It could cause intergalactic weather fronts such as geomagnetic storms, which can interfere with satellites and cause radio blackouts.
Nasa said in August that the change would happen in three to four months’ time, but it is impossible to give a more specific date. It is expected to be complete in the next few weeks.
During the 11-year solar cycle sunspots appear as blotches near the equator of the sun’s surface where there is intense magnetic activity.
Over a month these spots disintegrate and the magnetic activity migrates from the equator to one of the sun’s poles. As they move towards the poles, they erode the existing opposite polarity.
Todd Hoeksema, a scientist at Stanford University’s Wilcox Solar Observatory, which has monitored the phenomenon since 1975, said: ‘It’s kind of like a tide coming in or going out. Each little wave brings a little more water in, and eventually you get to the full reversal.’