Most of us live on a 24-hour cycle beginning with waking up, going about our jobs or other activities and then sleeping before waking up again. We call it the circadian rhythm from the Latin circa dia or “about a day.” The sun has a rhythm too, called the 11-year solar cycle. During that time, we see a rise in magnetic activity on the sun including sunspots (giant knots of concentrated solar magnetism), titanic solar storms called flares and blasts of plasma — clouds of electrons and protons — billowing from our star. The hubbub peaks and then gradually declines. Sunspot numbers plummet as do the intensity and frequency of solar flares and phenomena like the aurora borealis which is tied to solar activity.
After bottoming out at solar minimum, we head back up the curve to the next solar max and then back down to minimum. A full cycle from one minimum to the next takes about 11 years. Looking at the illustration above, you can see we’re clearly on the downward side of the curve, heading toward minimum sometime in 2020. Any amateur who’s been keeping track of the sun has noticed a sharp decline in the number of sunspots in the past year. This January alone, the sun’s face has been clean as a whistle for 10 days thus far.
Lately, I’ve been getting questions about where the aurora’s been hiding out. Here in the U.S., northern lights have been scarce for months save for now-and-again faint appearances in the middle of the night. Arctic locations always fare better because they sit directly under the permanent oval of aurora that rings Earth’s north magnetic pole like a wreath around hung on a nail. The rest of us take what comes our way, and there’s been little of that.
Yet there are a couple aurora triggers even during solar minimum including the occasional large flare or coronal mass ejection (plasma blast). These can occur at any time though with far less frequency than around maximum. The most reliable aurora-maker during low activity are coronal holes.
Holes are regions in the sun’s corona or atmosphere where billions of tons of material streams freely into space instead of being trapped by solar magnetic fields. Nearly all of it a swarm of single protons — basically hydrogen atoms sans their gnat-like electrons — mingled with free electrons and stamped with the sun’s magnetic field. Just like a bar magnet, this cloud of minute particles has a north and a south pole within it. Whooshing across the solar system at a million miles an hour, material from a hole can sometimes link directly into Earth’s magnetic field and dump some of that subatomic goodness into our upper atmosphere to spark an auroral display.
After many spotless days, the sun has recently perked up with the appearance of 3-4 modest sunspot groups and several C-class flares. C-class are fairly minor, but we’ll take anything on the ride to solar minimum. If you have a small scope equipped with a safe solar filter, you can watch the groups parade from east to west across the sun’s face in the coming week or two.
We’ve known about the sun’s ups and down for hundreds of years. The current solar cycle, dubbed Solar Cycle 24, is the 24th observed since close monitoring of the sun got underway around 1755. Cycle 24 began in early January 2008 and is on track to be one of the smallest in over a century with a peak sunspot number of 116 reached in April 2014. To find a similarly wimpy performance, we have to go back to Cycle 14 that peaked with a spot count of 107 in Feb. 1906. Click here to see graphs of each cycle.
Cycle 25 is expected to start in 2020, when activity ticks upward again after minimum, and peak sometime in 2025. But that peak is expected to be even lower that the one just past.
Astronomers attribute the recent decline in solar hustle and bustle to events occurring deep beneath the shiny photosphere, the “surface” of the sun we see with our eyes. There, two huge currents of fire (incandescent hydrogen gas) — one in the northern hemisphere and one in the southern — circulate like giant conveyor belts. Since the late 19th century those belts have been chugging along at a walking pace of about 3 feet or 1 meter a second. But in recent years, for reasons yet unknown, both have slowed down.
Like great ocean currents that can affect the weather of the continents, the solar conveyor belts control the “weather” on the sun. The sun’s conveyor drags the magnetic energy lingering around old and decaying sunspots deep beneath the surface, where they’re recharged, become buoyant and then float back up again as new sunspot groups capable of spawning flares. According to theory, the speed of the belts foretells solar activity some 20 years into the future. A slow belt means lower solar activity; a fast belt means stronger activity.
When we look up at the stars at night, they may appear serene, but they’re churning, flaring, bubbling and conveying just like our own star, the sun.
* If you’d like to learn more about stars and how to find the brighter constellations any time of year, pick up a copy of my new book Night Sky with the Naked Eye at Amazon or Barnes and Noble. It covers all the great things you can see at night with just your eyeballs. No equipment required!