Things get complicated if the magnet is not a simple shape. If you bend a bar magnet, the field lines will bend as well. If you take a dozen magnets, a hundred, and throw them together, the field lines can get very distorted, because each bit of the magnetic field is attached to the object generating it. Mess with one and you affect the other.
The magnetic field of the Sun is generated by moving currents of gas—currents that get twisted, distorted, and bent around just like rivers on the Earth. These field lines may be generated beneath the surface of the Sun, but they don’t stay down there; they pierce through the surface, looping upward and back down into the interior in an incredibly complex, interwoven, and interconnected way. These magnetic field lines can really get their knickers in a twist, becoming entwined and entangled. When this happens, there are profound changes on the surface of the Sun.
For one thing, since the field lines and the gas are coupled, when the lines get tangled and compressed, the gas has a harder time moving around as well. It’s like a giant net is thrown over the gas, preventing it from moving freely. Hotter gas welling up from below can’t reach the surface, and regions where the lines are particularly dense begin to cool off. Since the brightness of the Sun is due to its temperature, a cooler region becomes dimmer, forming a dark area on the Sun called a sunspot. Because sunspots are inherently magnetic phenomena (they are really a cross section of the magnetic field lines where they intersect the surface of the Sun), they always come in pairs with reversed magnetic polarity: one is like a magnet’s north pole, and the other is the south pole.
Sunspots can be small, barely visible to telescopes on Earth, and they can be huge, dwarfing the Earth itself, with some so large that they can be seen by the naked eye when the Sun is on the horizon.10
In fact, it was the observation of sunspots that first keyed astronomers into the Sun’s magnetic field. Heinrich Schwabe was a solar observer in the early nineteenth century who counted the number of sunspots every day for decades. He discovered that the number of spots waxes and wanes with a period of about eleven years from peak to peak—we now call this the sunspot cycle. At the time of the maximum, there can be well over a hundred sunspots on the Sun, but at the minimum that number drops to essentially zero.
Schwabe decided to publish his results in 1859, and it was quickly determined that the times of peak sunspot number also corresponded to the times of peak magnetic activity on the Earth, indicating a connection between sunspots and magnetism. In 1908, the astronomer George Ellery Hale discovered that the magnetic fields in sunspots can be thousands of times stronger than the Earth’s, indicating the presence of intense energies being stored there.
This is a typical sunspot, appearing darker than the surrounding solar surface because of its cooler material. This particular spot is far larger than the Earth. The graininess of the Sun’s surface around the spot is caused by convection, rising currents of hot material that cool and sink back down into the Sun.
STANFORD-LOCKHEED INSTITUTE FOR SPACE RESEARCH AND BIG BEAR OBSERVATORY
Which brings us back to balance. As the magnetic field lines tangle up, there is a balance struck between the pressure built up by the magnetic energy stored in them and the tension that exists in the lines. Imagine the magnetic field lines are like steel coil springs, all tangled together and interconnected. The springs are compressed and want to expand, but the tension of the intertwined mess keeps them from springing back. Now keep compressing them and adding more springs, again and again. The energy stored up would get pretty impressive.
What happens if you take a bolt cutter and snip one of the springs?
Right. Better stand back.
The same thing happens in a sunspot—in fact, much of the physics is pretty similar to a convoluted mess of coiled springs, with the analogous tension and pressure. As the field lines get more entangled, and more are added, the pressure builds up. Sometimes the pressure is relieved early in the process, and not much happens. But other times it builds, and builds . . .
Loops of extremely hot material flow up from the Sun’s surface, following along the magnetic field lines. When the loops get tangled or twisted, a flare or coronal mass ejection can be triggered.
TRACE TEAM/NASA
Something’s gotta give.
Eventually, something does. The field lines emerge from the Sun in tall, graceful loops, with one footprint being the magnetic north pole and the other the south. If the gas flow zigs instead of zags, for example, the footprints can be brought together, or twisted past each other. The pressure in the coil goes up, but the tension can’t compensate. The line snaps.
There is a lot of energy stored in the field line (just like the energy stored in a spring). When it snaps—what solar physicists call magnetic reconnection—the energy is released. A huge amount of energy. The explosion is titanic, but in general constrained to a local region, causing what’s known as a solar flare.
A FLARE FOR DANGER
By coincidence, a solar flare was first observed in 1859—the same year Heinrich Schwabe published his discovery of the sunspot cycle.
On September 1, 1859, astronomers Richard Carrington and Richard Hodgson were independently observing the Sun. Before their eyes, a small part of its normally calm disk suddenly exploded in intensity, becoming far brighter. This burst of emission lasted for five minutes, and even to this day may have been the most luminous flare ever observed. Within a few hours of the observations of the flare, magnetometers (instruments that measure the strength and direction of a magnetic field) on Earth went crazy, registering huge fluctuations in the Earth’s magnetic field.
The Solar and Heliospheric Observatory detected this massive flare from the Sun on November 4, 2003. It was one of three huge flares that surprised scientists that day; no such string of events had ever been witnessed before. They marked one of the most active weeks for the Sun ever recorded.
