GEOMAGNETIC SECULAR VARIATION IN CENTRAL MEXICO SINCE 1923 AD AND COMPARISON WITH 1945-1990 IGRF MODELS

Observatory data for temporal variations of the geomagnetic field in central Mexico are analyzed for the period since 1923 AD. Annual mean values of the intensity have decreased from 46,076 nT to 42,832 nT at a linear-regression rate of about 48 nT per year. The geomagnetic field direction has been varying from easterly to westerly declinations and towards steeper inclinations. During an early interval from 1923 to 1938, changes were mainly in inclination, this was followed by a brief period with low directional change from about 1938 to 1942, and since 1942, changes have been mainly in declination. Comparison of the observatory data with IGRF models for the period 1945 to 1990 shows that IGRF provides an excellent estimation of the geomagnetic parameters in this low latitude region (19.7-degrees-N). In particular this comparison study supports the existence in central Mexico of a relatively small secular variation as implied by the IGRF and observatory data.