Magnetic Apple Peeler
As anyone who’s ever used a compass knows, the earth is a magnet with north and south poles. It’s surrounded by invisible lines of magnetic force that bulge outward from either side of the planet. When the charged gas of a coronal mass ejection hits the planet’s magnetic field, the lines of magnetism become distorted. Electric fields produced by the distortion pull on the ionosphere, the charged layer of Earth’s upper atmosphere that makes long-distance radio communication possible and through which satellite transmissions must pass. A slice of the ionosphere peels off and streams upward, creating a structure of ions and electrons known as a plume. Imagine the earth is a giant spinning apple, Foster says, with the outer ionosphere as its peel. The ejection comes in like a giant parer, slicing off a strip of ionosphere as the earth rotates beneath it. The peel rises over the Caribbean and spreads northwest over North America as the world turns. Some of the material is lost to space, but much of it gets trapped by Earth’s magnetic field and redistributed.Like a squall in a snowstorm, the plume that rises over the U.S. can cause the worst problems because, says Foster, “It’s in a region of space where it really should not be.” The tempest can break up high-frequency signals used by the military or distort satellite transmissions beaming through the ionosphere. That includes Global Positioning System satellites, and intercontinental airline flights whose positions can’t be tracked accurately have to fly farther apart. “These are all the nasty space weather consequences of this magnetic apple peeler,” Foster says.
His efforts to understand the plume rely in large part on Haystack’s two radar dishes dedicated to ionospheric studies: a 68-meter fixed dish that stares straight up and a moveable 46-meter dish that Foster says is probably the world’s largest steerable radar antenna. There are six major antennas in all, including a research radio antenna that takes soundings of the ionosphere, much as sonar on a ship takes soundings of the ocean floor. A group of optical instruments measures temperature and motion in the atmosphere.
Foster uses these instruments, measurements of signal distortions from GPS receivers scattered across the globe, and images taken by satellites in the U.S. Department of Defense’s Defense Meteorological Satellite Program to create a map of the activity of electrons and ions in the upper atmosphere. The two dishes send radar pulses out into the atmosphere. The pulses strike the electrons in the ionosphere and are reflected. With proper processing, Foster’s team can determine the density of electrons at a given point in space, as well as whether they’re part of hydrogen, oxygen, or helium plasmas, how hot they are, and which direction they’re moving. In essence, the researchers can create a map of the upper atmosphere, showing the shifting shape of the plume. They can measure structures about 50 kilometers long and chart the variations in electron density throughout the visible ionosphere.
From this data, Foster produces computer animations that show what the various instruments see during a magnetic storm. One animation displays two maps of North America side by side, accompanied by a tiny clock. A magnetic storm progresses across each screen-one that occurred Nov. 20, 2003, and the other from two years earlier, April 11, 2001. Blobs of blue, yellow, and red indicate the different densities of electrons and ions in the atmosphere. As the clock reaches about 7:00 p.m. Greenwich mean time, or 2:00 p.m. eastern standard time, both storms produce a rust-colored plume over the Caribbean that moves northwest. Two and a half years apart, the storms are remarkably similar in appearance and behavior.
“These events are really repeatable and not at all random,” Foster says. “It happens like this consistently during every storm.”
Additional evidence from a magnetic storm that struck on March 31, 2001, helped corroborate the consistency of the plume phenomenon, showing that it occurred at a specific place over the earth at a specific time. During that storm, Foster’s team was pointing the radar dishes upward to gather data. Meanwhile, NASA’s IMAGE satellite was flying overhead, looking down on the same events. IMAGE was the first satellite to record such a plume of plasma evolving, and its data matched that which Foster’s team collected separately.
The formation of the plume, Foster believes, is abetted by a defect in the earth’s magnetic field, an area called the south Atlantic anomaly, centered 500 to 1,000 kilometers southeast of Brazil, where the magnetic field is about 30 percent weaker than everywhere else on the planet. If a storm reaches the planet just after the anomaly is directly facing the sun-early afternoons, eastern standard time-the effects of the storm are the most intense. Other, smaller plumes do happen elsewhere on the planet, but the largest, most disruptive ones occur over North America.
Other researchers have tracked plumes over the years, but the MIT observations are the first to show how electrons and ions spread to other parts of the atmosphere during a solar storm, an important component of anticipating the behavior and severity of magnetic storms. To this end, the Center for Integrated Space Weather Modeling is using Foster’s data and explanations to improve storm models. Its goal is to provide information so that officials can be ready to make adjustments when a storm hits. “You can find alternate routes to get your communications or know you have to leave more tolerances for your navigation,” Hughes says. “You might not get around it, but you know you have to react to it.”
Foster is furthering his own efforts by studying the path of the plume. Observers in Greenland and Scandinavia have seen plumes in their skies, but Foster now believes that they’re seeing the same North American plume crossing the North Pole and moving over Europe. That means when observers see the plume forming in the Caribbean in early afternoon, they know there will be disturbances in Europe around midnight. “It’s kind of neat if he’s now seen it going over the pole,” says Hughes.
Foster hopes to determine whether the severity of a plume can be correlated with aspects of a solar sunburst. Since sunspots appear on the sun’s surface days before a coronal mass ejection makes contact with Earth, scientists could have the means to provide ample warning about a storm and its intensity.
That sort of observation is part of the long, slow process of building up enough data to make statistically accurate models. In 10 years, Foster says, scientists will be able to predict the course of big storms and the specific effects they will have on human technologies.
“We need to see a thousand cases, and we need to base what we say on the results, and we’re just now getting to that point,” Foster says.
Here’s one prediction: the next big test of how Foster’s work has improved scientists’ ability to predict space weather will come in 2011, the next solar maximum, when plenty of sunspots are likely to flare again.