5.05-geomagnetic secular variation and its applications to the core

摘要: The purpose of this chapter is to review the origins of our current knowledge of the secular variation of the Earth’s magnetic field, that is, the slow changes that occur on timescales of years to centuries. There is clearly overlap with the description of the present geomagnetic field (Chapters 5.02 and 5.09), which treats changes in the field from centuries to millennia. The source of our knowledge on the timescales we are concerned with is primarily direct historical observations of the field; we review the available data, followed by the treatment of the data to generate mathematical models of the field in space and time. We then discuss interpretations of these models in terms of some of the physical processes occurring at the core surface. We stop short of describing the actual calculation of models of fluid flow at the core surface, as this is covered in detail in Volume 8, but we lay the groundwork by developing an exposition of the governing equations and the approximations that are frequently used.