A new method of estimating interplanetary sector polarity from geomagnetic activity is presented. The method is based on a linear multiregression between the By component of the interplanetary magnetic field and hourly values of the magnetic perturbation (ΔX,ΔY,ΔZ) at selected magnetic observatories. By simultaneous use of all three components, and by using all hours of local time, the method yields an improved accuracy compared to previous subjective assessments of the sector polarity. Using this method, it is possible to derive the interplanetary sector structure at least back to 1926, with an accuracy which is sufficient to follow the development of large-scale unipolar structures in the interplanetary medium. Furthermore, we present results indicating that it may be possible to derive sector polarity even further back in time to 1905, but with less accuracy. This is accomplished by including the two subauroral stations Sitka and Sodankyl¿, which have not previously been used for polarity determination. A major problem with this early polarity determination is a strong asymmetry favoring toward sectors. ¿ 2001 American Geophysical Union <