Abstract:
The stability of baseline is the most important criterion for evaluating the data quality of a ground magnetic
observatory. Theoretically, a baseline should be a straight line, provided, there are no error factors
affecting the absolute instruments, the variometer and the observational procedure. But in practice, we
observe that the baselines are affected by some errors in the form of random errors and long term baseline
drifts. It is known that temperature, pier tilts, aging of electronic components, etc. can affect the long
term stability of baselines, but in this paper we discuss a new type of error which affects the baseline in
the form of long term drifts due to the variation in the gradient field between the absolute room and the
variometer room. Even though, a site is selected with the least magnetic gradient for the establishment of
an observatory, in many cases, it is found that the magnetic gradient patterns are not permanent and
changes over the time. This slow gradient changes can distort the actual temporal magnetic variations
and thus affecting the purity of data recorded at a geomagnetic observatory. We have analytically shown
that an ideal baseline has to be a horizontal straight line and the RHS of the fundamental equation of an
observatory should be a constant. We have further shown that baseline instabilities are caused by variation
in gradient field between the absolute and variometer pillars in addition to the measurement errors
from absolute observations and variometer recording. This variation in the gradient field causes long
term drifts in baselines. We have derived the correction factor which can filter out the signals arising
out of variation in the gradient field. Finally we present how far the data quality can be improved by
applying this correction.
