Observatories, Intermagnet

Abstract

Geomagnetic observatories carry out continuous and accurate monitoring of the strength and direction of the Earth's magnetic field over many years, making measurements at least every minute. Observatory data reveal how the field is changing on a wide range of timescales from seconds to centuries, and this is important for understanding processes both inside and outside the Earth. It is estimated that there are approximately 180 observatories currently operating around the world (Figure O1) (for more information on observatories in particular countries or regions of the world see Observatories in. .. and Figure O2/Plate 3). The distribution of observatories is largely determined by the location of habitable land and by the availability of local expertise, funds, and energy supply, and as result, it is uneven and a little sparse in some regions. Many observatories have had to move because of encroaching urbanization. However , the continuity of their data series is generally maintained by simultaneous observations at the old site and new site over a period of time to allow site differences to be established. The distribution of data in time is shown in Figure O3. International scientific campaigns such as the first International Polar Year in 1882/1883, the second International Polar Year in 1932/1933, and the International Geophysical Year in 1957/1958 encouraged the opening of many observatories around the world. Geomagnetism is a cross-disciplinary science, and as a result, observatories are run by a wide variety of institutes whose interests range from geology, mapping, geophysics (including seismology and earthquake prediction), meteorology to solar-terrestrial physics, and astronomy. There are two main categories of instruments at an observatory. The first category comprises variometers that make continuous measurements of elements of the geomagnetic field vector but in arbitrary units, for example millimeters of photographic paper in the case of photographic systems or electrical voltage in the case of fluxgates (see Observatories, instrumentation and Observatories, automation). Both analog and digital variometers require temperature-controlled environments and installation on extremely stable platforms (though some modern systems are suspended and therefore compensate for platform tilt). Even with these precautions they can still be subject to drift. They operate with minimal manual intervention but the resulting data are not absolute. The second category comprises absolute instruments that can make measurements of the magnetic field in terms of absolute physical basic Figure O1 Locations of currently operating observatories.