Ringing to the beat of the solar cycle.

Ringing to the beat of the solar cycle 

The sun apparently changes its tune in time with the rise and fall in sunspot number and magnetic activity marking the solar cycle. Careful measurements of the acoustic frequencies at which the sun oscillates reveal that these frequencies increase slightly as the solar cycle goes from a minimum to a maximum. A detailed analysis of the data may provide important clues toward identifying the activities inside the sun responsible for the solar cycle.

"These are the most accurate measurements of the sun's acoustic modes that have been made so far,c says Kenneth G. Libbrecht of the California Institute of Technology in Pasadena. "These new measurements clearly show that [acoustic] frequencies change with time, presumably owing to changes in the structure of the sun during hte solar cycle." Libbrecht and Martin F. Woodard report their findings in the June 21 NATURE.

Ever since scientists learned more than a decade ago that the sun rings like a bell, they have suspected that fluctuations in the sun's interior during a solar cycle would affect the characteristic frequencies at which the sun vibrates. But previous measurements failed to pinpoint the changes.

Libbrecht and Woodard tracked the sun's oscillations for a four-month period in 1986, as the solar cycle reached its minimum, and for a similar period in 1988, as the cycle rose towards its peak. They discovered that the sun's acoustic vibration frequencies systematically increased by about one part in 10,000. The data also suggested that the structural changes responsible for these frequency shifts are concentrated in the sun's outermost layers.

"It is our view that the [acoustic] frequencies are responding to changes in the strength of solar magnetic activity near the the sun's surface," the researchers assert. But the origin of the changes isn't known yet.

"In the past three have been too few data to enable us to piece together a coherent picture of these frequency changes," Douglas Gough of the University of Cambridge in England comments in the same issue of NATURE. "It is likely that both magnetic activity and structural changes in the superficial layers of the sun are merely symptoms of a more deeply seated dynamical process."

Gleaning information about what happens in the sun's outer layers, Gough adds, "is an important step towards discovering the underlying mechanism that controls the solar cycle."