 I really have a fascination for the sun in many ways, and I hope to share some of that with you today. All right, so the sun is a star, as you probably all know. It's a very ordinary star. It's a very mediocre star, in fact. It's neither very bright nor very dim. It's not particularly large, not particularly small. In the context of all the stars in the universe, it's really not special at all. And if you were to ask a nighttime astronomer what they think about the sun, they'd probably tell you it's a really boring object to study. Why would you do that to yourself? But the sun, of course, is very special to us. It sits at the center of a solar system, and it provides the light and the heat that sustain life on Earth. And although there will come a day where the sun expands to engulf all the inner planets, probably including the Earth, that's not going to happen anytime soon. And it's currently enjoying a pretty, let's say, uneventful phase in its life. But the sun changes, and it changes in measurable ways, and it changes in ways that affect us. So that's how I'm going to start my talk. So just for context, this is not super interesting, but I thought I'd show you a slide with the structure of the sun and what we know about the structure of the sun. So maybe think of the sun as an onion. It's made up of layers, and in each one of these layers, different physical processes dominate so that the very inside of the sun, in the sun's core, the temperature and the pressure are so high that this nuclear reaction is taking place all of the time. And hydrogen atoms get combined to form helium atoms, and it releases vast amounts of energy in the process. Then this energy travels outwards, and it hits the next layer, the radiative zone. And in the radiative zone, the material is still very, very, very dense. It is so dense that it takes a photon, it's a particle of light, it takes a particle of light about a million years to travel through the radiative zone. And that's because it keeps colliding with the very, very dense material in that part of the sun. But eventually, photons make it through the radiative zone, and they hit the next layer. The next layer is called the convection zone. And in the convection zone, most of the heat is actually transported outwards by means of convection. Convection is what happens in a boiling pot of water. So you have hot material at the bottom of the pot, it becomes lighter, it rises, it releases heat at the top, and then it cools down, it gets dense, it plummets back in, and starts accruing heat again at the bottom of the pot. So this creates a cyclical process by which material that rises up and down kind of transports heat from the bottom to the top. So that's what happens in the convection zone of the sun. This is this area here. And the outer edge of the convection zone of the sun represents the outer edge of the sun's interior and the beginning of the sun's atmosphere. Now the sun is a gas, so the difference between the interior and the atmosphere has nothing to do with the interior being solid, like you would think in the Earth in some way. But the sun is a gas throughout. What makes the difference between the sun's interior and the sun's atmosphere is that by the time the energy reaches the first layer of the sun's atmosphere, which is called the photosphere, all of a sudden these photons, these particles of light, are free to escape into space. And they don't collide with an ethanol pretty much. So the sun's photosphere, this layer here, is a part of the sun, a part of the atmosphere of the sun from which most of the light that we see comes from. If we were to look at the sun with our naked eye, which I was reminded to tell you that you should never do, we probably know that, what you would see would be the sun's photosphere, basically. Above the photosphere sits a layer called the chromosphere. It's a very thin crust that only becomes visible to us during total solar eclipses. And it appears as a red ring glowing around the edge of the eclipse. And above that sits the solar corona. The solar corona is a very faint and very tenuous layer of the sun's atmosphere. And it expands many solar radii out into space. And it's also visible during, if you've ever seen a total solar eclipse, it's an amazing event. And you get to see the corona and its full glory. So I really recommend it if you ever have a chance. Next one in the US is in 2024. All right, so humanity realized a long time ago that the sun was not a perfect celestial body, that it was blemished with these dark spots that constantly appeared and disappeared off the surface of the sun. So we've known about sunspots for a while. The first written records of sunspots are from Chinese astronomers, several hundred years BC. But the observations, routine observations didn't start until the early 1600s with the invention of the telescope. And actually, at this point, maybe the longest, most continuous record of sunspot observations goes back to the time of Galileo, which is at that time at the early 1600s. At that point, astronomers didn't quite know what to make of these sunspots on the sun. Some thought there were shadows of planets that were undiscovered that were transiting across the sun. And some thought they were dark clouds in the sun's atmosphere itself. And they were obscuring the surface. But today, we know that sunspots are magnetic in nature. And they're just the footprints of very, very strong magnetic forces that pierce through the sun's surface and into its atmosphere. So sunspots look dark because they're cooler than their surroundings. And they're cooler than their surroundings because the strong magnetic forces that they harbor inhibit the propagation of heat to the surface. So that's what makes them dark. However, sunspots are not cold, by any means. Even the coldest of sunspots is still a couple of thousand degrees Celsius. But at the same time, it's still a couple of thousand degrees colder than its surroundings. So