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Secular variation in Coimbra (Portugal) since 1866

Authors: M. A. Pais and J. M. Miranda

Ref.: Journal of Geomagnetism and Geoelectricity 47, 267-282 (1995)

Abstract: We summarize the history of the Coimbra Observatory and identify the major discontinuities or gaps in the monthly mean values for the interval 1860 to 1990. Where possible, corrections are applied and gaps are filled to obtain a more complete time series. The Coimbra data must be spilt into two intervals (1873-1932 and 1951-1987) corresponding to different operating conditions. The external signal was removed from the geomagnetic series by the use of a deterministic model using sinusoidal terms to represent the annual and semi-annual fluctuations. and the aa index to characterize the effect of long-period external disturbing fields. The values obtained for the coefficients of this model are stable for the two intervals and support the conclusion that the disturbing field affects mainly the X-component with a negative contribution and the Y- and Z-components with small but positive contributions (as expected in moderate north latitudes). The remaining signal, thought to represent the core field and its secular variation (SV), was used to analyse the occurrence of geomagnetic jerks, and their associated time scale and amplitudes. The first differences of the ''internal'' signal display an approximately piecewise-linear structure, where the change of slope corresponds well to some of the previously proposed jerks. A comparison of the results obtained for Coimbra with existing studies for other european observatories shows similar features concerning the 196-70 jerk.