SOHO (ESA & NASA)
They didn’t know it then, but at that moment the study of space weather was born.
They also couldn’t have known that the flare was caused when tangled magnetic field lines on the surface of the Sun suddenly realigned themselves. The energy stored in them was released like a bomb—the equivalent of 15 billion one-megaton nuclear weapons, or 10 percent of the total energy output of the Sun every second concentrated into one spot—hurling high-energy photons (particles of light) and subatomic particles both upward into space and downward onto the surface of the Sun. A typical flare from the Sun ejects billions of tons of subatomic particles outward at speeds that reach five million miles per hour—and in 2005, one extraordinary flare launched a blast of protons that reached the Earth in just fifteen minutes, indicating they were traveling at one-third the speed of light. These subatomic particles blast outward, generally straight out from the center of the flare. Because of this, the particles launched upward and outward from the flare are generally not a problem to us on Earth: they are focused enough that they usually miss us, causing no grief.
But along with the particles shot into space, a huge pulse of particles is shot down, onto the surface of the Sun. This heats the gas there tremendously, and creates an incredibly strong pulse of light. Now, that may not sound like a big problem; after all, how bad can light be?
Bad. But it depends on the kind of light.
What we call “visible light” is a narrow slice of a much wider range of electromagnetic radiation. Infrared light, for example, has less energy than visible light, and radio waves have less energy still. Ultraviolet (UV) light has more energy than the light we can see. Still higher-energy light is X-rays, and on up to gamma rays. UV, X-, and gamma rays are dangerous in large quantities. Each photon carries so much energy that it can radically alter any atom it hits, stripping off the atom’s electron, ionizing it.
Flares give off a lot of this kind of light. And unlike the particles of matter emitted in a solar flare, this light spreads out. A flare on the edge of the Sun’s disk will almost certainly
miss us with its particles, but any flare anywhere on the visible surface of the Sun is a potential danger because of the high-energy light it emits.
Picture a solar flare on the Sun: the tangled magnetic field lines over a sunspot suddenly snap, rearranging themselves, and releasing their energy. They heat the local gas up to millions of degrees, and a blast of X-rays surges outward.
Traveling at the speed of light, the high-energy radiation takes a little over eight minutes to travel the 90 million or so miles to the Earth. When it does, it slams into everything in its way: satellites, astronauts, and even the Earth’s atmosphere.
On the Earth’s surface, we’re protected from this onslaught by the thick air over our heads. But an astronaut in orbit is essentially naked, exposed to the wave of photons. A spacewalker caught by surprise will absorb many of the incoming X-rays, getting the equivalent of hundreds or even thousands of chest X-rays in a single flash.
X-rays are dangerous because when absorbed, they deposit all their energy into tissue. This can lead to cell and DNA damage. When DNA is damaged, mutations can occur that can (but do not always) lead to cancer.
Radiation absorption is measured in units called rems.11 Natural radiation coming up from the Earth’s surface surrounds us all the time; you get a dose of about 0.3 rem per year just by existing on the Earth. In high-altitude locations, like Denver, that can be as high as 0.5 rem due to both terrestrial and extraterrestrial sources. A dental X-ray, by comparison, gives a dose of about 0.04 rem, one-tenth of your normal annual background dose. The U.S. government has guidelines for employees who work in elevated radiation environments: the maximum safe whole-body dose is set at 5 rems per year.
A mild flare may expose an astronaut to several dozen rems of radiation. While that sounds bad, in fact the body can heal itself fairly well after such a one-time radiation dose. Cells heal, and small amounts of damaged DNA can be eradicated by the body’s natural defenses. That’s not to say it’s fun: the problems associated with this kind of dose are irritated skin and a higher risk of developing skin cancer or other forms of cancer. Male astronauts might also experience a temporary sterility lasting for a few months, and hair loss in both sexes is possible.
But if too much tissue is damaged, the body cannot heal itself. In a major flare, an astronaut could absorb hundreds of rems of X-rays. This can be fatal: there is simply too much cell damage for the body to repair itself. Over the course of several hours and days the astronaut suffers a slow death as cells die, the intestinal lining sloughs off, ruptured cells leak fluid into their tissue . . .the effects are horrifying. NASA takes this threat very seriously. When a flare is seen on the Sun, astronauts on the International Space Station retreat to a section that is more protected, letting the station itself absorb the radiation to safeguard the humans inside.
When astronauts return to the Moon they’ll have to deal with this as well. Lunar rock is an excellent absorber of radiation, so it’s likely that lunar colonists will cover their habitats with two or three yards of rock and rubble. It’s not as romantic as glass domes on the surface, but being able to actually survive a flare may take precedence over our preconceived notions of what a colony should look like from watching science-fiction movies.12
In a major flare, though, not just humans are in danger: our satellites can be fried as well. When an X-ray or a gamma ray from a flare hits the metal in a satellite, the metal becomes ionized. A very high-energy gamma ray can ionize many atoms in the satellite, causing a cascade of electron “shrapnel” to fly off the atoms. Remember, moving electric charges create a magnetic field. This sudden strong pulse of magnetic energy can damage electronic components inside a satellite (just as a magnet can damage your computer’s drive). The electrons themselves might short-circuit the hardware too.
