Electrical Conductivity of the Upper Mantle Below Europe Inferred From Long Period Geomagnetic Observations

Geomagnetic variations from world magnetic observatories for the period range 8 hours to 360 days have been recently interpreted in terms of depth-dependent conductivity by Praus et al. (2004). The six daily harmonics were obtained by the analysis of data from the minimum solar activity year 1965. For periods longer than one day, time series from high (1958-60) and low (1963-5) solar activity years were used. The results suggest increase in conductivity by two orders of magnitude at depths around 700 km. As the observatory distribution in the database is heavily biased towards Europe, the formulation of the global EM induction problem by Martinec (1999) is used to interpret the observations in terms of 3-D regional electrical conductivity structure below Europe.