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Authors: Ph. D. Philip Plait

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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
BOOK: Death from the Skies!
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