April 9, 2009
Over the last few months, the Sun has been back in the news with new strong surges of flaring activity, expected to rise to a maximum predicted in 2012 or 2013. Concerns are spreading about possible devastating effects of this activity on the Earth.
Those in the solar industry have been concerned, especially with the media hype that solar flares may knock out everything electrical or all communications equipment. This is actually an overreaction to what amounts to a rather usual rise of a new solar activity cycle after the exceptionally long quiet period experienced in the past few years.
This is actually an overreaction to what amounts to a rather usual rise of a new solar activity cycle.
This eventless interval between 2007 and 2009 separated the end of the previous 11-year solar cycle (#23) and the one that finally comes to life now (#24). In fact, only one year ago, part of the solar physics community was wondering if a new cycle would start at all. Were we heading toward a new Grand minimum, when the regular 11-year activity cycle vanishes? In this case, would the Sun remain in “hibernation” during decades, with a resulting cooling of the Earth climate?
It’s happened before
In reality, when looking backward in time over timescales of centuries, we can find similar episodes of extended cycles and long very low activity minima. This happened in the late 19th century, with a last case equivalent to the last minimum in 1913. It tells us first that the current situation is not at all unprecedented. Moreover, if one considers the trends over the last 100 years, one notices immediately that we just went through a succession of particularly strong activity cycles over the last decades. Such an episode is unprecedented since observations of sunspots started, four centuries ago. Therefore, there’s likely a bias in our current vision and physical models of how the sun is working because of the tremendous but only recent increase in the number and diversity of solar data since the advent of the space age and radioastronomy techniques.
Figure 1: A panorama of all solar activity cycles of the last 250 years, with their maxima and minima (International Sunspot Index, SIDC, Royal Observatory of Belgium). It shows that the last 50 years were marked by a tight succession of exceptionally strong cycles, well above the long-term normal average.
What was taken as a “standard” Sun may actually be representative of a temporary regime of higher-than-average solar activity. The strange solar behavior that puzzled solar physicists over recent years may thus be simply a return to a more normal regime. Probably, the best illustration of this bias is the fact that among more than 80 published predictions of the upcoming solar cycle, as collected in 2009 by the Solar Cycle 24 Prediction Panel, only a handful have forecast a long minimum and late rise of the new cycle. The Sun took us by surprise!
Next cycle hard to predict
Now that cycle 24 has actually started, what can we expect? Recent predictions generally agree on a cycle that would peak in 2013 and would be weaker than the previous cycle. The peak amplitude, expressed in the sunspot index (long term measure based on counts of sunspots), would be only about 90, instead of 120 for the last maximum in 2000. The last time such an amplitude took place was in 1930, more than 80 years ago.
Some daring solar physicists even push their predictions further ahead in time and predict that the next cycle will be even weaker or may not occur at all (a new Grand Minimum, several centuries after the last one?). However, it is much too early to conclude. We need now to accumulate new data for a few more years as the current cycle unfolds.
Figure 2: A recent prediction for solar cycle #24 suggests a weaker maximum than all cycles since more than 80 years. (D. Hathaway, NASA/Marshall Space Flight Center)
About the Author
Dr. Frédéric Clette is solar physicist at the Royal Observatory of Belgium. He is an active researcher and science operator at the Solar Influences Data analysis Center (SIDC). He is also leading the development and operations of the Uccle solar patrol instruments (USET).