just a short digression to talk a little bit about magnetism. Magnetism, magnetic fields, magnetic forces, I will refer to all of these chains throughout my talk. But for all intents and purposes throughout the talk, they're the same thing. So magnetism is a force that is generated by an electrically charged particle in motion. And it's a force that is very intricately related to electric fields and electric forces. In fact, yeah, an electrically charged particle, when it moves, it creates a magnetic field. But a magnetic field that changes is capable of creating or creates an electric field and is capable of inducing an electric current. So why is this relevant to the sun? You may ask, well, the sun is, I said earlier that it's a gas, is something called a plasma, in fact. And a plasma is no more than a gas that is a mix of neutral and electrically charged particles. So because the plasma in the sun is constantly moving and churning and rotating, the electrically charged particles in this plasma are moving and churning and rotating. And they constantly generate and destroy magnetism. So the sun is full of magnetism everywhere. The lifetime of one given sunspot is typically a few weeks, anything between a few hours and actually a few months. And if you were to look at the sun for long enough, let's say for a few months in a row, the pattern of the movement of the sunspots across its surface reveals the sun's rotation. So the sun rotates, and it rotates with a 27 day period. It rotates once every 27 days, approximately. It rotates faster at the equator than it does at the poles. Again, because it's not a solid, it can do this. And that's, maybe it's obvious to you guys. But that actually results in interesting phenomena, too. So because the sun rotates, it generates very, very strong magnetic fields in its interior, very, very strong. And because it rotates differentially, because it rotates faster at the equator than it does at the pole, it drags these magnetic fields around its interior. And they kind of wrap around the interior of the sun, creating this kind of donut-like pattern. These are very, very strong magnetic forces. And sometimes strands of these magnetic forces become loose or separated from the main strands. And they're buoyant for reasons I'm not going to get into. But they will rise up to the sun's surface in the form of loops. And they will push through the surface and into the sun's atmosphere in the form of these loops. And the place with the foot points of these loops intersects the solar surface. That's where sunspots appear. And these, remember, are very, very strong magnetic fields. So just because they're there and they cross the sun's surface, they prevent the propagation of heat to the surface. And then they create these dark patches, which are the sunspots. Sunspots, because of this particular pattern of magnetism of the sun and because of the nature of magnetism itself, sunspots always appear in pairs of opposite magnetic polarities. They can't be just the north sunspot by itself. It has to be accompanied by a south magnetic polarity in one way or another. So sunspots typically appear in pairs or in groups where you have both magnetic polarities. The number of sunspots on the sun's surface waxes and wanes with approximately every 11 years. And this is what we call the solar cycle, if you've ever heard of it. So the two pictures on the top show the face of the sun, the surface of the sun in one day in April 2014 and one day in October 2019. And you can see a stark difference. In April 2014, the sun was full of sunspots. And pretty much all of the month of October was like that. There was nothing going on. And this is because April 2014 was close to the maximum of the solar cycle. And now we're pretty close to the minimum of the solar cycle. So if you look at the graph on the bottom, this is one of the two or three graphs that I have in my entire talk. I apologize. But this graph shows the area of the sun covered by sunspots at a given time as a function of time on this axis. I can't do this on this axis. Since 1875, approximately, up to 2016. And so each vertical bar tells you for each day in this period of time how much surface of the sun was occupied by sunspots. And you can see that although there's a lot of intrinsic variation from day to day and from year to year, there's a staggering repetitive pattern that repeats itself every 11 years, whether it's maxima followed by minima pretty much consistently since we've started observing sunspots with an exception that I will mention later on. So this is the solar cycle. Not only the number of sunspots varies with an 11-year periodicity, but there's a lot of other things that actually change with this 11-year pattern. One of them is interesting is the location at which sunspots appear on the sun's surface. That changes with the solar cycle. This graph on the top shows the location on the sun. So this would be the north pole. This here at the bottom would be the south pole. And the line across the middle would be the equator. And again, it's a function of time since 1875. So if you look at the beginning of a solar cycle, you will see that sunspots tend to appear at high latitudes, both in the northern hemisphere and in the southern hemisphere, but away from the equator. And as the cycle progresses, don't mean to do that, sunspots start appearing close and closer to the equator until they get to the end of the cycle, where they tend to appear in a narrow band around the equator. And then all of a sudden, pretty much will sunspots vanish from the surface of the sun for a little while until the new cycle begins. And sunspots start appearing again at high latitudes. So this is another quantity that changes with the solar cycle, but there's a lot of other ones, such as the amount of light that the sun emits also varies with the solar cycle, be a maximum during the maximum of sunspots, a minimum during the minimum of sunspots, and so on. So