Polar cap magnetic variations and their relationship with the interplanetary magnetic sector structure

The relationship between polar geomagnetic variations and the polarity of the interplanetary magnetic sectors has been studied for the quiet year 1965. It is found that during the day hours a system of ionospheric currents encircles the magnetic poles on every day. The current system may extend up to 15° from the pole but is strongest at 8°–10° invariant colatitude. The current direction as seen from near the magnetic poles is counterclockwise during interplanetary sectors with field pointing away from the sun and clockwise during toward sectors. The current strength is dependent on season, being strongest during local summer. When the magnetic pole is on the nightside of the earth, this polar cap current is absent or very weak. When the rotation of the earth brings the magnetic pole into the dayside, the polar cap current system develops, with the current being most concentrated in the part of the current system that is nearest to the noon meridian. The current increases its total intensity until the magnetic pole is rotated past the noon meridian; then the intensity decreases as the magnetic pole approaches the nightside again. The seasonal variation of the magnetic elements in the polar cap is discussed in view of the sector polarity effects. These effects introduce an important modulation of the seasonal variations of the geomagnetic polar field. The demonstration of current systems inside the polar caps encircling the magnetic poles during local day hours calls for a major revision of the generally accepted picture of polar cap geomagnetic variations. It also suggests a new framework for interpreting polar cap observations of geomagnetic and related phenomena.