Scientists have been puzzling in recent months over the sun's lack of spots, which signifies very low activity. The sun is in the midst of an expected low in its 11-year cycle of sunspots and storms but the lull has gone on much longer than expected.

Now researchers think they know why.

A jet stream deep inside the sun is migrating slower than usual through the star's interior, giving rise to the current lack of sunspots. It'll all be back to normal soon, according to a NASA statement.

Rachel Howe and Frank Hill of the National Solar Observatory (NSO) in Tucson, Arizona, used a technique called helioseismology to detect and track the jet stream down to depths of 7,000 km below the surface of the sun. The sun generates new jet streams near its poles every 11 years. The streams migrate slowly from the poles to the equator, and when a jet stream reaches the critical latitude of 22 degrees, new-cycle sunspots begin to appear.

Howe and Hill found that the stream associated with the next solar cycle has moved sluggishly, taking three years to cover a 10 degree range in latitude compared to only two years for the previous solar cycle, they explained today at an American Astronomical Society press conference in Boulder, Colorado.

The jet stream is now, finally, reaching the critical latitude, heralding a return of solar activity in the months and years ahead.

"It is exciting to see," Hill said, "that just as this sluggish stream reaches the usual active latitude of 22 degrees, a year late, we finally begin to see new groups of sunspots emerging."

The current solar minimum has been so long and deep, some scientists speculated that the sun might enter a long period with no sunspot activity at all, akin to the Maunder Minimum of the 17th century.

The new result dispells those concerns, according to the NASA statement. The sun's internal magnetic dynamo is still operating, and the sunspot cycle is not "broken."

Because it flows beneath the surface of the sun, the jet stream is not directly visible. Hill and Howe tracked its hidden motions via helioseismology. Shifting masses inside the sun send pressure waves rippling through the stellar interior. So-called "p modes" (p for pressure) bounce around the interior and cause the sun to ring like an enormous bell. By studying the vibrations of the sun's surface, it is possible to figure out what is happening inside. Similar techniques are used by geologists to map the interior of our planet.

In this case, researchers combined data from GONG and SOHO. GONG, short for "Global Oscillation Network Group," is an NSO-led network of telescopes that measures solar vibrations from various locations around Earth. SOHO, the Solar and Heliospheric Observatory, makes similar measurements from Earth orbit.

"This is an important discovery," said Dean Pesnell of NASA's Goddard Space Flight Center. "It shows how flows inside the sun are tied to the creation of sunspots and how jet streams can affect the timing of the solar cycle."