• ISSN 2097-1893
  • CN 10-1855/P

木星内磁层交换不稳定性理论分析

Theoretical analysis of interchange instability in Jupiter's inner magnetosphere

  • 摘要: 交换不稳定性广泛存在于工程和自然界中,被认为是木星磁盘内径向物质输运的主要机制之一. 本文基于理想磁流体力学理论,考虑了木星内磁层基本物理参数(Feng et al., 2025),获得了未考虑局域近似(扰动波长远小于特征长度)条件下的木星内磁层交换不稳定性色散关系和不稳定判据;通过理论分析获得了木星内磁层交换不稳定性的主要增长模式与增长率. 分析结果显示,在当前的参数条件下,在木卫一环内侧区域交换模式稳定;在木卫一环外侧存在交换模式不稳定区间,理论给出了不稳定性发生的空间位置. 结果还表明,在木卫一环外侧的离心力与密度梯度共同驱动了交换不稳定性,而偶极磁场曲率与熵梯度对交换模式起致稳作用. 当环向模数小于10时,交换不稳定性增长率随环向模数增大而增长;当环向模数大于10时,增长率趋于饱和. 交换不稳定性增长率随径向模数增大而减小. 理论分析获得的主导模式与数值模拟中增长率最大模式的环向模数(环向模数为m=13)接近. 与文献Newcomb(1961)和Ferrière等(1999)的理论结果对比显示,本文推导获得的理论结果与Ferrière理论模型的不稳定区域基本一致,但是与Newcomb理论模型给出的木卫一外侧全区域不稳定的结果有差异. 上述差异主要是因为Newcomb理论模型采用平板构型,未考虑弯曲磁场曲率制稳的影响. 对比分析显示,理论结果获得的交换不稳定性主导模式(环向模数13)的增长率与Ferrière理论模型增长率接近,这说明局域近似理论能较好地描述木星内磁层交换不稳定性增长过程. 与数值模拟结果相比,理论分析获得的主导模式增长率偏高约一个量级.

     

    Abstract: Interchange instability is widespread in both engineering applications and natural environments and is recognized as one of the primary mechanisms for radial mass transport within Jupiter's magnetodisk. In this study, the dispersion relation and the instability criteria of the interchange instability in Jupiter's inner magnetosphere are derived by considering the parameters of Jupiter’s inner magnetosphere (Feng et al., 2025) and applying the ideal magnetohydrodynamic (MHD) theory without assuming the local approximation (i.e., the perturbation wavelength is much smaller than the characteristic scale length). The mode number and growth rate of the dominant mode are obtained through theoretical analysis. Under the current parameters of Jupiter’s inner magnetosphere, the interchange mode is found to be stable within the Io plasma torus, whereas unstable regions exist outside the torus. The unstable occurrence regions are given by the theoretical model. It is also found that the centrifugal force and density gradient outside the Io plasma torus jointly drive the interchange instability, while the curvature of the dipole magnetic field and entropy gradient exhibit stabilizing effects on the interchange mode. The growth rate of instability increases with the azimuthal mode number for values below 10, beyond which it saturates. Additionally, the growth rate decreases with increasing radial mode number. Compared to theoretical results from Newcomb (1961) and Ferrière et al. (1999), our theoretical model predicts instability regions almost identical to those of Ferrière's theoretical model, but differs from Newcomb's theoretical model, which suggests that the entire region outside the Io torus is unstable. This difference mainly arises because Newcomb's theoretical model adopts a slab geometry, which neglects the stabilizing effect of the curved magnetic field. It also shows that the growth rate of the dominant interchange mode in our theoretical model is close to that of Ferrière's theoretical model, indicating that the local approximation is suitable for describing the interchange instability of Jupiter’s inner magnetosphere. However, the growth rate of the dominant mode in our theoretical model is approximately one order of magnitude higher than that of the numerical result.

     

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