Slow Start to Solar Cycle Tied to Sluggish Interior Stream

Abstract

A slowly moving river of gas meandering deep inside the Sun's interior may be the reason for the paucity of sunspots this year and the late start to the current cycle of solar activity. Sonograms of the Sun's surface reveal that the buried stream is moving much more sluggishly toward the equator than similar streams observed during the previous solar cycle. Such jet stream–like currents typically take 6–8 years to move from the pole of the Sun to the equator as they spiral east to west, explained Frank Hill, a solar physicist with the National Solar Observatory, in Tucson, Ariz. “For this cycle, it's taken one to two years longer,” he said. Hill and researcher Rachel Howe used 15-year data sets from the Michelson Doppler Imager (MDI) aboard the orbiting Solar and Heliospheric Observatory (SOHO) and from the Global Oscillation Network Group (GONG) to track a stream of gas moving at depths of up to 7000 kilometers beneath the Sun's surface (Figure 1). “It goes all the way around the Sun and penetrates about one third of the way into the interior,” Hill said. “The sunspot activity locations are highly associated with the location of the jet stream–like flow.” Sunspots are regions of geomagnetic disturbances on the Sun's surface that are linked to solar magnetic storms, which can interfere with Earth-orbiting satellites, radio systems, and other technologies. Hill and Howe found that the flows emanate from the Sun's polar regions and gradually work themselves toward the equatorial regions. When the stream reaches about 22° latitude, a new crop of sunspots typically appears. The flow for solar cycle 24 has taken much longer to move down to the critical latitudes. “The sluggishness of this flow explains the extended (solar) minimum,” Hill said. Prior to this discovery, scientists were puzzled by the late and slow start to the Sun's new cycle, which typically lasts 11 years. Because society is increasingly dependent on space-based technologies, determining the characteristics of solar cycles is growing in importance. But “none of the forecasting research groups actually predicted the extended delay in the new cycle,” said Dean Pesnell, with NASA Goddard Space Flight Center, in Greenbelt, Md. Last year, the Sun was spot-free for 266 days, marking the quietest period of solar activity in a century. In the first 9 months of this year, the Sun had 212 spotless days. Hill and Howe report that the interior jet stream has finally reached critical latitudes for sunspot development, and scientists expect to begin seeing an uptick in sunspot numbers soon. Solar activity is now expected to peak in May 2013. “What this does is provide us timing information,” Pesnell said, referring to the jet stream data. “Even if it doesn't predict the level of upcoming activity for the cycle, it's very nice to see the timing.” Irene Klotz is a freelance writer for the American Geophysical Union.