Verification of the geomagnetic secular variation models based on laboratory experiments and numerical simulations.

本文在室内实验和数值模拟的基础上，对几种地磁长期变化模型进行了验证。本项目包括：(1)观测地磁场在不同时间尺度上的变化；(2)对液态金属中热对流的湍流特性进行室内实验；(3)对导电流体壳中磁场的产生进行数值模拟。(1)在地磁场观测中，对伊豆大岛火山和富士山的历史熔岩流、日本中海和埃及喀伦湖的湖泊沉积物进行了古地磁研究，估计了近几千年来的地磁长期变化。(2)在室内实验中，研制了一种利用超声测速技术观察金属液层流体运动的新装置。利用新研制的仪器，继续对高瑞利数下的液镓层湍流对流运动进行了观测。在此基础上，我们发现了在瑞利数一定范围内对流模式的准周期性波动。图案的振荡频率随瑞利数的变化而变化，其中瑞利数越高，频率越高。(3)利用地球模拟器对高分辨率地球动力学模拟结果的检验表明，可以通过地磁场的时间变化推断流体壳内的湍流状态。在地球表面观测到的。将观测结果、室内实验结果和数值模拟结果综合起来，可以推断地磁场的起源及其变化。

Purpose of the present research is to verify several models for the geomagnetic secular variations based on laboratory experiments and the numerical simulations. This project consists of (1)observation of the variations of the geomagnetic fields in various time scales, (2)laboratory experiments on the turbulent properties of thermal convection in liquid metal, and (3)numerical simulations of generation of the magnetic field in a conducting fluid shell. (1)In the geomagnetic field observations, paleomagnetic studies on the historical lava flows from Izu-Oshima volcano and Mt.Fuji, and the lake sediments from Nakaumi, Japan and Lake Qarun, Egypt, were performed and the geomagnetic secular variations during the past several thousand years were estimated. (2)In the laboratory experiments, a new apparatus was developed for observing fluid motions in liquid metal layer by using the ultrasonic velocimetry technique. Observations of turbulent convective motions in liquid Gallium layer at high Rayleigh numbers have been continued by using the newly developed apparatus. Based of the present experiments, we found the quasi periodic fluctuation of convection patterns at a certain range of the Rayleigh number. The oscillation frequency of the patterns varies as a function of the Rayleigh numbers, where the higher frequency was observed at the higher Rayleigh number. (3)Examination of the results of the high resolution geodynamo simulations using the Earth Simulator revealed that the turbulent state in the fluid shell can be inferred from the temporal variation of geomagnetic fields. observed at the earth's surface. The results of the observations, laboratory experiments, and the numerical simulations can be integrated to infer the origin of the geomagnetic field and its variations.