space weather is kind of a hot topic nowadays. So space weather refers to the changes in the sun and on the sun and how they affect the interplanetary medium, and in particular, how they affect near-Earth space. And just like weather on Earth, space weather can come in many different phenomena, in many different shapes. And just like weather on Earth, there's storms. So solar storms are impulsive events in which the sun releases vast amounts of radiation and particles into space. And these particles can travel at really high speeds. So I'm going to show you some examples of solar storms. This one here is called a coronal mass ejection, or CME, for short. The video shows a composite of observations from three different telescopes, but these are actual observations of the sun. CME, or coronal mass ejection, is just a huge bubble of plasma threaded with magnetic fields that kind of is ejected from the sun's surface over the course of several hours. The amount of material in a large coronal mass ejection can reach the billion tons of mass. And the speeds at which this material travels in space can be anywhere from 300 kilometers per second to 3,000 kilometers per second. And realize that, as I said, per second, this is equivalent to saying multiple million miles per hour at the largest speeds. This storm here is a filament eruption. It's an amazing video. I love watching it. A filament is a large concentration of cool and dense material from the sun. This is this here, this big pointer, that one, that is suspended above the sun's surface, and it's confined and supported by magnetic forces. The magnetic forces that support it against the action of gravity are anchored below the sun's surface. And this filament, I'm just actually showing the eruption in this movie, but that filament has been sitting there for a while before it erupts. So these magnetic forces just supporting it and holding it against the action of gravity, and they anchored below the solar surface. And the motions of the plasma at the sun's surface and below the surface twist and stretch these magnetic fields, loading them up with energy. And sometimes they get to a point where they can't take it anymore, and they break. And they break, releasing all of the material and the energy that was trapped inside them, which is what is happening right there in that movie. This movie actually shows a different type of solar storm, too, just underneath the filament. When it's ejected, you see these two ribbons of light that appear. This is a solar flare. A solar flare is just a sudden and very, very intense brightening around the sun's surface. And all of these solar storm phenomena, coronal mass ejections, filament eruptions, and solar flares oftentimes come together, but they don't have to. They can also happen independently. So the sun's magnetism is not confined to its interior nor to its surface. It actually extends out into space. This is an old picture of a solar eclipse. And you see this is what you see. You see the corona over the sun in its magnificence. And it's kind of structured. So that is the magnetism of the sun extending out into space that structures that material. And it stands out into space and is carried away by the solar wind. So the sun has a wind. It's constantly and relentlessly expelling material into space. And it drags the magnetism of the sun away with it. And because the sun rotates, they both kind of get dragged out in this spiral pattern throughout the entire solar system. And this spiral pattern creates pathways along some of these solar storms can travel. The Earth has a magnetic field, too, as you know. And it creates a protective bubble around our planet called the magnetosphere. I don't know much about the magnetosphere. I'm going to tell you all I know about it. So the place where the magnetosphere and the interplanetary magnetic field from the sun, the spiral one, the place where these two meet is called the magnetopause. And at the magnetopause, the Earth's magnetic field points north. And it has done so for the last several hundred thousand years. I forget the numbers. But the Earth's magnetic field doesn't change very often. On the other hand, the sun's magnetic field changes all the time. And that interplanetary magnetic field at the magnetopause coming from the sun can be oriented in any direction and principle. When it happens to point south, it cancels out part of the Earth's magnetic field. And it peels off layers of the protective shield around the Earth. And it allows particles from the sun to penetrate into the Earth's atmosphere. This is one of the things that causes aurora, or northern lights or southern lights. These are energetic particles from the sun that penetrate through the Earth's magnetosphere, collide with the atoms and the molecules in the Earth's atmosphere, which get excited and release little bursts of light. And when seen all combined, they create these wonderful natural phenomena called the aurora. This is just one effect of space weather on Earth. But there's multiple other effects of space weather, some more disruptive than others. And in fact, different types of space weather can mess with different types of technologies on Earth. For instance, a solar flare can emit powerful x-rays that can disrupt radio communications. Solar energetic particles from the sun can cause failures in electronics on spacecraft and spacecraft themselves. And coronal mass ejections can create what are called geomagnetic storms. These are disturbances in the Earth's magnetic field. And these, in turn, can induce currents in power lines on the ground, disrupting power grid operations. These things are also responsible for disruption in the GPS navigation system, for instance. So the effects of space weather can make us really vulnerable, because we depend on a lot of these technologies. On September 1, 1989, Sir Richard Carrington was looking at the sun through his telescope. And all of a sudden, he saw this massive