Advances in Paleomagnetic Field Modeling: How does Earth's dipole grow and decay?

第1部分：对过去磁场的研究对地球科学的许多领域都很重要。偶极矩是我们抵御太阳风中高能粒子的主要屏障，控制着上层大气中宇宙成因同位素的产生。对过去偶极矩的估计用于推断宇宙成因同位素的产生，从而有助于古气候研究中太阳通量的代用物。它们还帮助我们了解与太空天气相关的预期危险程度。地磁场在时间和空间尺度上表现出很大的变化，但这里的重点是过去100 ka。从古地磁资料和考古地磁资料中得到的地磁时变特征表明，过去10天的地磁结构总体上具有地理不对称性，北半球的平均地磁较强，南半球的长期地磁变化较活跃。从大约公元前6000年到公元300年，偶极子有一个长期的增长，其中一个表现出特别强劲增长的时间段对应于黎凡特地区磁场强度的快速区域峰值的出现。自公元300年以来，轴向偶极子强度普遍从120 ZAm2左右的峰值下降到现在的80 ZAm2附近。黎凡特油田尖峰的区域范围和影响尚未完全阐明，类似的特征是否在其他地方和时间发生仍然是一个悬而未决的问题。这项提议有两个主要目标。第一个是探索类尖峰行为、偶极子生长和其他场性质之间的关系。第二是了解导致偶极矩减少间隔的变化，就像今天发生的那样，在以41 ka为中心的拉尚偏移期间以更戏剧性的形式发生。研究结果将有助于许多需要了解地磁场及其百年至百万年时间尺度变化的一般或专门知识的领域。EarthRef数字档案社区数据库中工具和产品的系统电子存档将使它们对其他美国和国际地磁和古磁学研究人员有用，并在多个层面上用于研究和教育目的。该项目将通过指导和培训研究生和本科生来加强未来的STEM活动和研究。第2部分：对数据不确定性的仔细分析和反演策略的改进使时变全球球谐古磁场模型在0-10 ka和0-100 ka时间范围内得到了极大的改进，这些模型在局部时间和空间复杂性方面受到限制，并且不能捕捉到黎凡特尖峰等特征的全部复杂性。然而，结果的鲁棒性表明它们可以作为对磁场动态结构演化的假设检验的基础。假设通量峰值起源于核-地幔边界的小尺度但大幅度的变化，可以量化在地球表面可以看到的变化的空间范围。0-10 ka和0-100 ka的古场模型将用于探索地磁偶极矩的两个变化时期：(i)围绕Laschamp偏移的20 ka的偶极矩衰减和随后的恢复，以及（ii）约公元前6000年至公元300年偶极矩的增长和过渡到现在场中看到的衰减。

Part 1: Studies of the past magnetic field are important to a broad range of the geosciences. The dipole moment is our primary shield against energetic particles in the solar wind, controlling the production of cosmogenic isotopes in the upper atmosphere. Estimates of past dipole moment are used to infer cosmogenic isotope production and hence contribute to proxies for solar flux in paleoclimate studies. They also help inform us about expected hazard levels associated with space weather. The geomagnetic field exhibits large changes on a range of temporal and spatial scales, but the focus here is on the past 100 ka. Time varying representations of the field derived from paleo and archeomagnetic data reveal an overall geographic asymmetry in field structure for the past 10 ky with stronger average fields in the northern hemisphere and more active secular variation in the southern hemisphere. There is a long-term increase in the dipole from about 6000 BC until 300 AD, and one interval exhibiting particularly strong growth corresponds to the rapid appearance of regional spikes in field strength in the Levantine region. Since 300 AD the axial dipole strength has generally declined from its peak of around 120 ZAm2 to the present day value near 80 ZAm2. The regional extent and impact of the Levantine field spikes has yet to be fully elucidated, and it remains an open question whether similar features occur at other places and times. This proposal has two major goals. The first is to explore the relationship between spike-like behavior, dipole growth, and other field properties. The second is to understand changes that lead to intervals of decreasing dipole moment as is happening today and occurred in more dramatic form during the Laschamp excursion centered about 41 ka. Results will contribute to numerous areas where general or specific knowledge is needed about the geomagnetic field and its variability on centennial to million year time scales. Systematic electronic archiving of tools and products in the EarthRef Digital Archive community database will make them useful to other US and international researchers in geo- and paleo-magnetism, and for research and educational purposes at multiple levels. This project will enhance future STEM activities and research through mentoring and training of both graduate and undergraduate students.Part 2: Careful analysis of data uncertainties and improvements in inversion strategies have resulted in greatly improved time varying global spherical harmonic paleomagnetic field models over the time spans from 0-10 ka and 0-100 ka, These models are limited in local temporal and spatial complexity, and do not capture the full complexity of features such as the Levantine spike. However, robust features in the results indicate they can be used as the basis for hypothesis testing about the dynamic structural evolution of the magnetic field. Assuming flux spikes originate as small scale but large amplitude changes at the core-mantle boundary allows quantification of the spatial extent over which changes should be visible at Earth's surface. Paleofield models for 0-10 ka and 0-100 ka will be used to explore two periods of change in geomagnetic dipole moment: (i) the dipole moment decay and subsequent recovery in a period of 20 ky surrounding the Laschamp excursion, and (ii) growth in dipole moment from about 6000 BC to 300 AD and transition to the decay seen in the present field.