外核黏性对地磁场发电机数值模型的影响

The effects of Earth′s outer core′s viscosity on Geodynamo models

地磁场发电机过程由数学方程组和输入参数所控制.为研究发电机参数对系统的影响,本文使用MoSST模型模拟了在外核黏性ν变化下发电机模型的输出.通过使用跨越近3个量级的ν值,着重研究了各物理场及其典型尺度随黏性的变化.发现黏性变化显著影响流场;磁场随ν增加而近乎单调减小,但变化幅度不超过30%;温度扰动随ν增加而小幅(6%)单调增加.经过拟合,得到外核流速u和黏性ν的比例关系:u~ν0.49.流速随黏性增加的现象本质上是由于黏性增加打破了Taylor-Proudman约束,使得临界Rayleigh数减小,从而在相同的驱动力下带来了流速的增加.此外,作用力平衡分析发现,随着ν的变化,系统在几种平衡模式间切换.通过与之前比例关系的研究对比,本研究支持在一定范围内,磁场与黏性关系不大的结论;但反对流速与外核黏性无关的假设.

Geodynamo process is controlled by mathematic equations and input parameters.To study effects of parameters on geodynamo system,MoSST model has been used to simulate geodynamo outputs under different outer core′s viscosityν.With spanningνfor nearly three orders when other parameters fixed,we studied the variation of each physical field and its typical length scale.We find that variation ofνaffects the velocity field intensely.The magnetic field almost decreases monotonically with increasing ofν,while the variation is no larger than 30%.The temperature perturbation increases monotonically withν,but by a very small magnitude（6%）.The averaged velocity field（u）of the liquid core increases withνas a simple fitted scaling relation：u~ν0.49.The phenomenon that uincreases withνis essentially that increasing ofνbreaks the Taylor-Proudman constraint and drops the critical Rayleigh number,and thus u increases under the same thermal driving force.Forces balance is analyzed and balance modeshifts with variation ofν. When compared with former studies of scaling laws,this study supports the conclusion that in a certain parameter range,the magnetic field strength doesn′t vary much with the viscosity,but opposes to the assumption that the velocity field has nothing to do with the outer core viscosity.