 Greetings and welcome to the Introduction to Astronomy. In this lecture, we will talk about the solar cycle, and this is the cycle of things that we see on the sun and solar activity and how that changes over time. So the main thing that is noticed with the solar cycle are the sunspots. What are these? Well, they are darker regions that we see on the surface of the sun. Now, they are darker, not because they are really dark, but because they are cooler than the surrounding surface. They are about 3,800 degrees as compared to the 5,800 degrees of the surface of the sun. So they look dark by comparison, but are really bright. Could you scoop that material up and put it out in space? It is quite possible that large sunspots may have been seen more than a thousand years ago, as it is possible to sometimes see them at sunrise or sunset when we are looking through a lot of atmosphere. However, the first recordings of them are from Galileo in the early 1600s. They last from a few hours to a few months and can be larger than Earth, so some of these large sunspots can be bigger than Earth itself. Now, the sunspot cycle is what we want to look at here, and we do see a cyclic pattern that the number of sunspots will rise and fall with a period of 11 years. So every 11 years or so, the peak will come, and we will get more sunspots, and here we see some of those. We see a peak here around 1991. We see another peak here around 2002, and then we see a peak here around 2013 to 2014. The actual period will vary a little bit from cycle to cycle and can be a year or so longer or shorter, but roughly every year we will get a peak of the cycle. Now note that the peak level is not always the same, that it can differ from year to year. Sometimes it can be much lower, other times it can have a very strong peak, and that is not something that can be predicted. We have to just see what comes out of each cycle. Now, this is actually related to the magnetic properties of the sun, and we're going to look at the magnetic field soon, and so it's really a 22-year cycle because at the end of each cycle, the sunspots will reverse their polarity, and what that means is that north becomes south, and south becomes north in terms of the magnetic field. Now, that does not affect the rotational axis of the sun, but its magnetic field will flip, and that makes the complete cycle 22 years long. So how do we know that sunspots are magnetic? Well, there's a couple things that we can see. One is the Zeeman effect, and that is where spectral lines can split in the presence of a strong magnetic field. So if we take a spectrum, and we look at it, the region near here is split into three lines, one straight down and one to either side, and therefore we know that that's a region where the magnetic field is strong, and that's right where the sunspot occurs. So that gives us evidence that there are strong magnetic fields associated with the sunspot. We don't see that splitting up here or down below when we're well away from the sunspot, but we do see it when we are at the location of the sunspot. Now, the other thing that we see is some of the magnetic loops. If we actually look at the surface of the sun, here we can see all these different loops of material, and that is plasma on the sun following the magnetic field lines that are present, and we can trace those back to sunspots. So we have learned from a couple of things that we have seen through the Zeeman effect and through these magnetic loops that connect various sunspots together, we know that sunspots are a magnetic phenomena. So they are caused by the magnetic fields of the sun and where they puncture through the surface, they inhibit the flow of their energy and therefore cool off that portion of the sun, giving us the sunspot. Now, the magnetic cycle, as I said, is a 22-year cycle. The number of sunspots changes with a period of 11 years, but not only do the number of sunspots, but their location on the sun also changes. This is what we call the Maunder butterfly diagram because it may look like a bunch of butterflies traveling in one direction here, and we see that the number of sunspots, again, about every 11 years, we see that those come and go. This shows us where they form on the sun. So here the sunspots at the beginning of the cycle form at a higher latitude and they form at a lower latitude close to the equator at the end. Then they disappear a little bit and come back at higher latitudes and again finish up at lower latitudes. And that process continues. So not only do the number of sunspots change, but their location changes as well. Now, the other thing that can flip is the polarity of the sunspots, and that means that what we see in a sunspot pair, you might see a north and a south one time, but if you saw a similar set of spots, 11 years later, they would then be flipped and the spot to the right would be south and the spot to the left would be north. So their polarities would change. Now, these would not be the same sunspots. The sunspots would not be visible after that period of time. Remember, they only last for a month or so at the most. However, the similar sunspots forming in that part of the sun would now have reversed polarities at the end of each cycle. So how does the sun get this magnetic field? Well, we talk about the solar dynamo. We had a dynamo on Earth, but we can also have a dynamo on the sun. There are layers of gas that can generate electric currents, which then form the magnetic field around the sun. Interestingly, like some of the Jovian planets, the sun undergoes differential rotation, which means it does not rotate like a solid object. It rotates like the ball of gas and plasmas that it is, and that is faster at the equator and slower at the poles. And here we see, over time, that the magnetic fields, as they start out nice and smooth, would then start to get tangled as they move faster at the equator. Those get stretched, the ones at the poles stay, and eventually you have material. You get these tangled up after a number of cycles. It takes about 25 days at the equator for a rotation, and it might take, say, 30 days near the pole. So that's five days difference, meaning after a few cycles, each cycle the equator picks up five days on a pole. It is rotated that much faster. So over a few months it has lapped the pole, and over a couple of years it really starts to tangle up, and then we get those areas where sunspots will occur, where it got so tangled that now the magnetic field lines pop out through the surface. And that will give us the sunspots and the magnetic regions that we see. Now we can also look at longer-term cycles here because, as we've said, there's a very relatively regular period that we've looked at for hundreds of years now. Going back into the mid-1700s, that every 11 years there's a peak of the sunspots. Sometimes they're low, sometimes those peaks are very high, but there's always a peak roughly 11 years apart. You'll note that there's a gap here where there aren't very few sunspots noted. Now that's not that we weren't looking for them. In fact, there were very few sunspots during this time, and this is actually related to cooler temperatures on Earth because there was a lack of activity for 50 years there that Earth cooled off a little bit. Now it was enough that it was definitely noticeable in the climate of Europe. That was a little ice age where it was unusually cold for a few decades. Now the question could be, how often does this occur? We don't know. There's only a very limited amount of data available. In fact, if you go back to Galileo, there's our data for sunspots starting in the early 1600s. And we don't know what the sunspot cycle was like before that. We don't know if this occurs every few centuries, or if it's once in a million years, or once in a million years, or once every 10,000 years. We don't know how often it occurs, or if it even occurs on any kind of regular basis. Simply we just do not have enough data going back far enough on the sunspot cycle to really be able to tell what this is like. But we do know that it does have an effect on us here, on Earth. So let's finish up here with our summary. And what we've looked at is that sunspots are the cool, dark regions that we see in the photosphere of the sun. The sunspots come and go in the 11-year cycle, but the magnetic cycle is 22 years long. And the sunspots and other solar activity are caused by twisting and tangling of the solar magnetic field. So that concludes this lecture on the solar cycle. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone. And I will see you in class.