Multiple jets in a rotating annulus model with an imposed azimuthal magnetic field

Zonal flows driven by rotation and convection are found commonly within astrophysical and geophysical bodies. Multiple jets are most famously observed on the surface of Jupiter, with prograde and retrograde zonal flows making up part of its banded structure of zones and belts. Non-magnetic studies of convection in a rotating annulus model have previously been shown to produce multiple jet structures, similar to those observed in atmospheres of giant planets such as Jupiter. These giant planets have strong magnetic fields which impact on convection in the electrically conducting regions of their atmospheres. Therefore the effect of a magnetic field on the multiple jet solutions of the annulus model is of astrophysical interest. In this work we impose a uniform azimuthal magnetic field to the annulus model and vary its strength and other control parameters to determine the parameter space where multiple jets exist. Zonal flows and multiple jet solutions are found at weak magnetic field strength and, indeed, the magnetic field can promote the production of these features. Simulations with a weak magnetic field or no magnetic field are also found to match well with the Rhines scaling theory. For cases with multiple jets, a strong inertial force must be present, combined with either a weak Lorentz force, or a Lorentz force entering the main balance with increased contributions from the viscous force. At strong magnetic field strengths two regimes are found. For weak magnetic diffusion zonal flows can be retained, albeit without the multiple jet structure. For larger magnetic diffusion non-axisymmetric solutions without clear bands are found. In these regimes, the Lorentz force appears in the primary balance of forces and zonal flows are only retained at strong driving when inertia is able to also enter the balance at leading order.

由自转与对流驱动的纬向流，普遍存在于天体物理与地球物理天体之中。其中尤以木星表面观测到的多喷流结构最为知名：顺行与逆行的纬向流共同构成了其特有的条带（区与带）结构。此前已有研究表明，针对旋转环形腔模型的非磁性对流分析，可生成与木星等巨行星大气中观测到的类似的多喷流结构。这类巨行星拥有强磁场，会对其大气导电区域内的对流过程产生影响。因此，磁场对环形腔模型多喷流解的调控效应，具备重要的天体物理研究价值。本研究通过向环形腔模型施加均匀的方位角磁场，并调整其强度与其他控制参数，以此探究能够生成多喷流结构的参数空间。在弱磁场环境下可观测到纬向流与多喷流解，且磁场确实能够促进这类结构的形成。无磁场或弱磁场的模拟结果，与莱茵斯标度理论（Rhines scaling theory）吻合度极佳。在存在多喷流结构的工况中，体系需具备强惯性力，同时搭配弱洛伦兹力，或是让洛伦兹力与粘性力贡献提升后共同主导受力平衡。当磁场强度较高时，存在两种不同的物理机制区域：在磁扩散较弱的区域，仍可保留纬向流结构，但不再具备多喷流特征；而在磁扩散较强的区域，则会出现无清晰条带的非轴对称解。在这类区域中，洛伦兹力成为主导受力项，且仅当强驱动条件下惯性力也能以主导阶次参与受力平衡时，纬向流结构才会得以保留。