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上海交通大学自然科学研究院最新成果:恒星潮汐及地球磁场

近期,天体物理和流体力学的主流杂志发表了上海交通大学自然科学研究院魏星老师在恒星潮汐和地磁场方面的工作。

关于恒星潮汐的响应和耗散的研究中,以前的研究只考虑了旋转和分层的影响,而未考虑磁场的影响;只考虑了线性波,而未考虑非线性波。ApJ(2016, 828:30)文章指出磁场在潮汐作用中的重要性:即便很弱的磁场也可以引起一阶扰动的磁流体波很强的响应和耗散。JFM(2016, 796:306-317)文章指出非线性在潮汐作用中的重要性:非线性惯性力在共振频率附近可以极大地抑制潮汐的响应和耗散。这两篇文章指出,磁场和非线性在潮汐问题的研究中都要考虑。

关于地磁场如何产生的研究中,以前的研究侧重对流引起的发电机效应,而近年来,进动的影响开始被关注。APJ(2016, 827:123)文章指出进动和对流同时存在时,通过三波共振机制可以使得流动变得更加不稳定,从而会更加容易产生地磁场。这篇文章指出,进动在地球磁场产生过程中是很重要的驱动力。

参考文献 Wei, X. “Calculating rotating hydrodynamic and magnetohydrodynamics waves to understand magnetic effects on dynamical tides”. Astrophysical Journal, 828: 30. 2016
Wei, X. “Linear and nonlinear responses to harmonic force in rotating flow”. Journal of Fluid Mechanics, 796: 306-317. 2016
Wei, X. “The combined effect of precession and convection on dynamo action”. Astrophysical Journal, 827: 123. 2016

In the aspect of astrophysical fluid, Xing found that magnetic effect broadens the range of tidal resonance and Ohmic dissipation can be stronger than viscous dissipation even at a weak magnetic field and that nonlinear effect suppresses the tidal response and dissipation near tidal resonance. This work suggests that magnetic fields and nonlinearity, both of which are neglected in the previous studies, are significant for dynamical tides. In the aspect of geophysical fluid, Xing found that the Earth’s precession, in addition to convection in the fluid core, can generate magnetic fields (precession dynamo) and interact with convection through triad resonance to facilitate the onset of dynamo instability. This work suggests that precession is significant for the Earth’s magnetic field.