 Greetings and welcome to the Introduction to Astronomy. In this lecture, we are going to talk about the solar cycle, so patterns of activity that we see on the surface of the sun, and leading into perhaps how those can affect us here on Earth. So let's take a look at the most visible sign of solar activity, which would be the sunspots. Sunspots, what are they? Well, they are dark regions on the surface of the sun. So when you look at an image of the sun, of the photosphere, you can pick out sunspots, large here, but smaller ones scattered around. And we can see all sorts of sunspots and even sunspot groupings present on the sun. Why are they dark? They're dark because they are cooler than the surrounding surface. They are still incredibly hot. They are almost 4,000 degrees as compared to 6,000 for the sun. But when you look at something cooler against the background of something much brighter, they are going to look dark. So if you could actually scoop out that material and put it out in space, it would glow an orangish red color. So they are not really dark areas or a view into the interior of the sun, but they are very, they are just cooler regions of the surface. Now, large sunspots may have actually been visible more than 1,000 years ago. A large enough sunspot could be seen with the naked eye, although it is very dangerous to look at the sun directly because it can damage your eyes. However, it is possible that some very large sunspots or sunspots groupings may have been seen more than 1,000 years ago. However, the most recent observations, modern detection of sunspots, was by Galileo with the telescope. These can last, these sunspots can last anywhere from a few hours to a few months. And they can be larger than the size of the Earth. So these can be very big compared to the size of the Earth even. Others can be smaller, but still they are very, all of them are going to be very good size objects. Now, the sunspots that we see aren't just random, they come and go with a pattern. So what we see is what we call the sunspot cycle. And the cycle means that there is a regular pattern, or in this case, a cyclic pattern, that we see where the number of sunspots will rise and fall with a period of about 11 years. So in our image here, we can see the sun in the background, and we can see cycle 22 peaking here, and about 11 years later, cycle 23 peaks, and about 11 years later or so, cycle 24 peaks. Actually, cycle 24 was a little bit later incoming than the previous cycle, so it's actually a little bit longer. So it's not precisely 11 years, but it averages to 11 years over our observations of the sun, so the actual period can vary a little bit. So it's not that it's precisely 11 years, but it's a pretty good predictor over a long-term time frames. The cycle is related to magnetic properties of the sun. And in reality, it's not just an 11-year cycle, which is when the sunspots come and go, but it's a 22-year cycle. Because at the end of each cycle, the sunspots reverse their polarity. What does that mean? Well, at one cycle, you might have a couple of sunspots in the southern hemisphere, and you might have the south spot leading the north spot. The next cycle, so after 11 years have passed, you might have another similar pair of sunspots, but now it is north and south. So even though they are the same spots in the same hemisphere of the sun, the polarity or essentially the entire magnetic field of the sun has reversed. It's flipped from the magnetic pole being aligned with the north rotational axis to the north magnetic pole being aligned with the south rotational axis. So not only the sunspots, but the entire sun reverses its magnetic field at the end of each cycle. Now, how do we know these are magnetic? Let's take a look at some of the things that we know. And there's two things here that we look at. One is the Zeeman effect, in which says that spectral lines can be split in the presence of a strong magnetic field. So when we take a spectrum of this and we take our observations right along here, so some parts of this are not in the sunspot and other parts are right in the sunspot, then we see the spectral line that we get. It's just one line when we're away from the magnetic field of the sunspot. One line over here, but it can split into three lines when you're right at the magnetic field. So that's one way that we can measure a magnetic field and we can actually measure the strength of the magnetic field in the sunspots because the amount of the splitting also depends on how strong the magnetic field is. Now, that's not the only way we can look at sunspots. We can, as being magnetic, we can also look at some of the magnetic loops that we get, which are the plasma flowing out from the sun and we can see little loops as we see here and here, which look much like we get four magnetic fields here on the Earth. If we have a bar magnet and some iron filings, you would get something quite similar to what we see here. So in this case, the magnetic field lines are invisible. We can't see those, but the plasma follows those lines and allows us to see them and how they connect sunspots together. So when you see little groupings like this, you can expect a sunspot group on either edge of those. And then the magnetic field line kind of looping in between them. Now, let's look at the pattern of that magnetic cycle and what we see here is, again, the magnetic cycle is 22 years, although the number of sunspots changes with a period of 11 years. Other things that change, not only the number of sunspots comes and goes with this period, but the location of the spots on the sun also changes. And this is seen in what we call the Maunder butterfly diagram. So down below, we can see that the sunspots change with the periods of about averaging 11 years or so. So about every 11 years, we get a peak of sunspots. However, the top one is showing where they appear during each cycle. So if you divide up these cycles here, then at the beginning of the cycle, they form at more