Because the polarity or direction of the interplanetary magnetic field (IMF) in the Sun’s northern hemisphere is opposite that of the field in the southern hemisphere, an ‘interplanetary current sheet’ or ‘heliospheric current sheet’ (an electric current that is confined to a curved plane) is created which separates regions with magnetic fields pointing in different directions.

This heliospheric current sheet extends to the outer reaches of the Solar System, and is both tilted (due to an offset between the Sun’s rotational and magnetic axes) and warped (because of a quadrupole moment in the solar magnetic field). It thus has a wave-like ‘ballerina skirt’ structure as it extends into interplanetary space, which changes in shape through the solar cycle due to the reversal of the Sun’s magnetic field approximately every 11 years.

As the Sun rotates, the current sheet wobbles up and down, like the folds in the skirt of a whirling dervish, sweeping regions of opposite magnetic polarity past the Earth. This means that, because the Earth is located sometimes above and sometimes below the rotating current sheet, it experiences regular, periodic changes in the polarity of the IMF.

These polarity changes occur two or more times per solar rotation. That is to say, because a solar rotation takes on average 27 days, the reversal of polarity in the IMF as experienced by the Earth occurs approximately every two weeks or less. The periods of alternating positive (away from the Sun) and negative (toward the Sun) polarity are known as magnetic sectors.

© Peter Dawkins