On the reduction of time variations for geomagnetic repeat station measurements

Abstract

For most previous geomagnetic surveys, the time variations of the observatory closest to the site of observation have been used for reduction. The accuracy of repeat station measurements depends upon the distance between the station and the observatory. Nowadays, the requirements for the accuracy of repeat station measurements are much higher, especially with respect to local secular variation anomalies. On the other hand, transportable variograph stations are available, by means of which time variations can be recorded closer to the station site.

The reduction steps for this method are:

• (1) Reduction of measured values to the base-line of the nearby variograph station;

• (2) Computation of the nearest quiet night value using the baseline of the variograph;

• (3) Computation of the difference between this value and the corresponding one at the observatory.

The problems are:

• - the stability of the base-line values of the variographs;

• - the question as to what degree a quiet night value represents the normal value at that time (e.g. influence of aD _st field and/or other fields).

A repeat station survey of the Federal Republic of Germany was carried out in 1982. During the same time, eleven stations recorded the time variations in that area.

We discuss the aforementioned aspects by way of an example taken from this survey.