IMF Polarities Inferred From Combined Observations of the Geomagnetic Field, Near the North and South Poles

We discuss a new algorithm, based on the Svalgaard-Mansurov (S-M) effect, to infer the polarity of the interplanetary magnetic field (IMF) from combined measurements of the geomagnetic field performed near the North and South Poles. Data are taken at Thule (near the geomagnetic North Pole) and at Vostok (near the geomagnetic South Pole) in a 40 year period extending from 1958 through 1998. Three phenomena limit the accuracy of the results which can be obtained using data from a single station: (a) the amplitude of the signal produced by the S-M effect is slightly different in periods of positive or negative IMF polarity; (b) during the local winter the S-M signal is much weaker than during the summer; (c) an intense geomagnetic activity favors the detection of the positive polarities at Thule and that of the negative polarities at Vostok. As a consequence, reliable polarities can be obtained for only a limited fraction of the time. On the other hand, using simultaneous observations at Thule and Vostok should remove most of the errors: in fact, at the North and South Poles the seasonal cycle is reversed, while the geomagnetic activity has opposite effects on the detection of the positive and negative polarities. Comparing the polarities inferred with our algorithm from Thule-Vostok magnetograms with those directly measured by spacecraft in the period 1965-1985 confirms this expectation: we obtained, in fact, a quite high success rate (95.6%) along with an almost continuous coverage (93% of the time).