How Does the Sun Affect the Earth?

The 1974 horror film "The Texas Chainsaw Massacre" kicks off with brilliant footage of solar flares and descends into violence and mayhem. While there's no evidence to suggest an actual link between increased solar activity and human violence, it can result in a great deal of earthbound devastation -- from city-destroying hurricanes to massive energy failures.

Two types of solar phenomena can affect the Earth in such a drastic manner: solar flares and coronal mass ejections. Scientists think that both types of events are caused by changes in the sun's magnetic field.

ANALYSIS: A solar storm could be responsible for some serious cable television interference after knocking out one of our vulnerable communications satellites in April.

In the case of solar flares, the magnetic field triggers a powerful explosion in the sun's atmosphere. This explosion accelerates subatomic particles near the speed of light, producing a broad range of electromagnetic radiation.

Coronal mass ejections (CMEs) on the other hand, involve the ejection of actual material from the sun's corona. Billions of tons of electrified gas fly away from the sun at incredibly high speeds.

"Those are the two kinds of space weather that have a direct effect on Earth," explains NASA solar astrophysicist C. Alex Young.

Summoning the Storms
To understand how space weather affects Earth weather, you first have to understand how the sun churns our atmosphere into motion.

"Because we're on a sphere, the sun heats up the equatorial regions more than the poles," explains Dr. Jeff Masters of Weather Underground, "so the Earth has to develop circulations to distribute the heat. This keeps the equator from getting hotter and hotter and the poles from getting colder and colder. The Earth's trying to balance out this uneven distribution of heat."

As the sun can only heat a portion of the globe at any given time, Earth's rotation causes additional east-to-west wind patterns to form. Weather as a whole comes down to the global circulation of hot and cold air. An increase in solar weather can result in an increase in the solar energy to reach the Earth's atmosphere. We can anticipate these changes by analyzing sunspots, which follow an 11-year cycle. An increase in sunspots signifies an increase in solar weather.

"Between the peak of the 11-year sunspot cycle and the minimum, you get a change in solar radiation of about 0.1 percent," Masters says, "and that 0.1 percent is enough to change the global surface temperature by 0.1 degrees Celsius (0.18 degrees Fahrenheit), which isn't a lot but it is noticeable. You can see that in the temperature record."

The sun has the greatest impact on Earth's lower stratosphere, the location of the ozone layer. Here, the atmosphere experiences a 0.4 percent temperature change due to the impact of ultraviolet light from the sun. 

"When you heat up the lower stratosphere, you also heat up the upper troposphere, the layer where weather occurs," Masters says. "The top of the troposphere is important because it controls the stability of the atmosphere. If you've got a very hot surface temperature and a very cold upper troposphere temperature, that's an unstable situation. It tends to lead to strong updrafts, which make for stronger storms and stronger hurricanes."

Hurricane Intensity
Masters explains that when the 11-year sunspot cycle is at its peak, there's a reduction in both updraft and hurricane strength. For the United States, this breaks down to just a 25 percent chance of a hurricane hitting the mainland. When the cycle is at a minimum, however, those chances skyrocket to 64 percent, according to a 2010 Florida State University study.

According to some estimates, the odds of the United States experiencing three or more hurricanes are 50 percent less likely when the cycle peaks.

At this moment, however, the solar cycle is at an 11-year minimum.

"If the theory holds true, then the U.S. is at an increased risk of getting hit by multiple hurricanes this year because the sun is currently at a solar minimum," says Masters, who attended the 29th Conference on Hurricanes and Tropical Meteorology this week.
 
Busting the Power Grid
In addition to affecting storm intensity, solar flares and coronal mass ejections can also wreak havoc on an increasingly technological Earth.  

"When a CME is directed toward the Earth, it can slam into the planet's protective shield, called the magnetosphere," explains Young. "This is essentially a magnetic bubble that sits around the Earth and helps to protect us from charged particles and plasma. Sometimes that material hits the protective shield and compresses it. Then it rebounds and it kind of rings like a bell."

This "ringing" of the magnetic shield can induce electrical currents both in the atmosphere and on the ground. A large enough shock can travel though electrical wiring or even along oil pipelines. 

"The power grids that we have in the U.S. and actually all over the world are interconnected and very fragile," Young says. "If the current's large enough, it can short out the largest of the transformers, which can knock out the power grid over the scale of a country, of a continent or even across the whole globe."

Canada's Hydro-Quebec power grid experienced just such a shock in 1989 from a particularly powerful sun storm. The grid went down for more than nine hours, resulting in revenue losses in the hundreds of millions of dollars. An 1859 solar flare caused telegraph system failures in Europe and North America.

Accelerated Particles
As you might imagine, satellites face an even greater risk of solar storm damage, as they lack the protection of Earth's atmosphere. For astronauts, the danger is ever greater.

"When a CME travels through space, it actually scoops up material and accelerates it in a kind of a snowplow-like effect," Young said. "These accelerated radiation particles will travel through human cells and cause disruption, genetic damage. If you're in space and you're not protected by the atmosphere, you can suffer dangerous or even lethal doses of this ionizing radiation."

Fortunately, scientists are becoming increasingly capable of predicting solar storms through the analysis of sunspots. If we know a powerful CME or sun flare has occurred, we can temporarily shut down vulnerable satellites and power grids in the same way a concerned homeowner might unplug a TV during a thunderstorm. For their part, astronauts can seek shelter against CME-accelerated protons inside the thick hull of their ship.

"We're getting there," Young said. "I've heard the analogy that we're maybe at the same level of prediction that terrestrial weather was back in the '50s. But we still have a long way to go. We still have a lot of research to do to be able to give a seven-day forecast. We're not to that point yet." 

Even more solar and terrestrial weather:
How Weather Works
Could an extremely powerful solar flare destroy all the electronics on Earth?
How the Sun Works
How the Earth Works
New Satellite to Lay Sun's Secrets Bare
NASA Launches Solar Observatory
SDO Shines Light on the Sun