Though the sun lies 93 million miles (149 million km) from Earth, its unceasing activity assures an impact on our planet far beyond the obvious light and heat. From a constant stream of particles in the form of solar wind to the unpredictable bombardment from solar flares and coronal mass ejections, Earth often feels the effects of its stellar companions. Less noticeable are the sunspots crossing the solar surface, though they are related to the more violent interactions. All of these fall under the definition of "space weather."
Watch a very large solar flare:
Studying the surface of the sun can reveal small, dark areas that vary in number and location. These sunspots, which tend to cluster in bands above and below the equator, result from the interaction of the sun's surface plasma with its magnetic field.
Sunspots are cooler regions that cap some of the intense energy below them. Here’s how they form:
The material at the solar equator travels significantly faster than the material at the poles. The magnetic field lines become warped. When the magnetic field is strong enough — and twisted enough — jet streams of flowing currents create ropes of magnetism. Most of the rope lays inside the sun, but part of it may break through the visible layer, where it is viewed in the form of two sunspots. The pair are polar opposites, literally; think of them as magnetic north and south, with the rope acting as the magnet in between. [Photos: Sunspots on Earth's Closest Star]
At temperatures of 3,800 kelvins, sunspot temperatures are nearly 2,000 K less than the rest of the sun. But don't let the numbers fool you. If a single sunspot stood alone in the night sky, it would be ten times brighter than the full moon.
Similarly, sunspots might seem small when compared to the 865,000-mile (1,392,000 km) diameter of the sun, because they typically cover less than 4 percent of its visible disk. However, ranging from 1,500 miles to 30,000 miles (2,500-50,000 km) in size, they can reach the width of the planet Neptune, the smallest of the gas planets. But with a lifetime of anywhere from a few days to a few weeks, sunspots are far less permanent.
Sunspots do not appear in random locations. They tend to be concentrated in two mid-latitude bands on either side of the equator. They begin appearing around 25 to 30 degrees north and south of the center. As the solar cycle progresses, new sunspots appear closer to the equator, with the last of them appearing at an average latitude of 5 to 10 degrees. Sunspots are almost never found at latitudes greater than 70 degrees.
It takes approximately 11 years for the sun to move through the solar cycle that is defined by an increasing and then decreasing number of sunspots. As it reaches the close of a cycle, new sunspots appear near the equator, while a new cycle produces sunspots in higher latitudes. The cycles overlap; sunspots from the previous cycle can still develop even after sunspots from the new cycle appear. So solar scientists have a very difficult time saying exactly when one cycle ends and the next begins.
As of early 2011, solar cycle 24 was under way, headed toward a peak of activity expected in 2013.
Scientists measure the activity of the sun by keeping track of the number of sunspots appearing on its surface. Since the invention of the telescope, sunspot counts have been relatively constant. In 1849, astronomers at the Zurich Observatory began observing and counting sunspots on a daily basis. The Solar Influence Data Analysis Center in Belgium and the U.S. National Oceanic and Atmosphere Administration are responsible for monitoring sunspot activity today.
The high magnetic fields in the sunspot-producing active regions also give rise to explosions known as solar flares. When the twisted field lines cross and reconnect, energy explodes outward with a force exceeding that of millions of hydrogen bombs. [The Sun's Wrath: Worst Solar Storms in History]
Temperatures in the outer layer of the sun, known as the corona, typically fall around a few million kelvins. As solar flares push through the corona, they heat its gas to anywhere from 10 to 20 million K, occasionally reaching as high as a hundred million.
Because solar flares form in the same active regions as sunspots, they are connected to these smaller, less violent events. Flares tend to follow the same 11-year cycle. At the peak of the cycle, several flares may occur each day, with an average lifetime of only 10 minutes.
The largest, X-class flares, have the most significant effect on Earth. They can cause long-lasting radiation storms in the upper atmosphere, and trigger radio blackouts. Medium-size M-class flares can cause brief radio blackouts in the polar regions and the occasional minor radiation storms. C-class flares have few noticeable consequences.
When the energized particles exploding from solar flares race toward us, they arrive in only eight minutes. Astronauts in space risk being hit by these hazardous particles, and manned missions to the moon or Mars must take this danger into account. Everyone else is shielded by the Earth's atmosphere and magnetic field. Sensitive electronic equipment in space can also be damaged by these energetic particles.
Absorbing X-rays affects the atmosphere. The increase in heat and energy result in an expansion of the Earth's ionosphere. Man-made radio waves travel through this portion of the upper atmosphere, so radio communications can be disturbed by its sudden unpredictable growth. Similarly, satellites previously circling through vacuum-free space can find themselves caught in the expanded sphere. The resulting friction slows down their orbit, and can bring them back to Earth sooner than intended.
