By Ron Cowen
Over the past 8 months, solar eruptions have increased in frequency and ferocity, a sign that the sun is at the peak of its 11-year activity cycle (SN: 1/13/01, p. 26). Now scientists have found another indicator that the so-called solar maximum has arrived–the polarity of the sun’s magnetic field has become more complex and a portion of it has flipped entirely.
As recently as last October, the sun’s magnetic north pole resided in the star’s northern hemisphere, as it had for the previous 11 years. But in late January, an analysis of magnetic measurements gathered since November by the U.S. National Solar Observatory atop Arizona’s Kitt Peak revealed that the field had flipped. The magnetic field in the sun’s northern hemisphere now points south, says solar physicist David H. Hathaway of NASA’s Marshall Space Flight Center in Huntsville, Ala.
Changes in the solar magnetic field drive the sun’s 11-year cycle. During solar maximum, magnetic activity increases, as indicated by the greater number of sunspots. These are the dark blotches on the surface of the sun that mark intense magnetic loops that arc into the sun’s outer atmosphere.
Ionized gas flows from the sunspots, which cluster at mid-latitudes, to the sun’s poles. The gas carries with it the magnetic field associated with the sunspots. Because of the present orientation of the sunspots, the flow transports a south-pointing magnetic field to the north magnetic pole and a north-pointing field to the south. For now, notes Hathaway, only the northern magnetic pole has flipped; the southern magnetic pole still points south but should switch soon.