flare in a group of sunspots. This is his drawing. And these two white bananas are his representation of the flare. A few hours later, there's records of aurora seen at really low latitudes, really close to the equator. It's far south of Cuba, in the Caribbean. And there's anecdotal records of miners in the Rocky Mountains. I had to look this up in Wikipedia. But miners in the Rocky Mountains are getting up to make breakfast in the middle of the night, because the aurora was so bright that I thought it was morning. So this is a very rare event. This doesn't happen very often. There's records of telegraph disruption all across North America and Europe, and even anecdotal evidence of telegraph operators getting electrical shocks because of the induced currents in telegraph wires. The measurements of the Earth's magnetic field disturbance indicate that, well, two things. This storm that happened when Sir Richard Carrington saw his flare took about 17 hours to travel from the sun to the Earth. That is a long way. So it was probably propagating at speeds over 5 million miles per hour. So that's a really fast solar storm. The other thing that we know is that its magnitude hitting the Earth has not yet been surpassed in the space age. There's statistical studies out there that say that the chance of a storm like this, of this magnitude happening within the next decade, is about 12%. But the sun is a spherical object, and storms can happen anywhere on the sun, and definitely do not have to happen in the line of sight of the Earth. They can happen anywhere on the sun and not hit us. So most of the times, it will probably not hit us. But it's something to take into account. So I hope that with this introduction, I've made it clear why the magnetism of the sun is important to the sun itself and obviously to space weather. So magnetism is at the root of all space weather. And I find it funny. It makes me feel important to this topic. And of course, we need to be able to forecast space weather in order to be able to protect ourselves from its effects. But in order to do that, we need to understand the magnetism of the sun. We need to quantify it. We need to measure it. So that's what I'm here to talk about. How do we measure the sun? And in particular, how do we measure its magnetism? The first thing I need to say is that solar physics and all of astronomy, pretty much, is a science based on remote sensing. We do not have the technology to travel to the sun. We measure things there directly. So we can only infer information about the physical conditions on the sun by looking at the light that it emits. Remote sensing, as you know, you're very familiar with it because it happens in everyday life. But it's the science and the art of measuring, identifying an object without coming to direct contact with it. So we measure light. And from the light, we infer physical properties. So I will constantly talk about measuring pressures, measuring densities, measuring we don't measure. We infer these things. It's just jargon. How do we do this? Well, we need three different elements. We need telescopes first. Telescopes help us capture the light and magnify the image of astronomical objects. And we need instruments which analyze the light, analyze the different properties of the light. And then we need detectors that record the light. Astronomers nowadays do not look through the eyepieces of the telescopes. They record the light on digital media. So telescopes. Telescopes are just arrangements of mirrors and or lenses. His purpose is to magnify the image of an object that is distant. The telescope was invented in the early 1600s. And although I used to think that Galileo was the inventor of the telescope, Galileo is not the inventor of the telescope. But he was certainly one of the first people to point it to the skies. So if we were to look at the sun through our human eyes, which we don't remember. All right, the human eye can discern any object that is bigger than, I forget, one minute of arc that subtends an angle larger than one minute of arc. This means that a human eye is able to discern something that is a third of a meter big at a distance of one kilometer, or larger, of course. The sun subtends in the sky about 31 minutes of arc. So with our resolution, with the eye resolution of one minute of arc, we can actually see details on the sun if we were to look at it. There's not just a point in the sky like all the other stars. However, if we look at the sun through a telescope, we get to see a lot more detail. This image here is from the HMI instrument on the Solar Dynamics Observatory. And it's just an image of the surface of the sun, and you can see a sun spot there. And as I zoom in, you will see how much more detail HMI can see than we would see with our eye. It's about 60 times more detail. The HMI telescope is able to see the equivalent of something that is half a centimeter big at a distance of one kilometer. So that is much better than what we could do. But if we were to look at this exact same sun spot with a different telescope, that is much more powerful. This is the Swedish Solar Telescope on the Canary Islands in Spain. You can actually see even way more detail in it. The Swedish Solar Telescope is about seven times better than the HMI instrument. So what determines the spatial resolution of a telescope? What determines the amount of detail that a telescope can see on the sun's surface? Well, there's many different factors, of course, that influence this. But the absolute limiting factor is the size of the telescope. What we call the aperture, which is the physical diameter, the physical size of its main mirror or its main lens that captures the light and converges, that creates the image. So the HMI instrument on board the Solar Dynamics Observatory sees the surface of the sun like this. It's a little blurry. If we look at it with the Swedish Solar Telescope, as I forget to say, HMI is about 14 centimeters big. That's