Nonadiabatic acceleration of protons in the near-earth magnetotail during substorm dipolarization
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摘要: 亚暴偶极化过程中离子加速是亚暴粒子注入的重要产生机制. 通过试验粒子的方法模拟研究了亚暴偶极化期间磁尾等离子体片-8Re～-5Re处超低频电磁波对质子的加速过程. 研究表明, 质子在大尺度偶极化电磁场的作用下向内磁层注入, 与质子回旋频率相近的超低频电磁波能够引起低能质子发生非绝热加速. 质子在偶极化前后的能量变化与质子的初始能量密切相关, 初始能量远小于截止能量的质子, 末能量要比初始能量显著增加, 其值与扰动波频率相关, 且量级与偶极化造成的低能氧离子能量增加量级基本相当; 初始能量在截止能量以上的质子受超低频电磁波影响不大, 注入过程能量基本保持不变.Abstract: Using test particle simulation, the acceleration of -Re～-5Re magnetotail protons is studied during substorm-associated dipolarization events. The effects of the ultra-low frequency waves on the proton acceleration process in the vicinity of equatorial plane are investigated. It is shown that particles are injected earthward during dipolarization. Protons with relatively low initial energies may not conserve their magnetic moment, and they are nonadiabatically accelerated by the electromagnetic wave whose frequency is close to the proton gyrofrequency. In addition, the final energies of the particles after the event are related to its initial energies at the dipolarization onset. For the protons whose initial energies are much lower than the cutoff energy, their energies are increased significantly after the event. The particles' energy increase have the same order of magnitude with the O+ ions' in the dipolarization process, and it is related to the electromagnetic wave frequencies. However, for the protons whose initial energies are higher than the cutoff energy, their final energy level is not influenced by the wave and the energies are stable during the injection.
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Key words:
- Substorm /
- Dipolarization /
- Nonadiabatic acceleration
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