| Autor: |
Cheng, Kun, Liu, Kaijun, Mousavi, Ameneh, Cowee, Misa, Zheng, Xianming, Zhou, Jingyi, Wang, Yan, Liu, Yuqi, Wang, Xinye |
| Předmět: |
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| Zdroj: |
Geophysical Research Letters; 11/16/2024, Vol. 51 Issue 21, p1-9, 9p |
| Abstrakt: |
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. Plain Language Summary: Without the shielding of a global intrinsic magnetic field, the Martian exosphere interacts directly with the solar wind. Pickup ions are generated when neutrals in the exosphere are ionized and "picked up" by the solar wind. The instabilities driven by these pickup ions lead to the generation of proton cyclotron waves upstream of Mars. Despite that linear analysis indicates higher maximum growth rates for pickup ion ring distributions compared with beam distributions, observations show that the excitation of observable waves upstream of Mars is relatively rare with pickup ion ring distributions. The present study carries out linear instability analysis for pickup ions with a ring distribution. The results reveal that the ion cyclotron, ion Bernstein, and mirror instabilities are unstable. The subsequent particle‐in‐cell simulation validates the linear theory prediction and illustrates the scattering of the pickup ions by the enhanced waves. Interestingly, although the ion cyclotron instability has the largest maximum growth rate, it eventually diminishes and transforms into Alfvén waves. Consequently, the simulation is dominated by mirror waves in the late stage. More importantly, the wave saturation energy for pickup ion ring distributions is much lower than for beam distributions. Key Points: A pickup ion ring distribution is unstable to the ion cyclotron, ion Bernstein, and mirror instabilitiesThe instability saturation level is lower for a ring distribution than a corresponding beam distributionThe mirror waves have the maximum power spectrum eventually in the simulation [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
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