Despite their size and high energy, solar flares are almost never visible optically. The bright emission of the surrounding photosphere, where the sun's light originates, tends to overshadow even these explosive phenomena. Radio and optical emissions can be observed on Earth. However, since X-rays and gamma rays fail to penetrate the atmosphere, only space-based telescopes can detect their signatures.
Coronal Mass Ejections
The magnetic field lines that twist up to form solar flares occasionally become so warped that, like rubber bands under tension, they snap and break, then reconnect at other points. The gaps that form no longer hold the sun's plasma on its surface. Freed, the plasma explodes into space as a coronal mass ejection (CME). [Stunning Photos of Solar Flares & Sun Storms]
It takes several hours for the CME to detach itself from the sun, but once it does, it races away at speeds of up to 1,000 km (more than 7 million miles per hour). The cloud of hot plasma and charged particles may be up to a hundred billion kilograms (220 billion pounds) in size.
As with solar flares, if the CME is aimed in our direction, it takes the particles eight minutes to reach Earth. However, the particles take anywhere from one to five days to travel the distance to our planet. The solar wind, a constant stream of charged particles ejected by the sun, acts on the cloud like a current on a boat. Faster CMEs feel the drag of the wind and slow down, while those with low initial velocities speed up.
- Auroras (Northern Lights): When the energy from a solar storm reaches the vicinity of Earth, charged particles in our planet’s upper atmosphere interact with air molecules to create aurora. These Northern Lights, as they are also called, can be fantastic displays of color. The solar wind also generates a near-constant but less spectacular display of aurora.
Many solar storms aren't aimed toward us. At the high point of the solar cycle, the sun may produce as many as five CMEs in a given day; even at the low point, it averages one a day. The spherical shape of the sun means that most of them miss the Earth completely. In fact, we can't even observe all of the ejections; those emerging directly opposite our planet are undetectable.
However, when the sun does eject a cloud of plasma and gas directly toward us, the incoming matter seems to surround the sun. Much like a baseball falling from the direction of the sun can seem to grow larger and dwarf the star, the so-called "halo coronal mass ejection" can appear to overshadow its source. Such ejections cause the most problems for the people on Earth.
Like solar flares, CMEs bring an increase in radiation to astronauts and electronics in space. But unlike flares, they also bring charged particles of matter that interact with the field surrounding our planet. The results vary depending on the size, speed and magnetic strength of the particles.
When the particles reach the Earth's magnetic sphere, they stretch and distort. Much like a tree in a strong wind, the day side — the first side affected — is compressed, while the night side is stretched out like a tail. When it reconnects on the night side, it releases the energy found in a bolt of lightning. While lightning lasts on the order of microseconds, however, the magnetic storm created lasts far longer. It races back toward Earth's upper atmosphere.
The sudden increase in power can damage sensitive electronic equipment. Power transformers can overload, causing long-lasting blackouts. Long metal structures like oil and gas pipelines can carry currents, which can enhance their corrosion over time and lead to devastating effects if proper safety measures are not in place. The resulting variations in the ionosophere can disrupt GPS signals, giving inaccurate readings.
On Sept. 1, 1859, Richard Carrington and Richard Hodgson, both amateur English astronomers, independently made the first observations of a solar flare, one which resulted in the largest geomagnetic storm ever recorded. Auroras, which normally occupy the polar regions, were visible in tropical latitudes. Telegraph operators reported being shocked — literally — by their instruments. Even after unhooking them from the power supply, messages could still be transmitted, powered by the currents in the atmosphere.
The so-called Carrington Event would be far more devastating if it happened today, given the greater reliance on electronics and the expanded power supply. However, thus far, it is the strongest storm yet recorded.
Observing the Sun
NASA is currently implementing Project Solar Shield to provide warnings to vital systems after an Earth-affecting CME occurs. This allows satellites and power transformers to be shut down if necessary for a short period of time. The result is a short- term, controlled blackout rather than a longer one caused by the destruction of vital equipment. [Amazing New Sun Photos from Space]
Similarly, several satellites keep the entire sun under constant observation. NASA’s Solar & Heliospheric Observatory (SOHO) spacecraft studies the sun, while the Solar Dynamic Observatory (SDO) focuses on solar atmosphere. And the Advanced Composition Explorer (ACE) samples particles from the sun as they stream toward our planet. These programs will help bring a greater understanding of the subject of space weather on Earth.