Nap time for the Sun: solar cycles

Extreme close-up of the Sun's visible surface,
showing 'bubbling' cells of convecting gas--each the size of
Northern California. credit: Hinode JAXA/NASA/PPARCBy all accounts, a new cycle-Cycle 24-in solar activity has begun... something you probably didn't notice since the beginning of a solar cycle is quite subtle....

First things first: what is a solar cycle, and why is this one number 24? You've probably heard of sunspots and solar flares and disturbances in radio communications caused by solar activity, but had you noticed NOT hearing much about these things in the last two or three years?

The Sun exhibits a cyclic rise and fall in its level of magnetic activity. Being an enormous ball of roiling, circulating plasma (electrically charged gas), the Sun generates powerful magnetic fields in a way similar to how the circulating electricity in an electromagnet creates one.

Over the course of a solar cycle, the intensity and amount of magnetism generated by the Sun increases, like soup warming up on the stove, reaching a violent climax in which twisting, tangling magnetic fields break loose and release their energy in the form of solar flare explosions, coronal mass ejections, and tremendous heating of the solar atmosphere.

Sunspots are surface features formed by the presence of strong magnetic fields, and in general the number of sunspots that can be seen and counted indicate the level of magnetic activity on the Sun. For 400 years, since Galileo first started counting sunspots through his telescope, observers have kept track of sunspot counts, and over time a pattern in their number emerged. On average, the number of sunspot activity peaks every 11 years at a time called solar maximum.

I remember when I first started working at Chabot Space & Science Center, back in 1999/2000, during the last solar maximum. Using our Sunspotter telescopes on public observing days, in teacher workshops, and in my solar summer camp, we could easily count many sunspots-sometimes as many as 20 or more! Those were the days!

In the past two or three summers, however, it's a lucky week to spot just a single sunspot! Most of the time, the Sun's face has been a bland disk with few discernible surface features.

That status quo should start to change, now that we have allegedly reached solar minimum and are stepping onto the uphill slope toward the next maximum, which should happen sometime around 2011 or 2012. If you want to keep tabs on the rising solar activity, and you like lots of graphs and numbers and stuff like that, check out the Solar Cycle 24 website.

Oh, why is this Cycle 24? A 19^th Century astronomer who studied the then newly discovered sunspot cycle, Rudolf Wolf, established the cycle that spanned 1755 to 1766 as Cycle 1...and they've been counting up ever since.

But even in this "nap time" of the Sun, today's modern solar observatories and spacecraft, with their arrays of high-tech cameras and sensors, see plenty on the Sun to keep them busy.

Japan's Hinode spacecraft, launched in 2006, has returned libraries of amazing pictures and movies of solar flares, activity around sunspots, circulating hot gases, fine details of the life and times of magnetic fields...and all of this during solar minimum! I can't wait until the Sun really gets going and Hinode becomes like a camera-happy tourist in Tahiti....

Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.