Critical component of the interplanetary magnetic field responsible for large geomagnetic effects in the polar cap

Abstract

Abstract.An observed influence of the interplanetary magnetic sectorstructure on the geomagnetic variations in the polar cap appearsto be due to the component of the interplanetary magnetic fieldnear the ecliptic perpendicular to the earth-sun direction. Thissuggests that the observed effect on the ground originates in thefront of the magnetosphere.A clear correspondence between the sector polarity of the inter­planetary magnetic field and the geomagnetic variations in the polarcap has been found to exist by Svalgaard (1968) and by Mansurov (1969).For a positive polarity of the interplanetary magnetic field (a direc­tion away from the sun) the vertical component, Z, of the magneticfield at Thule (INVL, invariant latitude, = 86.8°N) is depressedduring several hours around magnetic noon and the horizontal compo­nent,H, at Godhavn (INVL = 77.5 0 N) 1S increased. Correspondingly anegative sector polarity gives r1se to an increase of the Z-componentat Thule and a decrease of the H-component at Godhavn. This correlationbetween the interplanetary magnetic field and the geomagnetic field 1nthe polar cap is so distinct that it has been possible to infer thesector polarity from the geomagnetic field variations at the polar capobservatories Thule and Godhavn (Friis-Christensenet al., 1971),(Svalgaard, 1972).But a more detailed examination of the observed relationship (Friis­Christensen, 1971) revealed a few cases in which the geomagnetic var1a­tions at Thule and Godhavn apparently disagreed with the sector polarity.Especially May 29th and June 21st in 1968 showed evidence of beingsituated in a sector of opposite polarity to the one observed by space­craft (Wilcox and Colburn, 1970). As these days occurred around shiftsin the sector polarity a possibility of a time delay was at firstlooked for. The fact, however, that the shift in the type of dailyvariation of the geomagnetic field in the polar cap did occur nearly