Inferring the interplanetary magnetic field by observing the polar geomagnetic field

L. Svalgaard and S. M. Mansurov have shown that it is possible to infer the polarity of the interplanetary magnetic field quite reliably from observations of the diurnal variation of polar geomagnetic fields. The effect is most prominent in the vertical component of geomagnetic observatories near the geomagnetic poles during several hours near noon. The interplanetary magnetic field observed with spacecraft near the earth is very similar to the mean solar magnetic field (i.e., the sun observed as though it were a star); thus the fact that observations of the polar geomagnetic field have existed without interruption since 1926 at the Danish Meteorological Institute station at Godhavn, Greenland, means that in effect the inferred solar magnetic field during five sunspot cycles is available for analysis. The specific nature of the relation between the interplanetary field and the polar geomagnetic field should aid toward physical understanding of the interaction between the two fields. It has already been shown that it is the direction of the azimuthal (i.e., not the radial or north‐south) component of the interplanetary field that is significant in causing this effect.