What’s up with that crazy giant hole on the sun? That’s the question I was addressing during my short appearance on Fox News last week. Or rather, it’s the question I was trying to address. My explanation contained a few poor word choices, which resulted in a confusing and misleading description of solar activity. That is the danger of live television: once a conversation goes off track, it’s hard to get back in real time, with only 100 seconds to go.
The coronal hole, as seen by the SOHO spacecraft on July 18. (Credit: ESA/NASA/SOHO) These days nothing on TV really vanishes, of course. My clip is preserved online, where anyone can react and critique it—and boy did I get some critiques. The good news is that the story of the hole on the sun is a fascinating one, and the queries and criticisms it inspired point the way to a much deeper understanding both of how the sun works and how it affects us here on Earth. I gave a lemon of an interview. Time to make some lemonade.The Sun is a Mass of Incandescent…Plasma Let’s start with the image itself. It was taken with the Extreme Ultraviolet Telescope aboard the Solar Heliospheric Observatory, or SOHO. The observatory was created jointly by NASA and the European Space Agency. It’s been operating since 1995, making it one of the older solar space telescopes; some commentators criticized me for using an “outdated” image of the sun but this one was released just last month by NASA, and for good reason: SOHO is still doing great work, and there’s a lot to appreciate in this one view. One important thing to know in interpreting the image is that the sun is not actually made of gas. Under intense heat, atoms lose some or all of their electrons and become plasma, sometimes called the fourth state of matter. It is technically correct to say that there are no atoms in the sun—the entire mass of the sun is ionized. Plasma is electrically charged and so can hold a magnetic field, not unlike an electromagnet. That explains a lot about how the sun works, and about why it looks the way that it does. In the sun’s upper atmosphere, or corona, magnetic waves heat the plasma to extreme temperatures of 3 million degrees Fahrenheit or even higher, far hotter than the 10,000-degree F. temperature of the photosphere, the visible surface of the sun. How exactly that mechanism works is an area of active study. (Another point of contention: in my Fox News segment I referred to the temperature in degrees without specifying the scale. Solar physicists typically express temperatures in the Kelvin scale. But if I’m speaking to a lay audience, I always assume Fahrenheit: When was the last time you heard someone say, “Man it’s broiling today—it must be 310 K outside”?) SOHO’s Extreme Ultraviolet Telescope picks up high-energy radiation from that superheated coronal plasma, detecting several different wavelengths of extreme ultraviolet radiation. Shorter, more energetic wavelengths come from hotter parts of the corona. Almost all of the radiation detectable to the telescope comes from the corona. The photosphere appears black at these wavelengths, because that region is not nearly hot enough to shine brightly in extreme ultraviolet. Anatomy of a Coronal Hole Now we get to the heart of the story. The sun’s corona is a seething mass of magnetic fields. In most places the field loops out of the sun and back, trapping and heating the plasma. But in some locations the field lines are open; think of them as straws sticking straight out of the sun rather than bending around and inward. The places where the field lines are open are known as “coronal holes.”
The coronal hole, as seen by the newer Solar Dynamics Observatory one solar rotation earlier. (Credit: NASA/SDO) That is what you are seeing in this SOHO image: The huge blank part of the sun is a region where the temperatures and densities in the corona are relatively low, so it appears dark in the SOHO image. The corona leaks out from the sun in all directions, creating a flow known as the solar wind. But coronal holes are associated with particularly high-speed gusts of wind that travel at up to 500 miles per second—nearly 2 million miles per hour. Those strong winds contribute to space weather, the movement of particles and magnetic fields in space that can disrupt power and communications on Earth. Here is where I got in trouble in my TV appearance. I described the dark part of the SOHO image as a chunk of the sun that is missing because it is flying our way at 2 million miles per hour. Error #1 was using the term “chunk.” I think of the sun as chunky because magnetized plasma holds together in a way that ordinary gases and liquids do not; the loops, filaments, sunspots, and other structures on the sun are evidence of that property. But chunk implies a solid, and the plasma in the corona is very far from a solid. One indication of just how thin the coronal plasma really is: Small, sungrazing comets pass right through it without getting blown apart by solar winds. They are damaged far more by the sun’s gravity and radiant heat. And the corona itself carries surprisingly little heat energy, even though it is at a temperature of millions of degrees, because it is so sparse. If you could somehow shield a person from the sun’s direct rays, the heat flux from the corona would be about the same as it is at home at room temperature, according to astronomer John Brown of the University of Glasgow. The molecules in Earth’s atmosphere each carry far less energy, but ordinary air is far far denser than the corona. Error #2 was saying that part of the sun is “missing.” I was thinking about the coronal hole as a depleted region of the corona. Temperatures and densities are lower there than elsewhere in the corona, because the sun’s plasma is not trapped in closed tubes of magnetic field. But again, there is a serious problem of