Weird sense of calm: The sunspot cycle from 1995 to the present. The jagged curve traces actual sunspot counts. Smooth curves are fits to the data and one forecaster's predictions of future activity.
Credit: David Hathaway, NASA/MSFC
All these lows have sparked a debate about whether the ongoing minimum is 'weird', 'extreme' or just an overdue 'market correction' following a string of unusually intense solar maxima.
"Since the Space Age began in the 1950s, solar activity has been generally high," notes Hathaway. "Five of the 10 most intense solar cycles on record have occurred in the last 50 years. We're just not used to this kind of deep calm."
Deep calm was fairly common 100 years ago. The solar minima of 1901 and 1913, for instance, were even longer than the one we're experiencing now. To match those minima in terms of depth and longevity, the current minimum will have to last at least another year.
In a way, the calm is exciting, says Pesnell. "For the first time in history, we're getting to see what a deep solar minimum is really like."
A fleet of spacecraft, including the Solar and Heliospheric Observatory (SOHO), the twin STEREO probes, the five THEMIS probes, Hinode, ACE, Wind, TRACE, AIM, TIMED, Geotail and others are studying the Sun and its effects on Earth 24/7 using technology that didn't exist a century ago.
Their measurements of solar wind, cosmic rays, irradiance and magnetic fields show that solar minimum is much more interesting and profound than anyone expected.
Modern technology cannot, however, predict what comes next. Competing models by dozens of top solar physicists disagree, sometimes sharply, on when this solar minimum will end and how big the next solar maximum will be.
Pesnell has surveyed the scientific literature and prepared a 'piano plot' showing the range of predictions. The great uncertainty stems from one simple fact: no-one fully understands the underlying physics of the sunspot cycle.
He believes sunspot counts will pick up again soon though, "possibly by the end of the year," to be followed by a solar maximum of below-average intensity in 2012 or 2013.
But like other forecasters, he knows he could be wrong. Bull or bear? Stay tuned for updates.
Dr Tony Phillips is a writer for the U.S. space agency, NASA.
This is an edited version of a story first published on the Science@NASA web site.
