ISSN:
1365-246X
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
Notes:
Geomagnetic secular variation is a result of inductive effects of fluid motion at the top of the Earth's liquid core, which can be mapped using the frozen-flux approximation and inverting mathematical models of the secular variation. Non-uniqueness may be removed by assuming the flow to be stationary. The use of an intermediate mathematical model of secular variation makes comparison of the derived models with observation unsatisfactory, and here we devise a new method, inverting the geomagnetic measurements directly to obtain steady core motion. The method is applied to observatory annual mean data for the 30 yr period 1960–1990 to produce six separate models of steady flow: three that are stationary over 10 yr intervals (1960–1970, 1970–1980 and 1980–1990), two 20 yr models (1960–1980 and 1970–1990), and one 30 yr model (1960–1990). The method is not restricted to continuous time series or linear (X, Y, Z) data. The 10 yr models fit the weighted data with misfits of 1.0–1.3, the 20 yr models fit less well, and the 30 yr model has a misfit of 1.7 when constrained to have the same norm as the 10 yr models. All models appear to fit the central decade with a gross misfit close to unity, suggesting that changes in core motion may be associated with the two geomagnetic jerks that occurred around 1969 and 1978. Furthermore, close inspection of the fits at individual observatories shows that steady core motions with this norm cannot produce the secular accelerations required at a large number of high-quality observatories. Steady flows can predict magnetic fields that follow the trend at most observatories, but cannot follow the detailed time variations of the past three decades.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-246X.1995.tb03556.x
