Investigation of the Causes of Geomagnetic Dipole Moment Change

本研究将数值发电机模型和发电机理论与现今地磁和古地磁测量相结合，探讨地磁偶极子在地核变化的起源。偶极矩在短期（10-1000年）、中期（1-10万年）和长期（1-10亿年）时间尺度上的变化正在分别进行研究。在与研究生的合作中，PI正在描绘和解释观测到的在核-幔边界上的短期地磁场变化，这些变化是当今地磁偶极矩快速减少的原因，偶极倾斜角的减少更为迅速，以及这些正在进行的磁场变化与流体外核一般循环之间的关系。特别注意了在地核-地幔边界上具有反向磁场的区域的生长和运动，包括北大西洋和北美东海岸下面正在发展的一些反向磁场区域，它们在减小当今的偶极矩和偶极倾斜中起着重要作用。这些磁结构提供了关于当今发电机波动原因的实时证据，以及关于地磁极性反转如何开始的当代证据。PI正在使用数值发电机模型来研究1-10万年中间时间尺度偶极矩振荡的原因，这种振荡在古地磁记录中无处不在。本研究检验了古磁偶极子振荡起源的各种理论，包括波动理论、流动振荡理论和进动理论。利用高分辨率数值发电机模型研究了极长时间尺度（1-10亿年）地磁场变化与地核能量平衡之间的联系，以推导出时间平均偶极矩与发电机能量源之间的比例关系。然后将这些尺度关系外推到地球深处的极端条件，将地磁偶极矩在地质时间尺度上的缓慢变化与控制地核演化的基本物理和化学过程联系起来，包括固体内核的形成和生长，放射性同位素衰变的内部加热，以及地幔羽流从地核中带走热量。

This research combines numerical dynamo models and dynamo theory with measurements of the present-day geomagnetic and the ancient paleomagnetic fields to investigate the origins of geomagnetic dipole change in Earth's core. Separate studies of dipole moment variations over short (10-1000 year), intermediate (1-100 thousand year), and long (1-1000 million year) time-scales are being conducted. In collaboration with graduate students, the PI is delineating and interpreting the observed short-term geomagnetic field changes on the core-mantle boundary that are responsible for the present-day rapid decrease of the geomagnetic dipole moment, the even more rapid decrease of the dipole tilt angle, and the relationship between these on-going magnetic field changes and the general circulation in the fluid outer core. Special attention is given to the growth and movement of regions on the core-mantle boundary with reversed magnetic field, including some reversed field regions now developing beneath the North Atlantic and the east coast of North America, which are playing major roles in reducing the present-day dipole moment and the dipole tilt. These magnetic structures provide real-time evidence on the causes of present-day dynamo fluctuations, as well as contemporary evidence on how geomagnetic polarity reversals are initiated. The PI is using numerical dynamo models to investigate the causes of the 1-100 thousand year, intermediate time-scale dipole moment oscillations that are ubiquitous in the paleomagnetic record. This research tests the competing theories for the origin of these paleomagnetic dipole oscillations, including wave, flow vacillation, and precession theories. The connection between geomagnetic field changes on very long time-scales (1-1000 million years) and the energy balance of the Earth's core is being investigated using high-resolution numerical dynamo models to derive scaling relationships between the time-average dipole moment and dynamo energy sources. These scaling relationships are then extrapolated to the extreme conditions of Earth's deep interior to connect the slow change of the geomagnetic dipole moment over geological time-scales to fundamental physical and chemical processes that control the evolution of the core, including the formation and growth of the solid inner core, internal heating by decay of radiogenic isotopes, and the removal of heat from the core by mantle plumes.