Study of the geomagnetic variation and its relation to climate change

我们研究了地球磁场强度在很宽的时间尺度上的变化，并研究了它与气候变化的关系。对于太古代至元古代的古地震强度，我们从加拿大、澳大利亚和非洲的稳定火山岩中采集了样品，并用Thellier方法进行了古地震强度测量。结果表明，近20亿年来古地磁强度的平均值与现今地磁强度相当接近，而25亿年前的地磁强度变化很大。在过去的20万年里，观测到了几次短暂的强度增加或减少。这些变化是由于偶极矩的变化。强度下降到目前的1/10左右，对应于磁场的漂移时间，并且已知与气候变化有关。另一方面，强度的瞬时增加与气候变化之间的关系并不明显。在过去的2000年里，我们获得了日本场强变化的标准曲线。在这个时间尺度上，磁场强度的变化主要受地磁场非偶极分量变化的控制，尽管在500 AC左右磁场强度的突然增加可能是由于偶极矩的变化。对于磁场强度的近期变化，我们提出了一种新的方法，通过球谐分析将观测磁场分解为核心起源的外部分量、感应分量和内部分量，并确定了几年时间尺度的核心磁场的短周期变化。

We studied the variation of the intensity of the Earth's magnetic field for a wide range of time scale, and examined its relation to climate change. For the paleointensities of the Archean to Proterozoic period, we collected samples from the stable cratons in Canada, Australia, and Africa and the paleointensity measurements were made by the Thellier's method. The results indicate that the average value of paleointensity during the last 2 billion years are rather similar to the geomagnetic intensity of the present field, whereas the variation of the intensity around 2.5 billion years ago was found to be very large. During the last 200,000 years, several transient increases or decreases of the intensity were observed. The variations are attributed to the change of the dipole moment. The decreases of the intensity to about 1/10 of the present field corresponds to the excursion time of the magnetic field, and are known to relate to climate change. On the other hand, the relation between the transient increases of the intensity and the climate change is not evident. For the last 2000 years, we obtained a standard curve of the field intensity variation in Japan. The variation of the field intensity for this time scale is mainly controlled by the change of the non- dipole component of the geomagnetic field, although an abrupt increase of the field intensity at about 500 AC is probably due to the change of the dipole moment. As for the recent variation of the field intensity, we developed a new method for decomposing the observed magnetic field into the external, induced, and internal component of core origin by using spherical harmonic analysis, and short period variations of core field with a time scale of several years are identified.