Base line stability of the magnetic main registration at the Göttingen observatory

Abstract

Since November 1984 a regional study of the Earth's magnetic field has been carried out, measuring absolute values ofF, D, andI. The aim is to find the behaviour of the secular variation at about 40 field sites with maximum distances of 50 km from Gottingen. After a long search for the least disturbed points, the first set of measurements was taken in 1987. For the reduction according to the normal 2h UT level the Gottingen observatory (GTT) records are used for each field point. However, by comparing the simultaneously registered magnetic values of the observatories Wingst (WNG) and GTT it turned out that there is an annual periodic drift in the GTT registrations. Besides the temperature coefficients of the variometers the whole micro-climate of temperature and humidity inside and outside the building and of the underground has to be considered. It causes a motion of the bottom plate (on which the variometers are standing) relative to the foundations of the walls (which house the camera and the lamps for the photographic registration). These motions can be misinterpreted as long period magnetic variations. Variations of the micro-climate and consequently the time dependence of the motions cannot be predicted. Apparently the long-term stability of the whole recording system, which is essential for reducing regional measurements and for the investigation of the secular variation, had not been considered important as the building was set-up. To overcome that lack of stability two ways are necessary: firstly, to perform regular absolute measurements at the observatory (more than once a week) to get a better knowledge of the base line of each component, secondly, to refer the GTT registrations to the momentary values of WNG which are assumed to be stable, to get by this way a proper reduction of the field measurements.