By Ron Cowen
The more things change, the more they stay the same. That old adage accurately describes the behavior of the solar magnetic field revealed by a new study. Waxing and waning, even flipping direction, the sun’s magnetic field undergoes dramatic upheavals yet always returns to its original shape and position.
“The sun’s magnetic field has a memory and [has returned] to approximately the same configuration” during each of the past three solar-activity cycles, says Marcia Neugebauer of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. She and her JPL colleagues base their finding, described in the Feb. 1 Journal Of Geophysical Research, on an analysis of 250,000 hours of solar data compiled by spacecraft between 1960 and 1998.
Current theories, she notes, suggest that random, churning motions of charged particles within the sun generate the solar magnetic field, so it should have no long-term memory. “Despite this expectation, the underlying magnetic structure remains fixed at the same solar longitude,” Neugebauer reports.
“It is a surprise,” comments Peter A. Sturrock of Stanford University.
Neugebauer’s team analyzed data collected by spacecraft studying the solar wind, the breeze of charged particles that carries the sun’s magnetic field into space.
Scientists had already found that some features of the wind repeat about every 27 days, matching the sun’s rotation. The new analysis shows that the magnetic field’s intensity has a fixed pattern that rotates with the sun every 27 days and 43 minutes. Moreover, the sun has kept up this rhythm for at least 38 years.
Surprisingly, the pattern persists for more than one solar-activity cycle. During each 11-year cycle, the sun’s north and south magnetic poles flip. Even so, the field maintains the same structure and placement.
The explanation may lie beneath the solar surface. One-third of the way to the solar core lies the boundary between two seething layers. From the inner layer, the radiative zone, the sun’s energy travels out as radiation. Above lies the convection zone, where heat converts to motion and the bulk of the sun’s magnetic field is believed to originate. Material within the radiative zone may not be evenly distributed, Neugebauer suggests, imposing a particular orientation on the field generated in the layer above it. Alternatively, the magnetic field’s structure could date back to the sun’s birth, she says.
Sturrock notes that the intensity of neutrinos emitted by the sun exhibits a periodicity that might also be tied to the sun’s magnetic activity. His team reported the neutrino fluctuations in the Oct. 1, 1999 Astrophysical Journal Letters.
Other sunlike stars may have similar magnetic behavior. Among several of these stars, magnetically regulated emissions from calcium atoms wax and wane in sync with stellar rotation, says Neugebauer.
She notes that the sun’s magnetic pattern stands out most clearly at peak solar activity and in the years immediately following it. Astronomers expect the current cycle to peak in a few months. Indeed, last week several observers glimpsed a sign of what may be in store, when they recorded the most energetic solar flare, so far, of the current cycle. The flare packed 10 million times more energy than a volcanic explosion, NASA scientists calculate.
