Targeted palaeomagnetic studies and geomagnetic field modelling to probe dynamics of the geodynamo

地球的磁场是由外核（地球发电机）中的流体运动产生的。虽然我们在地球表面测量的磁场可以近似于地球中心的条形磁铁（大致与地理北极和南极对齐），但实际上要复杂得多。该领域在许多不同的时间尺度上变化（例如，年，几十年，世纪，千年）。通过研究地磁场的变化，这告诉我们地球深处的磁场产生的地方。因此，地表测量和由此衍生的模型为深入地球提供了一个窗口。在这个项目中，我们建议调查的核心动力学和核幔相互作用的影响，影响地磁场的千年至百万年的时间尺度。虽然现在的磁场通过卫星和直接测量非常有名，但为了了解过去的变化，我们必须查看燃烧的考古和地质材料中包含的记录-古地磁学研究。问题是古地磁数据不像直接观测那样在地球仪范围内数量众多或分布均匀。事实上，大多数古地磁数据来自北方，并且方向性数据远远多于磁场强度（古强度）数据，这使得模型的建模和地球物理推断更具挑战性。随着时间的推移，一致的地磁场特征被解释为地幔对地球发电机的控制。在当今和过去400年的场模型中清楚地看到的两个特征是：在核心地幔边界的模型中看到的关于赤道对称的高纬度四个高强度通量斑块;其次，在地球表面的太平洋（与大西洋相反）半球观察到低长期变化。在这个项目中，我们将使用一个有针对性的收购古地磁数据和地磁建模方法相结合的方法，以确定：通量瓣对称的赤道？它们的生命力有多强，寿命有多长？太平洋半球长期低变化的持续性如何，其程度如何？这将允许确定这些特征是地磁场的临时特征还是基本特征。通量瓣被假定为整个核心过程的签名，而不是本地化的核心表面，通过确定对称性之间的北方和南半球，这将被测试。我们将从西南太平洋地区生成高质量的古地磁数据（如果可能的话，方向和强度;否则，集中在古强度）。西南太平洋已成为目标，因为它远离夏威夷，目前大多数太平洋数据都来自夏威夷，直接受到其中一个通量瓣的影响，目前是一个非常高的非偶极场区域。我们将研究来自西南太平洋岛屿的陶瓷，来自新西兰和澳大利亚的火山岩，集中在过去10，000年的材料上，但也包括5 Ma的材料。将进行详细的研究，以确定样品对古强度实验的适用性，因为虽然该方法背后的基础在实践中很简单，但有许多因素可能破坏或偏差实验。相关项目的学生将研究不同的古强度方法，包括使用单晶，而不是通常的整个岩石样品。将我们新的古地磁数据与存档数据相结合，构建地磁场模型。我们将测试是否古地磁数据允许，一个模型可以科普，一些额外的约束，如合成数据，有对称的通量瓣。生成的古地磁数据和新的场模型不仅对那些对地球磁场和地球深部过程感兴趣的人有用，而且对地球物理学家、气候学家和考古学家也有用。

The Earth's magnetic field is generated by fluid motions in the outer core (the geodynamo). Whilst the magnetic field we measure at the Earth's surface can be approximated to that of a bar magnet at the centre of the Earth (roughly aligned with the geographic North and South Pole) it is in fact much more complicated than this. The field varies on many different timescales (e.g. years, decades, centuries, millennia). By studying variations in the geomagnetic field this tells us about the deep Earth where the field is generated. Thus surface measurements, and models derived from them, provide a window in to the deep Earth. In this project we propose to investigate dynamics of the core and the influence of core mantle interactions that affect the geomagnetic field on the thousand to million year timescale. Whilst the present day field is very well known from satellite and direct measurements, in order to see variations in the past we have to look at the record contained in burnt archaeological and geological material - the study of palaeomagnetism. The problem is that palaeomagnetic data are not as numerous or as well distributed over the globe as direct observations. In fact, most palaeomagnetic data are from the northern hemisphere and there are far more directional than field strength (palaeointensity) data which makes modelling and geophysical inferences from the models more challenging. Consistent features of the geomagnetic field seen over time have been interpreted as mantle control on the geodynamo. Two features clearly seen in field models of the present day and over the last 400 years are: four high intensity patches of flux at high latitudes symmetric about the equator seen in models at the core mantle boundary and; secondly, low secular variation seen in the Pacific (as opposed to the Atlantic) Hemisphere at the Earth's surface. In this project we will use a combined approach of targeted acquisition of palaeomagnetic data and geomagnetic modelling methods in order to determine: are flux lobes symmetric about the equator? How dynamic and long lived are they? How persistent and what is the extent of low secular variation in the Pacific Hemisphere? This will allow determination of whether these features are temporary or fundamental characteristics of the geomagnetic field. Flux lobes are assumed to be signatures of whole core processes rather than localised at the core surface; by determining symmetry between northern and southern hemispheres this will be tested. We will generate high quality palaeomagnetic data (directions and intensity if possible; otherwise, concentrating on palaeointensity) from the SW Pacific region. The SW Pacific has been targeted as it is away from Hawaii where most Pacific data currently stems, is directly influenced by one of the flux lobes and is currently a region of very high non dipole field. We will study ceramics from the SW Pacific Islands, volcanic rocks from New Zealand and Australia concentrating on well dated material from the last 10,000 years but also including less well dated material up to 5Ma. Detailed studies will be carried out to determine the suitability of samples for palaeointensity experiments as whilst the basis behind the method is simple in practise there are many factors that can ruin or bias experiments. An associated project student will investigate different palaeointensity methods including using single crystals as opposed to the usual whole rock samples. Combining our new palaeomagnetic data with archived data, geomagnetic field models will be constructed. We will test whether the palaeomagnetic data allow, and a model can cope with, some additional constraint such as synthetic data which does have symmetric flux lobes. The generated palaeomagnetic data and the new field models will be of use not only to those interested in the Earth's magnetic field and deep Earth processes but also geophysicists, climatologists and archaeologists.