11-year sunspot cycle


The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe, who after 17 years of observations noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk.[3] Rudolf Wolf compiled and studied these and other observations, reconstructing the cycle back to 1745, eventually pushing these reconstructions to the earliest observations of sunspots by Galileo and contemporaries in the early seventeenth century. Starting with Wolf, solar astronomers have found it useful to define a standard sunspot number index, which continues to be used today.

Until recently it was thought that there were 28 cycles in the 309 years between 1699 and 2008, giving an average length of 11.04 years, but recent research has showed that the longest of these (1784–1799) seems actually to have been two cycles,[4][5] so that the average length is only around 10.66 years. Cycles as short as 9 years and as long as 14 years have been observed, and in the double cycle of 1784-1799 one of the two component cycles had to be less than 8 years in length. Significant variations in amplitude also occur. Solar maximum and solar minimum refer respectively to epochs of maximum and minimum sunspot counts. Individual sunspot cycles are partitioned from one minimum to the next.

Following the numbering scheme established by Wolf, the 1755–1766 cycle is traditionally numbered “1″. The period between 1645 and 1715, a time during which very few sunspots were observed, is a real feature, as opposed to an artifact due to missing data.[6] This epoch is now known as the Maunder minimum, after Edward Walter Maunder, who extensively researched this peculiar event, first noted by Gustav Spörer. In the second half of the nineteenth century it was also noted (independently) by Richard Carrington and by Spörer that as the cycle progresses, sunspots appear first at mid-latitudes, and then closer and closer to the equator until solar minimum is reached.



Years ago, in 2008 and 2009 an eerie quiet descended on the sun.  Sunspot counts dropped to historically-low levels and solar flares ceased altogether.  As the longest and deepest solar minimum in a century unfolded, bored solar physicists wondered when “Solar Max” would ever return.

They can stop wondering. “It’s back,” says Dean Pesnell of the Goddard Space Flight Center.  “Solar Max has arrived.”


A new ScienceCast video examines the curious Solar Max of 2014.  Play it

Pesnell is a leading member of the NOAA/NASA Solar Cycle Prediction Panel, a blue-ribbon group of solar physicists who meet from time to time to forecast future solar cycles.  It’s not as easy as it sounds. Although textbooks call it the “11-year solar cycle,” the actual cycle can take anywhere from 9 to 14 years to complete.  Some Solar Maxes are strong, others weak, and, sometimes, as happened for nearly 70 years in the 17th century, the solar cycle can vanish altogether.

Pesnell points to a number of factors that signal Solar Max conditions in 2014: “The sun’s magnetic field has flipped; we are starting to see the development of long coronal holes; and, oh yes, sunspot counts are cresting.”

Another panelist, Doug Bieseker of the NOAA Space Weather Prediction Center, agrees with Pesnell: “Solar Maximum is here …. Finally.” According to an analysis Bieseker presented at NOAA’s Space Weather Workshop in April, the sunspot number for Solar Cycle 24 is near its peak right now.

They agree on another point, too:  It is not very impressive.

“This solar cycle continues to rank among the weakest on record,” comments Ron Turner of Analytic Services, Inc. who serves as a Senior Science Advisor to NASA’s Innovative Advanced Concepts program.  To illustrate the point, he plotted the smoothed sunspot number of Cycle 24 vs. the previous 23 cycles since 1755. “In the historical record, there are only a few Solar Maxima weaker than this one.”

As a result, many researchers have started calling the ongoing peak a “Mini-Max.”


This plot prepared by Ron Turner of Analytic Services, Inc., shows the smoothed sunspot number of Cycle 24 (red) vs. the previous 23 cycles since 1755.

Pesnell believes that “Solar Cycle 24, such as it is, will probably start fading by 2015.” Ironically, that is when some of the bigger flares and magnetic storms could occur.  Biesecker has analyzed historical records of solar activity and he finds that most large events such as strong flares and significant geomagnetic storms typically occur in the declining phase of solar cycles—even weak ones.

Indeed, this “Mini-Max” has already unleashed one of the strongest storms in recorded history.  On July 23, 2012, a plasma cloud or “CME” rocketed away from the sun as fast as 3000 km/s, more than four times faster than a typical eruption. The storm tore through Earth orbit, but fortunately Earth wasn’t there. Instead it hit NASA’s STEREO-A spacecraft, which recorded the event for analysis.  Researchers now believe the eruption was as significant as the iconic Carrington Event of 1859—a solar storm that set telegraph offices on fire and sparked Northern Lights as far south as Hawaii. If the 2012 “superstorm” had hit Earth, the damage to power grids and satellites would have been significant.

It all adds up to one thing: “We’re not out of the woods yet,” says Pesnell.  Even a “Mini-Max” can stir up major space weather—and there’s more to come as the cycle declines.

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