Asymmetries in Geomagnetic Field Structure and Variability

地磁场结构和变化的不对称性

• 批准号: 1246826
• 负责人: Catherine Constable
• 金额: $ 27万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-01-01 至 2015-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1246826&HistoricalAwards=false
• 关键词: Asymmetries; Geomagnetic; Field; Structure; Variability

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.

地磁场的主要来源是由地球液态外核中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，有助于保持大气的挥发性部分，并保护地球表面免受太空天气的最严重影响。 对公元1590年以来地磁场的直接观测显示，地磁场的时间变化具有显著的地理变异性。南半球比北半球更活跃，自世纪中期以来，强烈的变化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。 该提案将通过绘制过去10万年来随时间变化的地磁场来研究内部磁场的空间和时间变化。 这将使用从高积累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。 拟议的工作是重要的，在广泛的地球物理学科，解决古地磁工作中使用的基本假设，提供具体相关的结果，地球发电机建模，一般地震，地球化学和矿物物理研究的核幔边界区域和深部内部。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方面提供重要服务，以及提供地磁场模型，动画和相关产品。现有的一万年前的磁场研究表明，南半球的地磁变化比北半球大，平均磁场强度较低。这种不对称性在较长时间尺度上的持久性尚未得到证实，尽管有迹象表明它发生在熔岩流记录中，可追溯到2 Ma。持续的不对称性将对广泛应用于古地磁学和板块构造重建的地心轴偶极子假说产生重要影响。 目前的时变全球场模型仅限于轴向偶极子场的变化或Laschamp偏移（40 ka）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，偶极子的变化表现出一个有趣的时间不对称的增长和衰减率的地磁偶极子在超过25千公里的周期，这表明可能的情节期的亚临界发电机活动的领域是由扩散过程为主，其次是短暂的强增长的轴向偶极子。 建立了0-100 ka的时变地磁场模型。 数据和模拟结果将用于（1）测试长期变化和时间平均场的半球不对称性是否在该时间尺度上持续存在;（2）以比目前从PADM 2 M获得的更好的时间分辨率对轴向偶极子的增长与衰减速率的差异进行详细研究;（3）提高对1-50~ky周期的地磁功率谱的认识。