the size of the telescope. The Swedish Solar Telescope is about one meter big, and it can do this much better. So one meter, more or less. The US is currently building the largest solar telescope in the world ever. And it's going to be four meters in diameter. It's huge. From here, just the main mirror, not talking about the building. The building is a lot bigger than that. So scientists are really, really excited in the solar physics community to see what the telescope is going to reveal. Because it's undoubtedly going to answer some scientific questions that we've not been able to answer before. But it's also undoubtedly going to bring up new scientific questions. Because we're going to be able to see the sun have never seen it before in a lot more detail. So my last slide about telescopes. I'm going to try to explain why we put telescopes in space. So sometimes we put telescopes in space, on board a spacecraft. And it's really, really, really expensive to put a telescope in space compared to putting it on the ground. It's also a lot riskier. And it's hard to upgrade or hard to fix instruments if they happen to fail. So why would we do that in the first place? Well, there's several reasons that they all have to do with the Earth's atmosphere. So the Earth's atmosphere gets in the way of astronomical observations. It blurs them. If you've ever looked through the hot air above the flames of a campfire to an object behind a campfire, you see how it flickers and shimmers. That's because the hot pockets and cold pockets are there above that campfire and making a blur in your image. And that's exactly what the Earth's atmosphere does to astronomical observations. You can go outside right now, look at the nighttime sky, and you will see all those stars that kind of flicker. They don't really flicker. It's an optical illusion created by the Earth's atmosphere. So by sending a telescope into space, we bypass that. We get rid of the Earth's atmosphere. The other reason we send a telescope into space has also to do a lot with the Earth's atmosphere. The Earth's atmosphere absorbs different types of light, some types of light that come from the sun. The very same ozone layer that protects us from ultraviolet radiation, have to look at you, prevents that ultraviolet radiation from reaching the ground. And that's a shame in some ways because there's a lot of diagnostic potential about the physical conditions in the sun in the ultraviolet light that emits. So if we want to measure this ultraviolet light, we have to get outside the Earth's atmosphere to be able to detect it. So that's another reason we send telescopes to space. But both ground-based and space-based observations have their advantages and disadvantages and they're very complementary. So they're both important to do. So the next element for observing the sun is instruments. And I can't speak about instruments without speaking about the properties of light. These two first properties that I'm highlighting here, we're all very used to, they're kind of intuitive because the human eye can perceive them. Talking about brightness or intensity, which is self-explanatory, I would say, and then color or frequency or wavelength, they're all equivalent. So color is a little bit more interesting because the human eye is not able to see all colors. They're certainly able to see the colors in the rainbow. This is the visible light, the colors that we're used to see in. But there's a lot of colors of light that we do not see, such as radio waves, microwaves, x-rays and gamma rays. These are all types of light, but we're not able to see them. The human eye is just not sensitive to them. There's a third property of light that we're maybe a little bit less familiar with just because the human eye cannot, it was not sensitive to it or barely sensitive to it. And it's polarization. Polarization is a little bit hard to explain, but it's a property of light as a wave because light is an electromagnetic wave. So imagine you're holding a skipping rope and you're holding the skipping rope and you're shaking it side to side, like this. And you're now creating a wave in this horizontal plane. That's the type of linear polarization. It's horizontal linear polarization. Now take that same skipping rope and move your arm in a circle like this. And you create this spiral pattern that kind of propagates along the rope, okay? That is called a circularly polarized wave because you move in your arm in a circular motion. Circularly polarized waves can happen in two directions, counterclockwise or clockwise. A linearly polarized wave can be horizontal, vertical, or any angle in between, of course. And then you can have a combination of all of these. You don't necessarily have to happen separately. Talk a little bit more about polarization later, but first let me talk about the light spectrum, about the colors of light. So if you were to take the light from the sun and break it down in all of its different colors, what you would get is the sun spectrum. Sun spectrum looks like this, at least the visible part of the spectrum. Now if you look carefully, you see it's littered with dark lines. These dark lines are called spectral lines and they're just the footprints of the atoms and the molecules that form the sun's atmosphere. So each type of atom and each type of molecule in the sun's atmosphere, anyway, imprints a very, very specific pattern of dark lines in the spectrum of light. So just by measuring the spectrum and by virtue of looking at where these lines are, we can begin to find out what the chemical composition of the sun is because the patterns of line tells us what elements are present in the sun. Even beyond this, it actually tells us how many of each type of atom and each type of molecule we have in the sun, even more than this. If you look at these lines, some are broad, some are narrow, some are very faint, some are very dark, they have different shapes and the shapes of these spectral lines