northerly latitudes. At the end of the cycle, they form at more closer to the equator. So we get this pattern when we map out where the sunspots occur each time. So what we're seeing, again, is the positioning on the sun is changing. So where these sunspots actually occur will change over the course of the cycle. So very early in the cycle, they're very high and very late in the cycle, they're much closer to the equator, giving us this distinct pattern that we see each time and giving the impression, if you look at it sideways, of a bunch of butterflies in a line there, giving it its name as the butterfly diagram. Now, when we look at some of these sunspots, we see that they do tend to come in pairs and we do see them where they are. Now, here's where we see, you know, are they north or south? So let's take a look here. In this case, we can't see it, but you'd have perhaps the north spot leading and the south spot trailing. If we wait one more cycle and come around again, then we would see the next cycle, then we would get the south spot leading and the north spot trailing. And then after another cycle, we would get the same thing again. So because of this, we call the entire cycle is lasting 22 years, even though the number of sunspots come and go every 11 years. So let's look a little bit about how this works. What is going on inside the sun that is actually causing this to occur? And what we see is that the sun's magnetic field, we said that was the underlying cause of all of this activity, is formed by what we call the solar dynamo. And this is the layers of gas underneath the sun, which generate the electric currents, which when rotating form a magnetic field. Now, typically a magnetic field looks something like this. You have the south magnetic pole and you have the north magnetic pole and the magnetic field lines just form big loops around. And for something like the earth, that's exactly what happens. And you have everything rotating together. But remember that the sun rotates differentially. And what that means is that it's spinning faster at the equator. So over time, the areas, the magnetic field lines near the equator are moving faster. Those near the poles are moving slower. And the equator will lap the poles because of this differential rotation. Again, it's faster at the equator, slower at the poles. And over time, this tangles up the magnetic field. If it takes about, what is it? It's about 25 days for the sun to rotate at the equator and it takes about 36 days at the poles. So if we look at this, if we look at this rotation, then if we think about that, for every three rotations that occur here near the equator, we've only had two near the poles. So every 75 days or so, the equator has lapped the poles. And that pulls and tugs on those magnetic field lines. After another 75 days, it's lapped them again. And over the course of a year, it can lap them four or five times. And that starts to twist and tangle up the sun's magnetic field. And eventually, various parts of it will get so tangled up that they'll burst through, forming little regions of sunspot activity. And when those magnetic field lines burst through, they will slow down the energy transport or decrease it cooling off those areas of the photosphere, giving rise to the sunspots. Now when we look overall at the sunspot activity, it does have a very long-term changes as well. So what are the long-term cycles? When we look here, we do see that 11-year pattern. But we also note that there are some years where the sunspot activity was tremendous. And other years where while there was a peak, there wasn't very much occurring. There were not a lot of sunspots even at the peak. However, there is this one section here between about 1650 and 1700, which we call the Maunder Minimum. Typically, the minimum is what occurs in between the sunspot cycles when there's very few sunspots. However, in this area, there were essentially no sunspots on the sun for a period of many decades. So during that 50-year period, we had very limited numbers of sunspots. If you can look there, you might see a few little spots, but nothing much. Now we knew they were being looked for because we had detected spots, all sorts of that, all sorts of kinds before, going back to the time of Galileo in the early 1600s. And it was simply a time when there were no sunspots seen on the sun. This also gave us cooler temperatures on the earth at the time. So the solar activity, the more active the sun is, the warmer it is going to be, the more energy it is putting out. So when we get an area of low activity, then we are going to get correspondingly cooler temperatures here on the earth. So this was what is known as the little ice age in Europe. The little ice age in Europe. So we did see this correspondingly a decrease in temperatures here on the earth, which then of course thought as the solar activity started to pick up again later on. So how often does something like this occur? We have no idea. It's a big question mark there because we've only been able to really observe sunspots for about 400 years now. That's about all we know. We know that there are times when there are less and times when there are more and we have a somewhat regular pattern, but we don't know over tens of thousands of years accurately how often something like this would actually occur. So let's finish up with our summary. And what we find is that we talked about sunspots and they are the cool, dark regions on the photosphere of the sun. So on the visible surface, we see them as darker areas. They come and go in an 11 year cycle. However, we really talk about the entire magnetic cycle of the sun being 22 years long. And sunspots and other solar activity are all caused by the twisting and tangling of the solar magnetic field. So the differential rotation causes the magnetic field to twist and tangle, and that gives rise not only to sunspots, but other solar activity. So that concludes our 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.