Historic and paleomagnetic secular variation and the Earth's core dynamo process

This paper attempts to summarize briefly the recent and ongoing efforts of the geomagnetism and paleomagnetism community to understand both the earth's magnetic‐field secular variation, and its implications for the core dynamo process. Secular variation is defined in this paper as the general spatio‐temporal variation of the field; secular change will refer to the first time derivative of the spatial field (strict geomagnetic definition of secular variation). We will emphasize here the long‐term (10¹ year or longer) behavior of the historic secular variation (HSV), for this is the variation that is comparable with the paleomagnetic secular variation (PSV). Two important related topics, the 1969/1970 geomagnetic ‘jerk’ in secular acceleration and the spatial geomagnetic field components due to crustal or mantle sources, are treated more fully in other reviews.