so give us information about the physical conditions in the sun's atmosphere. And it's not worth going into how because it's a very complicated science but we can look at these spectra and extract information about the pressure, about the density, about the temperature, we can extract information about how the plasma is moving and so on and so forth. So it has an incredible diagnostic capacity. This is also the reason why if we observe the sun in different lights, in different colors, we get to probe different physical conditions. So some colors are sensitive to high temperatures, some colors are sensitive to low temperatures, some colors are sensitive to high pressures, some are sensitive to low pressures. So each color is probes a different physical regime in the sun's atmosphere and this is equivalent to saying that each color probes a different layer of that onion, of that sun's atmosphere. So different colors help us probe the physical conditions in the sun's atmosphere as a function of height. All right, polarization. I said it was a property of light as a wave, right? Most sources of light are unpolarized. This is equivalent to shaking that skipping rope in random directions and not creating an organized pattern in the movement. But there are some physical mechanisms that can polarize the light. The one that you're probably most familiar with is reflection off of flat, shiny surfaces. So if you drive in east in the morning, when the sun is low and it reflects on the road where you're driving on, it produces this crazy glare and it's really annoying. So you put on your polarized sunglasses and you eliminate the glare. So what's happening there? The reflection of the road, of that parallel surface, becomes polarized horizontally because reflections create polarization. When you put your sunglasses on, they eliminate all the horizontal polarization and they only allow the vertical polarization to pass through to your eyes. So you're eliminating only the reflection and you're keeping everything else. So that's how you eliminate the glare of shiny surfaces, reflective surfaces. In the sun, there's a different mechanism that creates polarization and that's the presence of magnetism. So magnetism creates polarization in the light that is emitted from the sun. So if you're looking at an area of the sun that contains magnetic forces, the light emitted from that area will be polarized. And the strength and the type of polarization will depend on the strength and the orientation of that magnetic force. So just by looking at the types of polarization at different locations on the sun, you can begin to infer information about the magnetic field, both in strength and what direction in space they're pointing. And this is in summary what we do with, what I do with instruments, how I measure the sun, there's a lot more instruments out there, but the instruments I typically work with are first polarimeters. Polarimeters allow us to measure the polarization of light. They allow us to select different polarizations and then measure that light. And then on the bottom, instruments that allow us to measure the spectrum in different ways. And I've highlighted two here. There's way more than these, there's a combination of these. A spectrograph is a little bit like a prism. You shine the light from the sun through it and it spreads it out into the spectrum. And then you put a camera behind it and you take a photograph of the spectrum. So you take a photograph of all the colors. A filtograph is a type of instrument that allows us to select one specific color and then take a picture of the sun in that specific color. And both instruments are very useful and complementary in what would they do. So this is what I do. And this is what Lorena introduced me. This is a science I do, it's called spectropolarimetry. Spectropolarimetry, if you look at the etymology of the word, is the measurement metric of the intensity and the polarization of light as a function of color, function of the spectrum. And why do I do this? Well, because it's a really incredible, powerful tool for diagnosing the physical conditions in the sun's atmosphere, including its magnetism, which is where my heart lies, I guess. So what does the sun look like in polarized light? This picture here is another image taken by the HMI instrument on board the Solar Dynamics Observatory. And what it's showing, it's called a magnetogram, by the way, it's showing circular polarization measurement of the sun, of the surface of the sun in circular polarization. So gray means no polarization, and black and white means the two circular polarizations, the right-handed and the left-handed. And you may infer, from what I said earlier, where there's polarization, there's magnetism, because that is the main mechanism in the sun that generates polarization. So we know that these areas with strong black and white have magnetic fields. So are they sunspots, you may ask? If we compare the magnetogram to a picture of the sun in visible light, we can see several things. First, the concentrations, the very, very strong concentrations of black and white correspond to locations of sunspots, always. However, the circular polarization always extends beyond the location where there's sunspots, and that's because there's magnetism also outside of sunspots. It's just not strong enough to make it dark. So magnetism is around sunspots, but it's just weaker, and it doesn't create the sunspot area around it. The other thing that you can notice is that there's kind of a salt and pepper pattern everywhere away from sunspots in the magnetogram. So if we were to zoom in with a more powerful telescope and a more sensitive instrument to an area on the sun where there's apparently nothing, we would see that there's circular polarization everywhere on the sun, and it changes with time, and it changes all the time. It's constantly emerging and submerging, being created, and being destroyed. So the sun is riddled