the discrete Fourier transform of the complex magnetic field S(x, t) = By (x, t) + i Bz(x, t). The two columns from left to right display the results calculated for the wave magnetic field recorded during two different intervals as labeled. The ellipticity values in panels (c, d) are shown for only the waves with power spectral values above 10-7.6 and 10-6 (arbitrary unit) in panels (a, b), respectively, to highlight the enhanced waves in the simulation.
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