connotation. Missing implies that something was there and is suddenly gone. In reality, coronal holes evolve over many months, and even years, as the sun’s 11-year cycle of magnetic activity rises and falls. The coronal hole in the SOHO image is part of a long-lasting gusty patch of solar wind. As for "flying our way," I'll give myself partial credit there. The hole in question is not aligned with Earth (although other coronal holes certainly are), so its high-speed wind mostly passed over us. And any wind from the date when the SOHO image was taken would have already passed Earth, but the same region of the sun comes around each time the sun rotates, roughly once a month. Strictly speaking, the corona is flying off all the time and this coronal hole region is a place where it is flying off faster and more efficiently than in other regions of the sun. Plasma from the coronal hole region really is flying out at speeds of 1 million to 2 million miles per hour. That high-speed component of the solar wind blows past Earth and jostles our planet’s magnetic field, contributing to stormy space weather. But it is a very different thing than an explosive event like a solar flare or a coronal mass ejection, a bona fide eruption in which a tremendous mass of material is ejected all at once. A Realistic Measure of Risk Stepping back, I am encouraged that so many people took me to task for my sloppy statements. There are active communities of astronomy enthusiasts, and even of specific space-weather enthusiasts, on Twitter, Facebook, and YouTube. Facebook's Space Weather Trackers do a particularly good job. They stepped up to critique my Fox News comments, fill in the missing parts of the story, and point readers to sources of authoritative information. (They also had some colorful things to say about me personally. I’m not used to being called an “idiot” so often by people who are not family, and I do believe this is the first time I’ve ever been accused of being both a Fox News shill and an Obama clone—at the same time, by the same person. Not to mention quite a number of unprintable words. Nothing like reading comments on the Internet to keep a man humble.) My biggest concern is that my Fox News comments miscommunicated the real risks of solar activity (“fearmongering” as some of my critics called it). I noted that coronal holes happen all the time, but that is only part of the story. Because holes are an ongoing source of the high-speed component of the solar wind, they contribute to the background effects of space weather. The real risks come from those explosive events, the flares and coronal mass ejections, or CMEs—the equivalents to hurricanes and tornadoes as opposed to a day of high winds. Flares and CMEs are associated with many of the biggest costs of space weather. And those costs are not insignificant. The National Research Council estimates that space weather causes $200 million to $400 million in damage each year in the United States. The effects of high solar activity show up in all kinds of places. It can disrupt airplane communications, disrupt GPS signals, speed the corrosion of pipelines, and shorten the life of satellites through radiation damage or by hastening the rate at which their orbits decay. Lloyds, the British insurance company, put together a bracing summary of the risks. The real concern, which I highlighted at the end of my Fox appearance, is that the sun might experience a superflare: an extreme explosive event far more intense even than the typical flare, but also far rarer. I pointed out that the last true superflare happened in 1859, an eruption known as the Carrington event; milder ones happened in 1921 and 1960. Such a solar eruption is quite different than a coronal hole. In fact, it’s essentially the opposite. A coronal hole is a slow, steady release of solar plasma from a magnetically open region of the sun. A superflare is a fast, explosive release from a place of extreme magnetic confinement. A superflare could wreak havoc on electronic technology. Damage to communications satellites alone would total tens or even hundreds of billions of dollars. The even bigger concern is that a superflare would induce intense power surges in the electrical grid, possibly overloading transformers and triggering a blackout across a large part of the world. Fixing all those transformers could take many weeks or months—time without routine power for hospitals, computers, factories, etc. The National Research Council put a possible $1-2 trillion price tag on such an event. Where We Go from Here
Solar Probe Plus, illustrated here, will approach within 4 million miles of the sun, protected by a carbon-foam shield that can withstand temperatures of 2,600 degrees F. (Credit: NASA/JHU-APL) Fortunately, better models of solar activity and a better understanding of space weather can go a long way toward ameliorating those risks. That SOHO image is just one small part of the story. Newer space telescopes like STEREO and the Solar Dynamics Observatory are providing much better readings of what the sun is doing. Even more information will come from NASA's daredevil Solar Probe Plus mission, set for a 2018 launch. Space weather forecasts help satellite operators and electrical utilities to prepare for power and radiation surges. Concerns about worst-case scenarios will help make sure that they never come to pass. The sun itself may be cooperating with us, at least in the short run. Solar activity has been trending somewhat downward over the past half-century, for reasons poorly understood. Then again, the Carrington event occurred during a cycle when solar activity was not unusually intense. Scientists still have a long way to go toward fully making sense of how the sun works. In the future, I’ll aim to do a better job communicating that effort. Follow me on Twitter: @coreyspowell
