GEOMAGNETIC-FIELD ANALYSIS .5. DETERMINING STEADY CORE-SURFACE FLOWS DIRECTLY FROM GEOMAGNETIC OBSERVATIONS

Geomagnetic secular variation is a result of inductive effects of flui d motion at the top of the Earth's liquid core, which can be mapped us ing the frozen-flux approximation and inverting mathematical models of the secular variation. Nonuniqueness may be removed by assuming the f low to be stationary. The use of an intermediate mathematical model of secular variation makes comparison of the derived models with observa tion unsatisfactory, and here we devise a new method, inverting the ge omagnetic measurements directly to obtain steady core motion. The meth od is applied to observatory annual mean data for the 30 yr period 196 0-1990 to produce six separate models of steady flow: three that are s tationary over 10 yr intervals (1960-1970, 1970-1980 and 1980-1990), t wo 20 yr models (1960-1980 and 1970-1990), and one 30 yr model (1960-1 990). 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 o f 1.0-1.3, the 20 yr models fit less well, and the 30 yr model has a m isfit of 1.7 when constrained to have the same norm as the 10 yr model s. All models appear to fit the central decade with a gross misfit clo se to unity, suggesting that changes in core motion may be associated with the two geomagnetic jerks that occurred around 1969 and 1978. Fur thermore, close inspection of the fits at individual observatories sho ws 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 mo st observatories, but cannot follow the detailed time variations of th e past three decades.