Many civilian satellites have been lost in solar flare events. Military satellites are in many cases protected from this damage, and such radiation-hardened satellites can still operate even if there is a major flare. The effects of a nearby nuclear blast are similar to those of a flare, so these satellites may also survive a nuclear detonation in space (as long as debris and heat from the blast doesn’t get them).
Moreover, the Earth’s atmosphere absorbs the incoming high-energy light. While that protects us on the surface, the upper atmosphere can heat up from this and “puff up” like a hot-air balloon. If the atmosphere expands enough, it can actually reach the height of some satellite orbits. A satellite normally orbiting in a near-vacuum environment may suddenly find itself experiencing drag as it plows through the very thin extended atmosphere. This lowers the satellite’s orbit, dropping it into even thicker air, where it drops more, and so on. Even if it survives the initial flare, it may still be destroyed when it burns up in the Earth’s atmosphere! Many low-orbiting satellites are lost every solar cycle because of this effect. The American space station Skylab was destroyed this way in 1979.
Because of this, space agencies and commercial satellite owners watch for flares very closely. Flares are linked to the eleven-year sunspot cycle, tending to occur on or around the solar sunspot maximum, though for reasons still not well understood, the most energetic flares usually happen about a year after maximum. Incidentally, the 1859 flare, perhaps the brightest of all time, occurred a year or so before the sunspot maximum.
That flare induced quite a bit of magnetic activity on the Earth. While the flare itself probably did have some direct effect on the Earth, it’s now thought that it had some help.
HALO, HOW YOU DOING?
Normally, there is a relatively constant flow of material from the Sun. Called the solar wind, it’s a stream of subatomic particles accelerated by the usual suspect: the solar magnetic field. The solar wind blows off the Sun in all directions, and continues outward for billions of miles, well past the orbit of the Earth around the Sun. Near the surface of the Sun, the particles can be seen as a faint pearly glow called the corona. The corona is incredibly hot—billions of degrees—but extremely tenuous, like a laboratory-grade vacuum. But over the trillions of cubic miles of solar surface, even something so diffuse can add up to a lot of mass. Astronomers think of the corona as the atmosphere of the Sun, so, in a very real sense, we live in the atmosphere of a star.
This has some disadvantages. Atmospheres sometimes have bad weather.
When a flare erupts from the surface of the Sun, needless to say, it tends to have an effect on its environment. The blast of energy and particles from the flare goes upward, of course, away from the Sun, but it also goes downward, onto the surface. This creates a seismic wave on the surface of the Sun with tens of thousands of times the energy of the strongest terrestrial earthquakes. The Sun’s surface ripples as waves of energy are slammed into it. The magnetic field lines surrounding the energy get an enormous jolt as well, and many times this is enough to disrupt them. The lines going in and out of the Sun’s surface in the area reconnect, release energy, and disrupt more lines around them. More and more energy is released as the effect spreads and more lines reconnect.
As this occurs, the matter that was previously constrained by those magnetic fields suddenly finds itself able to expand under the intense pressure. Instead of a single coil springing open as in a flare, it’s as if they are all free to expand. The matter suddenly bursts outward in a coronal mass ejection, or CME.13
The energy of a CME goes more into accelerating particles than it does into giving off light, so the event is actually difficult to detect initially. In fact, while the first flare was seen almost two hundred years ago, CMEs weren’t first seen until the 1970s!
However, their effect is profound. Unlike flares, which are basically a local disturbance, CMEs involve a gigantic area of the Sun. If flares are like tornadoes—local, intense, brief, and focused—CMEs are solar hurricanes. The effect is not as intense, but much, much larger: as much as a hundred billion tons of matter are hurled into space at a million miles per hour, and that can do far more damage on a far bigger scale.
Once the CME erupts, it can cover the distance from the Sun to the Earth in one to four days. That’s all the warning we get.
It’s possible to see the actual event when it occurs. When you try to look at an airplane flying near the Sun, what do you do? You put up your hand to block the Sun, allowing you to see the plane. Astronomers do the same thing. They equip sunward-pointing telescopes with coronagraphs—generally very simple masks of metal that block the fierce light coming from the Sun’s surface—that allow fainter objects nearby to be seen. When a CME occurs, it can be seen by these telescopes as an expanding puff of light coming out from the Sun. If a CME is seen coming from the side of the Sun, astronomers breathe a sigh of relief: it will miss the Earth because it was aimed sufficiently far away from us. But sometimes the Sun is not so agreeable, and it sends a hundred billion tons of million-degree plasma screaming our way. This is seen as an expanding halo of light, because we are looking down the throat of an advancing front of subatomic particles accelerated to mad speeds.
Death From the Skies!: These Are the Ways the World Will End... Page 5