引力振荡的核-幔模态
The core-mantle mode of gravitational oscillation
- University of Alberta（阿尔伯塔大学）
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
本研究揭示一种新的地核-地幔引力振荡模态（CMG），其周期约40-100年，可能共振放大流体核驱动的日长多年代变化。
AI中文摘要：
我们展示了一种此前未被报道过的地球地幔、流体外核和固体内核之间的轴向振荡模态的特征。该模态涉及切线圆柱（TC）外部流体外核的准刚性旋转，通过三步过程与地幔交换角动量。首先，通过与TC内部流体的磁力矩作用；其次，通过后者与内核之间的磁力矩作用；最后，通过内核与地幔之间的引力力矩作用。尽管引力力矩纯粹作用于地幔和内核之间，该模态涉及整个核的振荡，我们将其称为核-幔引力（CMG）模态。这种引力振荡形式发生在地核内磁场足够强，使得阿尔文波的传播时间短于模态周期的情况下，而地球正是这种情况。我们展示了CMG模态的周期、品质因数$Q$和结构如何依赖于引力力矩的强度和内核的粘性松弛时间$\ au_i$。对于地球，CMG模态的周期应在40至100年范围内，但内核的粘性松弛可能意味着较小的$Q$，如果$\ au_i<10$年，则$Q$低于1。我们的结果表明，CMG模态可能通过共振放大（尽管幅度不大）由流体核内纬向加速度驱动的多年代尺度日长变化。
英文摘要：
We present the characteristics of a mode of axial oscillation between Earth's mantle, fluid core and solid inner core that has not been previously reported. The mode involves a quasi-rigid rotation of the fluid core outside the tangent cylinder (TC) exchanging its angular momentum with the mantle via a three step process. First, by a magnetic torque with the fluid inside the TC; second by a magnetic torque between the latter and the inner core; and finally by a gravitational torque between the inner core and mantle. Although the gravitational torque is purely between the mantle and inner core, the mode involves an oscillation of the whole of the core, and we refer to it as the core-mantle gravitational (CMG) mode. This form of gravitational oscillation occurs when the magnetic field within the core is sufficiently strong that the propagation time of Alfvén waves is shorter than the mode period, which is the case for Earth. We show how the period, quality factor $Q$ and structure of the CMG mode depends on the strength of the gravitational torque and viscous relaxation time $τ_i$ of the inner core. For Earth, the CMG mode period should be in the range of 40 to 100 years, but viscous relaxation of the inner core likely implies a small $Q$, below 1 if $τ_i<10$ years. Our results suggest that the CMG mode may act to amplify resonantly, though only modestly, multi-decadal changes in the length of day driven by zonal accelerations in the fluid core.
