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。过去偶极矩的估计被用来推断宇宙成因的同位素生产，因此有助于在古气候研究中替代太阳通量。它们还帮助我们了解与空间天气有关的预期危险程度。地磁场在一系列时间和空间尺度上表现出很大的变化，但这里的焦点是过去的100ka。由古地磁和古地磁资料得到的磁场的时变表示表明，过去10年的磁场结构总体上存在地理不对称，北半球平均场较强，南半球长期变化较活跃。从公元前6000年到公元300年，偶极子有一个长期的增加，其中一个增长特别强劲的区间对应于莱万丁地区场强的区域尖峰迅速出现。自公元300年以来，轴向偶极强度普遍从120Zam2左右的峰值下降到现在的80Zam2附近。莱万丁油田尖峰的区域范围和影响尚未完全阐明，类似的特征是否会在其他地点和时间出现也是一个悬而未决的问题。这项提议有两个主要目标。第一个是探索类棘波行为、偶极子生长和其他场特性之间的关系。第二是理解导致偶极矩递减间隔的变化，就像今天发生的，在以41ka为中心的拉尚漂移期间以更戏剧性的形式发生。结果将有助于许多领域需要关于地磁场及其在百年至百万年时间尺度上的变异性的一般或具体知识。对EarthRef数字档案馆社区数据库中的工具和产品进行系统的电子存档，将使它们对其他美国和国际地磁和古磁学研究人员以及多层次的研究和教育目的有用。该项目将通过对研究生和本科生的指导和培训来加强未来的STEM活动和研究。第二部分：对数据不确定性的仔细分析和反演策略的改进导致了从0-10ka和0-100ka的时间跨度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上受到限制，并且没有捕捉到诸如Levantine尖峰等特征的全部复杂性。然而，结果中的稳健特征表明，它们可以作为关于磁场动态结构演化的假设检验的基础。假设通量尖峰起源于小尺度，但在核-地幔边界的大幅度变化允许量化在地球表面应该可见的变化的空间范围。利用0-10ka和0-100ka的古场模型，探讨了地磁偶极矩的两个变化周期：(I)拉尚漂移前后20Ky周期内偶极矩的衰减和随后的恢复；(Ii)从公元前6000年到公元3000年的偶极矩的增长和向现今磁场中看到的衰变的过渡。

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.