Sun-weather: a new kind of connection

Sunspots, flares, and other solar activity have long tempted climatologists to link their 11- and 22-year cycles with Earth's climate. But all such theories beg the question of what physical connection this action on the sun could have with weather on Earth.

Solar physicist Sabatino Sofia says he thinks he has a plausible answer. He and his colleagues at the NASA Goddard Space Flight Center are developing a theory of solar magnetism which suggests the sun expands and contracts during its cycles, thus significantly altering the solar energy which powers Earth's weather.

Solar cycles are rooted in the sun's magnetic activity. Sunspots, flares, and other forms of activity at the sun's surface rise to a crescendo and subside into quiescence over roughly an 11-year period. The solar magnetic field also reverses itself during this time. Thus it takes two such periods for the magnetic field to return to its original orientation--a 22-year cycle.

How the surface activity with its 11-year cycle is related to the magnetic field with its 22-year cycle is a longstanding puzzle. Sophia's theory builds on the main theory advanced to explain it.

The magnetic fields at the sun's surface are normally rather weak - about 10 times the strength of Earth's field. But near sunspots and other centers of action, they can reach strengths hundreds of thousands of times greater. Physicists attribute such strengthening to twisting and tightening of lines of magnetic force by turbulent motions in the sun's outer layer and to that layer's rotation.

Different regions of the layer rotate at different speeds. Material in the equatorial zone goes around in 26 days, while matter near the poles takes 37 days. This differential rotation twists the magnetic field, gradually strengthening it. Eventually, the strengthened magnetic field erupts into sunspots and other activity. As its energy is spent, the activity subsides. This is the familiar solar cycle.

As he explained at the recent annual meeting of the American Association for the Advancement of Science, Sophia believes there is more to the story than this. As the magnetic field strengthens, he said, its pressure lifts the outer layer, causing an increase in the solar radius. Likewise, as the 11-year cycle ends with dissipation of the magnetic field's energy in flares and other action, the outer layer subsides and the sun shrinks.

This change in radius changes the sun's luminosity. It is this change in solar energy output that could have a direct effect on weather, Sophia said. Measurements of solar output by satellites have begun to detect some small short-term changes. Also, there is some suggestion in astronomical records of small changes in solar radius.

If the theory is at all correct, it would at least begin to explain why meteorologists find echoes of the 11-year and 22-year solar cycles in climatic records. And whether it is right or wrong, it at least proposes a direct physical link between solar activity and weather--something most other sun-weather theories have failed to do.