Asymmetries in Geomagnetic Field Structure and Variability

地磁场的主要来源是由地球液态外核的流体运动产生的自持发电机。这个磁场提供了对太阳风的保护，有助于保留大气中易挥发的部分，并保护地球表面免受太空天气的最坏影响。对公元1590年以来地磁场的直接观测表明，其时间变化具有显著的地理变异性。南半球比北半球更活跃，自19世纪中期以来，强变率特别集中在被称为南大西洋磁异常的区域周围，引发了有关其寿命、与长期地磁场强度的关系以及对空间天气和低地球轨道卫星辐射暴露的后果的问题。本提案将通过绘制过去10万年的时变地磁场来研究内部磁场的时空变化。这将利用从高堆积率沉积物和火山岩中提取的全球分布的古地磁记录来完成。这项工作在广泛的地球物理学科中具有重要意义，解决了古地磁工作中使用的基本假设，提供了与地球动力学建模，以及核-地幔边界区域和深部内部的一般地震学，地球化学和矿物物理学研究的具体相关结果。PI在培训和指导年轻科学家方面有着良好的记录，并计划继续在开发社区数据库领域提供重要的服务，以及提供地磁场模型、动画和相关产品。现有的实地研究可以追溯到1万年前，表明南半球的地磁变异性比北半球大，平均磁场强度也较低。这种不对称在更长的时间尺度上的持久性还没有得到证实，尽管有迹象表明它发生在2 Ma的熔岩流记录中。持续的不对称性将对古地磁和板块构造重建中广泛使用的地心轴向偶极子假说产生重要影响。目前10~ka以外的时变全球场模型仅限于轴向偶极子场变化或拉尚偏移（40ka）和最近的反转（780 ka）周围的短片段，不能用于研究潜在的长期空间不对称性。然而，偶极子的变化在超过25ky的周期内显示出一种有趣的时间不对称性，即地磁偶极子的生长和衰减率，这表明可能存在亚临界发电机活动的偶发性周期，在此期间磁场主要由扩散过程主导，随后是轴向偶极子的短暂强生长期。建立了0 ~ 100 ka的时变地磁场模型。数据和建模结果将用于(1)测试半球不对称的长期变化和时间平均场是否在这个时间尺度上持续存在；(2)以比PADM2M更好的时间分辨率，对轴向偶极子的生长和衰减率差异进行详细研究；(3)提高对1 ~ 50~ky周期地磁功率谱的认识。

The main source of the geomagnetic field is a self-sustaining dynamo produced by fluid motions in Earth's liquid outer core. This magnetic field provides protection from the solar wind, helps retain volatile parts of the atmosphere, and protects Earth's surface from the worst impacts of space weather. Direct observations of the geomagnetic field back to 1590 AD show significant geographic variability in its time variations. The southern hemisphere is more active than the north, and since the mid nineteenth century strong variability has been particularly focused around the region known as the South Atlantic Magnetic Anomaly prompting questions about its longevity, relation to long term geomagnetic field strength, and consequences for space weather and radiation exposure for low-earth orbiting satellites. This proposal will study spatial and temporal changes in the internal magnetic field by mapping the time-varying geomagnetic field over the past 100 thousand years. This will be accomplished using globally distributed paleomagnetic records drawn from high accumulation rate sediments and from volcanic rocks. The proposed work is of importance in a broad range of geophysical disciplines, addressing fundamental assumptions used in paleomagnetic work, providing results of specific relevance to geodynamo modeling, and to general seismological, geochemical, and mineral physics studies of the core-mantle boundary regions and deep interior. The PI has a track record of training and mentoring junior scientists, and plans to continue significant service in the area of developing community databases, as well as making available geomagnetic field models, animations, and related products. Existing field studies extending back 10 thousand years show greater geomagnetic variability in the southern hemisphere than in the north, and lower average field strength. The persistence of this asymmetry on longer time scales has not been confirmed, although there are hints that it occurs in lava flow records back to 2 Ma. Persistent asymmetry would have important consequences for the geocentric axial dipole hypothesis widely used in paleomagnetism and plate tectonic reconstructions. Current time-varying global field models extending beyond 10~ka are limited to axial dipole field variations or to short snippets surrounding the Laschamp excursion (at 40ka) and the most recent reversal (780 ka), and cannot be used to study potential long term spatial asymmetries. However, dipole variations show an interesting temporal asymmetry in growth and decay rates for the geomagnetic dipole at periods longer than 25 ky, suggesting the possibility of episodic periods of subcritical dynamo activity where the field is dominated by diffusive processes, followed by transient periods of strong growth of the axial dipole. A time-varying geomagnetic field model for the period 0-100 ka will be constructed. Data and modeling results will be used to (1) test whether hemispheric asymmetry in secular variation and the time-averaged field persists on this time scale; (2) conduct a detailed study of differences in growth versus decay rates for the axial dipole at better temporal resolution than currently available from PADM2M; (3) improve knowledge of the geomagnetic power spectrum at 1-50~ky periods.