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

第一部分：对过去磁场的研究对广泛的地球科学都很重要。偶极矩是我们抵御太阳风中高能粒子的主要屏障，控制着高层大气中宇宙成因同位素的产生。过去的偶极矩的估计被用来推断宇宙成因的同位素生产，从而有助于代理太阳通量在古气候研究。它们还帮助我们了解与空间天气相关的预期危险水平。地磁场在时间和空间尺度上都有很大的变化，但这里的重点是过去100 ka。从古地磁和古地磁资料中得到的磁场随时间变化的描述揭示了过去10 ky磁场结构的总体地理不对称性，北方半球的平均磁场较强，南半球的长期变化较活跃。从公元前6000年到公元300年，偶极子有一个长期的增长，一个时间段表现出特别强的增长对应于地中海东部地区场强的区域尖峰的快速出现。自公元300年以来，轴向偶极子强度一般从其峰值约120 ZAm 2下降到目前的值接近80 ZAm 2。 黎凡特场尖峰的区域范围和影响尚未完全阐明，而且在其他地方和时间是否发生类似的特征仍然是一个悬而未决的问题。这项建议有两个主要目标。第一个是探索尖峰状行为，偶极子的增长，和其他领域的属性之间的关系。第二个是了解导致像今天这样的偶极矩减少间隔的变化，以及在大约41 ka的Laschamp偏移期间以更戏剧性的形式发生的变化。 结果将有助于许多领域的一般或具体的知识，需要有关地磁场及其变化百年到百万年的时间尺度。 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其对其他美国和国际地质和古磁学研究人员有用，并用于多层次的研究和教育目的。该项目将通过对研究生和本科生的指导和培训，加强未来的STEM活动和研究。对数据不确定性的仔细分析和反演策略的改进导致了0-10 ka和0-100 ka时间跨度上的时变全球球谐古地磁场模型的极大改进。这些模型在局部时间和空间复杂性方面受到限制，并且不能捕获诸如黎凡特尖峰的特征的全部复杂性。然而，在结果中的强大功能表明，它们可以作为假设检验的基础上的动态结构演化的磁场。 假设通量峰值起源于小规模，但大幅度的变化，在核幔边界允许量化的空间范围内的变化应该是可见的地球表面。将使用0-10 ka和0-100 ka的古场模型来探索地磁偶极矩变化的两个时期：（i）在Laschamp偏移周围的20 ky期间，偶极矩衰减和随后的恢复，以及（ii）从大约公元前6000年到公元300年的偶极矩增长和过渡到现在的磁场中看到的衰减。

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.