Solar Cycle 23 - Still Going Strong
Posted on March 28th, 2008 by blue collar scientistBack when this blog was young, and being read by five to ten people a day, I wrote about a hyped-up article announcing that Solar Cycle 24 had “officially” begun - a claim I found overblown and exaggerated.
The solar cycles in question are roughly 11 year cycles of increased and decreased solar activity. They vary quite a bit - sometimes they are ten years, sometimes twelve. And their boundaries are a bit fuzzy. But they average out to about 11 years, and the “11 year cycle” is an elementary part of solar physics.
The most visible solar activity that follows this 11 year cycle are sunspots and other magnetically-related phenomena - prominences, large flares, and so on. During solar maximum, there are lots of sunspots, they form in groups, and they are big; during minimum, the sun can be free of sunspots for weeks or months at a time, and when there are sunspots, they tend to be small and isolated.
One way of distinguishing that a cycle is ending and a new one is beginning is that the magnetic polarity of the sunspots reverses, and these new magnetically reversed sunspots appear in a high solar latitude.
As I went on to point out, scientists determine when the transition from one cycle to the next took place based on a retrospective look at the data. The hype in early January had to do with some small, magnetically reversed sunspots that had been discovered two days earlier. Real solar astronomers will have a working hypothesis about when the solar cycle changes, but nobody’s going to rush to publication claiming the solar cycle has “officially” changed after a couple small sunspots have been around for two days. You can’t clear peer review that fast, anyway.
And today, there’s a great example of why solar astronomers take their time about this sort of thing. As PhysOrg reports, three large sunspots have appeared on the sun that are not magnetically reversed - in other words, they are spots of solar cycle 23, which is ending, and not of solar cycle 24, about which hyped-up claims were made back in January.
“This week, three big sunspots appeared and they are all old cycle spots,” says NASA solar physicist David Hathaway. “We know this because of their magnetic polarity.”
As Hathaway points out, what’s really happening here is that both cycles are in progress, and we’re going to continue to get a mix of cycle 23 and cycle 24 spots for a while yet. What still isn’t clear, and won’t be clear for some time, is when cycle 24 actually began. Hathaway seems inclined to rule that the magnetically reversed spots of mid-2006 were not part of cycle 24, and he’s probably right. But it is important to remember that the 11-year solar cycle is merely the visible manifestation of periodic changes in the way the sun works. We’ll have a much better sense of just when cycle 24 began once all the data has been incorporated into the model. That’s just another way of saying that when that happens, we’ll have a much better sense of what’s actually going on with the sun.
Remember, we’re just talking about observations here. That’s just the first step in the scientific process: have a look around and see what’s going on. It is in the following steps - coming up with explanations, and then testing the proposed explanations against other observations and experiments - where things get interesting.
Tags: magnetic polarity, new solar cycle, solar activity, solar cycle, solar cycle 23, solar cycle 24, sunspot
March 28th, 2008 at 1:29 pm
I didn’t realize that sunspots came in cycles. How can you tell where one cycle ends and another begins?
March 28th, 2008 at 4:33 pm
Was Cylcle 23 unusually active? I’ve heard of some correlation between Sun spot activity and Earth temperature. Since some of the hottest years have been recorded within the last 11 year, I wonder if this cycle could be part of the recently hot years.
March 29th, 2008 at 5:22 pm
Zach, the end of one cycle and the begging of the next can be determined in a variety of ways. The “classic” method is to observe the latitude and magnetic polarity of sunspots. When sunspots at high latitude and opposite magnetic polarity are seen, that is a sign that the cycle is changing.
Kevin, cycle 23 was less active than typical. Changes in solar activity are not enough to significantly affect Earth temperature; over the course of a solar cycle the amount of energy received from the sun varies between 1366.6 Watts per square meter at maximum, to 1365.5 Watts per square meter at minimum. That’s a change of 7/100 of one percent, which is not nearly enough to account for the observed temperature increases at the Earth’s surface which are on the order of several percent. It isn’t solar activity, but solar brightness, that would have an effect on Earth temperature; but the sun hasn’t gotten any brighter than normal over the last few decades either.
March 30th, 2008 at 1:08 am
How do you measure the polarity of a sun spot?
March 30th, 2008 at 3:50 pm
Martin, there is something called the Zeeman effect, which describes how a spectral line will be separated into multiple lines in the presence of a magnetic field. The splitting is different if the magnetic field is + or -, so if you can observe the Zeeman effect, you can deduce the magnetic polarity.
As to how you do it, I’m only familiar with one method, and that’s the old-fashioned way, completely obsolete in today’s astronomy. That method would be to use a parallel-plate micrometer to measure the Zeeman splitting of a spectral line, usually the 5250-angstrom spectral line. (That would be the Iron I line, for those keeping score at home.) I suspect that modern methods rely on the same principle, but I’m sure they don’t rely on a micrometer.