Magnetic Field Variation Caused by Rotational Speed Change in a Magnetohydrodynamic Dynamo

Abstract

We have performed numerical magnetohydrodynamic dynamo simulations in a spherical shell with rotational speed or length-of-day (LOD) variation, which is motivated by correlations between geomagnetic field and climatic variations with ice and non-ice ages. The results show that LOD variation leads to magnetic field variation whose amplitude is considerably larger than that of LOD variation. The heat flux at the outer sphere and the zonal flow also change. The mechanism of the magnetic field variation due to LOD variation is also found. The keys are changes of dynamo activity and Joule heating.

Libration-driven multipolar instabilities

“…On the Earth, the change in polar ice sheets, between ice and non-ice ages, modifies the inertia tensor of the Earth, which leads to oscillations of the mantle rotation rate (see e.g. Miyagoshi & Hamano 2013, where the consequences on the Earth magnetic field are discussed). Note that these oscillations are called length-of-day (LOD) variations, rather than libration, for non-synchronized planets like the Earth.…”

Eccentricity-paced geomagnetic field and monsoon rainfall variations over the last 870 kyr

“…As for geomagnetic field, the main geomagnetic field is generated by movement of liquid iron in Earth’s outer core, a process known as the geodynamo ( 64 , 65 ). The thermochemical convection is the main power source in the core, but changes of Earth’s motion state play a role in controlling the patterns of convection, hence, the generation of the geomagnetic field ( 65 , 66 ). Earth’s eccentricity-paced ~100 kyr glacial cycles ( 54 ) lead to redistribution of global water between global oceans and polar ice sheets ( 61 , 67 ).…”

“…LOD correspondingly increases as these ice caps recede redistributing mass away from the poles. Decadal to millennial scale variations in Earth’s rotation rate (or LOD) are suggested to be linked to geomagnetic secular variation and geodynamo simulations suggest that small periodic changes in LOD can manifest as amplified variations in dipole field strength with a similar period ( 64 , 66 ). We speculate that the shifting glacial mass distribution over the eccentricity-paced glacial-interglacial cycles may have a sufficiently large impact on Earth’s rotation rate to manifest as a ~100 kyr signal in the geomagnetic field strength ( 11 , 61 ).…”

What causes geomagnetic reversals?

“…地磁倒转时, 虚拟地磁极(virtual geomagnetic pole, VGP)的移动路径更青睐美洲和东 亚地区 [27] , 这种情况也说明下地幔的热状态对地磁 场的控制作用, 因为这两个地区正位于太平洋板块 俯 冲带 , 对应 着下 地幔的 地震 波高速 区 (低 温异常 区). Amit和Olson [28] Yoshida 和 Hamano [31] 以 及 Miyagoshi 和 Hamano [32] 认为, 地球自转速度(日长)的改变也许是另一个触发 地磁倒转的原因. 他们发现地球自转速度变化可以 影响地磁场的西向漂移, 改变地磁场的强度, 并且地 磁 场 强 度 的 相 对 变 化 幅 度 比 自 转 速 度 变 化 更 大 .…”