  Sunspots are dark spots on the solar surface associated with strong magnetic
fields. The number, area, and brightness of sunspots are supposed to reflect
the intensity of the solar magnetic fields and are often used as proxies for
their long-term variations. However, the correlations between the sunspot
parameters and solar magnetic fields are not constant, and the causes and the
time profiles of the variations in these correlations are not quite clear.
Therefore, the sunspot data alone cannot be used as proxy for deriving the
variations of the sunspot magnetic fields for periods when no instrumental
measurements are available. But the Earth is a sort of a probe reacting to
interplanetary disturbances which are manifestation of the solar magnetic
fields, so records of the geomagnetic activity can be used as diagnostic tools
for reconstructing past solar magnetic fields evolution. In the present study
we combine sunspot and geomagnetic data to estimate the long-term variations of
sunspot magnetic fields.