with magnetic activity at the smallest spatial scales, not only at the large spatial scales. All right, so we now know how to measure the spectrum of the sun, and we know how to measure its polarization. So how do we interpret these measurements? How do we turn them into magnetic field or pressure or temperature without going deep into this at all? And this is the only equation of my talk, I swear. But we use theory, and we use equations to interpret the measurements. The most important equation for this particular problem is called the radiative transfer equation, and it's that one. And the radiative transfer equation tells us how light is generated and how its spectrum and its polarization get changed as the light travels through the sun's atmosphere and towards us. So if we understand how the light travels, how it's generated, and how its spectrum and its polarization travel, we can begin to understand how to interpret the measurements of polarization. And we also use computer codes. Computer codes call spectral line inversion codes. These codes implement these equations that we need for interpretation, and they solve them because these equations are really hard to solve with pen and paper. And they solve them in a way that they try to make the solution of the equation match the observations that we make with telescopes and instruments. And when they are able to match these two things, what we get is our best guess or our best estimate over the physical conditions in the sun's atmosphere that led to the generation of that specific observation. So the video I'm about to show you is shows an observation of polarization from the HMI instrument in the Solar Dynamics Observatory. I did work for this project, so that's why I quoted so much. But the measurements of polarization that I'm going to show have already been interpreted in terms of magnetism. So they've already been interpreted through this equation and these computer codes. And what you will see is a couple of small sunspots on the sun's surface. These ones here. I'm going to zoom into this area. And all of a sudden, new magnetism is going to start emerging from the sun's interior onto the surface and interacting with those two sunspots. So here you are. We zoom in. These are these two sunspots. And now all this new stuff is coming up, and it started interacting with already other sunspots. So here what I'm showing, I told you it was already interpreted in terms of magnetism. So red and blue are showing the main magnetic polarities, north and south. North is magnetism that is mainly pointing towards you. And south is magnetism that is mainly pointing away from you into the screen. But then each one of these little arrows, each one of those tiny little things, actually shows the actual direction of the magnetic field for each point in the solar surface in the group of these sunspots. And what you can see is the new magnetism emerges. Kind of they separate. They pull away. They rotate. The sunspots rotate. And they drag the magnetism with them. And creating these very, so you can get the sense of how they stretch and how they twist as they move along. And that is what's happening to the magnetism in these sunspot regions. So what are these measurements for? When we do all of this, why do we do it? I'm going to show you a list of examples of use cases for these kind of measurements. Well, first of all, just to repeat the same thing over and over again, measurements of polarization on the sun allow us to identify magnetized regions in the sun. They allow us to quantify the magnetism in sunspot regions. And they allow us to quantify how they change with time. And this allows scientists to, for instance, see how the magnetism gets rearranged during a solar storm and how it changes during an impulsive event from the sun. And this, in turn, may eventually allow us to try to find clues as to what indicators there are pre-eruption that tell us that an eruption is going to happen. So this has an interesting potential from the forecast perspective. How can we predict a solar storm? Is there anything in the magnetism before the storm that is going to tell us that a storm is about to happen? Then measurements of the sun over decades and even centuries allow us to study long-term changes on the sun. They allow us to study things like the solar cycle, that 11-year variation of the sun. It also allows us to study things like epochs of grand minima. I didn't mention this earlier, but there are times, spans of several decades on the sun at a time where it doesn't show a solar cycle. And we don't know why. It doesn't have sunspots. The first one and last one recorded, in fact. This was very close to the beginning, the invention of a telescope and routine observations of sunspots. But there was this period in the late 1600s and the early 1700s where there were barely any sunspots for decades on the sun's surface. And it's called the Monde Minimum. And there's a lot we don't know about these epochs. On the other hand, very fast cadence observations of the sun will allow us to study solar storms in detail before, during, and after. And it will allow us to understand the sequence of events, the chain of events that happens before and during a solar storm. Measurements of the sun's magnetism at the sun's surface allow us to make a three-dimensional reconstruction of the magnetism of the outer layers of the sun's atmosphere and into space. We do this using theoretical equations, Maxwell's equations. And we do this because it's really hard to measure the magnetism in the sun's corona. I mean, some people do it, but there's a fair amount of uncertainty associated to those measurements. So if you want to know what the magnetism of the sun is in the outer layers in the corona and beyond, we need to aid ourselves with this kind of tool, with magnetic field extrapolations, that are based on measurements at the surface and then a certain set of assumptions and some equations to extrapolate