Imagine a threat to the global community with the potential to damage communication satellites, interrupt navigation systems, shut down regional power grids, impede oil and gas exploration, expose aircraft crews to high levels of radiation, and endanger the lives of astronauts.

That threat exists, but it's not from any well-organized terrorist group. It's from the sun.

"Ultraviolet and X-ray radiation and particle emissions from the sun affect the ionosphere and [the Earth's magnetic field] and can cause lots of problems to space and ground assets," explained Russell Howard, an astrophysicist at the Naval Research Laboratory, in a Dec. 21 interview on Pentagon Web Radio's audio webcast "Armed with Science: Research and Applications for the Modern Military."

Howard was joined in the interview by George Doschek, head of the Naval Research Laboratory's Solar Terrestrial Branch, who explained that the "solar wind," a stream of charged particles and radiation constantly blowing toward the Earth, is intensified by disturbances in the sun's magnetic field, such as sunspots, solar flares, and coronal mass ejections.

"All the activity on the sun is produced when the sun's magnetic field is converted into particle emissions and acceleration, and radiation," Doschek said. He explained that the magnetic field of the sun actually stores energy, which is released in bursts when the structure of the field suddenly changes to a configuration that holds less energy.

"When that happens," he said, "we think that the excess energy goes into radiation and accelerating particles."

Howard, who holds a doctorate in chemical physics, said the release of electromagnetic radiation in the form of X-rays, ultraviolet rays, and gamma rays, interacts with the Earth's ionosphere.

"The ionosphere is an electrically charged layer of the Earth's atmosphere," he said. "It's most important, because it reflects radio waves, and that's what allows us to propagate radio waves around the Earth," said Doschek, who holds a doctorate in physics. Radiation affects the ionized particles in the ionosphere, causing them to absorb radio waves, causing communication fade-outs, he explained.

"That's the issue of the satellites themselves," Howard said. "But with ionospheric disturbances, you are also getting an increase in electron density in certain areas, and this can cause failures in GPS." He added that there were about 30 minutes of complete GPS outage in December 2006.

In addition to GPS, Howard noted that the effects of solar storms on communications satellites extend to such things as cell phones, pagers, television, the Internet and streaming video. "Society is becoming increasingly dependent on space-based assets," he commented.

Howard added that strong ionospheric disturbances can also cause ground controllers to lose track of low orbit satellites. "Electromagnetic energy comes in and heats the atmosphere," he said. "When you heat the atmosphere, you get increased density at spacecraft altitude, and that causes an increase in drag, and you can lose track of them."

Solar radiation also can pose a threat to humans. "The radiation can damage astronauts, or if you're flying [in an aircraft] over the polar regions, you have to worry about getting too much radiation from X-rays and ultraviolet radiation," he said.

"The transpolar routes are becoming extremely popular for the airlines," Howard added, "so the crews have to wear radiation dosimeters to measure how much exposure they are getting."

High-energy particles also pose a hazard to anyone working high in the atmosphere or in space, Howard said. "Particles can also be released, and they're coming at fantastic speeds, 500 times that of a bullet, and their combined mass is a million times that of a Nimitz-class carrier," he explained.

"With a coronal mass ejection, you can get a billion tons of matter moving 1 million miles an hour toward the Earth," Doschek added. He noted that coronal mass ejections were first identified by researchers at NRL in 1971.

The solar wind interacts with the Earth's magnetic field, Doschek said. "When solar particles get into the magnetosphere, they're trapped there, and that's when they can do a lot of damage," he noted.

In a large solar storm, the particles also can damage equipment. "You can get energetic particles at hundreds of electron volts of energy, and these can damage electronics in our space assets," Howard said.

"They can cause electrical discharges inside the spacecraft and destroy the circuitry," Doschek added, "and they can cause disruptions in the software and communication links in the satellite until it has to be rebooted."

The solar wind can directly affect people on Earth, as well.

Howard described the impact of the solar wind on the Earth's magnetosphere as a force that puts pressure on the magnetic field. With large solar storms, the pressure intensifies and distorts the shape of the field.

"The magnetosphere, when it gets compressed, induces a current in the Earth's crust," he explained, "and power transmission lines can get a huge amount of back current into transformers that actually burns them up. I've seen pictures of copper straps that are two inches thick that are melted. It's just amazing."

A mass ejection in 1989 shut down the Quebec power grid, which is connected to power grids along the entire East Coast of the United States, Howard said. Quick action on the part of an engineer disconnected the Quebec grid from the other grids. "It was within seconds before it would have taken out the power for the entire northeast part of the U.S.," Howard said.

In addition to the loss of transformers, which cost about $10 million to replace, the disruption of power was estimated to be a loss of $2 billion of gross national product, he said.

These induced currents in the Earth's crust also can affect oil and gas exploration. Howard explained that oil prospecting often is done by trailing a magnetometer behind a ship to look for changes in the magnetic field structure. "A huge oil or gas deposit would be indicated by a change in the field properties," he said, "but if one of these storms comes along, you've completely lost that activity."

While it may not be possible to stop the solar storms, it would be useful to know when they are coming to better prepare for them. To do that, researchers need to have a better understanding of their nature, and NRL has been conducting solar research since 1946, Doschek said.

"We try to understand what is causing the atmosphere to do what it's doing," he said, "which means that we want to understand the mechanisms by which the sun's magnetic field, and the energy within that field, can be converted to particles."

Doschek explained that most solar research needs to be accomplished above the atmosphere, using remote sensing instruments carried on spacecraft.

"We use spectrometers," he said, "to determine the temperature and density, and even the motions within the sun's atmosphere. We have another instrument, called a coronagraph, which blocks out the main radiation from the sun and looks at the outer part of the atmosphere of the corona. With this instrument, we can see hot gases and coronal mass ejections as they come toward the Earth."

Reserachers have made some progress in developing notice of solar activity. "We have instruments that are actually on two NASA spacecraft that are in orbit around the sun, "Howard said. "It's called the STEREO mission. They're looking at the sun and the region between the sun and Earth from two different viewpoints."

Howard explained that these sensors, located more than 100 million miles from Earth, are able to observe solar activity as it happens and more precisely pinpoint the time the charged particles will reach the Earth. But not all solar events send high-pressure streams of dangerous particles towards the Earth.

"Part of our research is to determine the parameters that we need to be studying in order to say whether this will have a powerful impact on Earth or not," he noted.

Another approach to forecasting solar events is with the use of computer modeling. Doschek described three-dimensional numerical simulation models that attempt to portray how changes in the sun's magnetic field get converted into thermal energy based on complex circulations on the sun's surface, and observed phenomena like sunspots.

"The magnetic field is part of a dynamo," he explained, "and when sunspots appear - these are regions of strong magnetic field - they get fed into the model and the field moves around the sun."

From these models, the researchers have developed a predictive algorithm for the solar wind. "That works on the basis of how the magnetic field originates on the sun," Doschek said.

Howard acknowledged that the modeling effort is in the infant stages, but noted that the observations and measurements being made by space-based sensors are providing a foundation for improving the models.

"Hopefully, in 10 to 15, maybe 20 years, we'll be much better than we are today," he said.

(Bob Freeman works in the Office of the Oceanographer of the Navy.)