into outer layers. Of course, because we're making assumptions, our assumptions could be right, could be wrong. So there's a limitation to these kind of tools. Observations help us constrain numerical simulations. Numerical simulations are just computer implementations of mathematical models that describe physical systems. And they allow us to study physical systems that are really too hard to understand analytically. And by this, I mean, it's really hard to solve the equations that describe these physical systems just using pen and paper. We have to use the process and power of computers to be able to solve them. These equations are typically very general, and they could be applied to a variety of physical scenarios. So by constraining them with observations, we can make them produce solutions in the physical scenario that we're interested in, or at least in the range of parameters of physical scenario we're interested in. This is a simulation that I love by one of my colleagues, Suhum Fang. It's a simulation of a coronal mass ejection. And here, gray represents the sun's surface and black and white at two magnetic polarities, like in a magnetogram. They're probably two sunspots. But remember, this is a simulation, not an observation. And these red and yellow lines represent the magnetic structure in the atmosphere of the sun. And as new stuff emerges, new magnetism emerges down here, and new material emerges from under the surface, represented in blue, kind of it slowly rises, and then all of a sudden it reaches this point where it just explodes catastrophically. This is a coronal mass ejection. Observations, lastly, also help us test the robustness of models. Models ideally mimic observations of the sun, observations of the actual behavior of the sun. If a model is able to mimic an observation, whether it's in a specific way or in a statistical way, in a general way, then we can assume that the physical assumptions in the model and the physical processes in the model are describing the physical processes in the sun. But if a model is not able to describe an observation, then we know that there's something wrong with the model. And I say this from the perspective of an observational solar physicist. There's never anything wrong with observations. It's always the models. It's always, you know. But anyway, that would tell us that we need to take a model back to the drawing board and figure out what physical ingredients are missing. What are we missing that is not describing the observations that we make? Okay, so there's a lot of usefulness for these observations, of course, because they represent or they show reality in one way or another. This particular numerical simulation, this is numerical simulation, not an observation. Sometimes it's really hard to tell because it's so good. Basically shows a solar flare in the sun's atmosphere. And this was done by my colleague, Matthias Rempel, and his collaborators. So this takes me, okay, frontiers of data interpretation, I don't know what to call this, but in the last few years, the field of solar physics and many other fields have been resorting to artificial intelligence to address some of the problems and certain types of artificial intelligence, in particular machine learning. And these have been, I guess it's yet to prove, but they're really powerful tools to help us analyze the large volumes of data that are coming from telescopes and from models these days. And it's humanly impossible to do with a human brain, or many of them actually. And then they're also really good. These techniques are also really good at detecting subtle patterns hidden in the numbers. So there's this very promising avenue for diagnostics of data in machine learning that I don't know anything about, so please do not ask questions. So this takes me, I'm really close to the end. This is what my lab does. My lab does all of these things and many other things, actually. The High Altitude Observatory of ENCA studies basically the sun and earth connections. We want to understand how the sun's variability affects the Earth's atmosphere. So the science that we do spans all the disciplines from solar and heliospheric physics all the way to the Earth's ionosphere, magnetosphere, ionosphere, and Earth's atmosphere. And we do this with the mission of trying to inform and improve space weather forecasting. And we combine theory, observations, and numerical simulations to create this big picture understanding of the sun Earth system. We do not do space weather forecasting. The Space Weather Forecasting Center over at NOAA off of Broadway in 28th do space weather forecasting. And I don't know much about that, but I was listening to this talk a few years ago at the HEO and it was this forecaster from the Space Weather Forecaster, Space Weather Prediction Center, sorry. And he was saying that space weather forecasting lies several decades behind Earth's weather forecasting, both in accuracy and in sophistication. And there is a strong need for more reliable and more timely forecasts of space weather because there's a lot of technologies in Earth that depend on this. And space weather makes us vulnerable. So with this, I wanted to finish me talk with this image which I really like. It puts into perspective our place in the universe in my opinion. This is a picture taken from the Boyer year one spacecraft when it was about to leave the solar system. And that inside that blue circle is the Earth. And it looks just like a speck of dust and it just looks really fragile and really vulnerable. And whether you want it or not, we live inside the atmosphere of this star and we are subject to its temperamental behavior. And magnetism is at the root of all of space weather, larger, small or relentless or impulsive. Magnetism is the culprit. So only by quantifying the magnetism and the evolving magnetism of the sun, we can begin to be able to predict its behavior and to protect ourselves from its effects. So thank you.