Until recently, researchers knew that electromagnetic disturbances in space called “substorms” caused the colorful streaks of light, but they didn’t know what triggered those storms. Now, with the help of a string of five satellites acting as giant stopwatches and a network of ground-based observatories, they have begun to find out.
They’ve traced the storms’ beginnings to a spot about one-third of the distance between the earth and the moon’s orbit, where the earth’s magnetic field lines snap together like rubber bands, flinging charged particles back toward earth.
“We finally had the right instruments in the right place at the right time, and it’s allowed scientists to make the right measurements to solve this heated debate,” said Nicola Fox, a senior scientist at the Johns Hopkins University Applied Physics Laboratory.
NASA launched the project, called THEMIS (Time History of Events and Macroscale Interactions in Substorms), in February 2007.
Space substorms happen as often as every few days, Fox said. In addition to causing the northern lights to dance, they can disrupt satellites, pose a danger to astronauts and even interfere with power lines on earth. Understanding the causes of substorms will also help scientists understand even larger, rarer magnetic storms that are more likely to cause those problems, she said.
“Just like the meteorologists who study tornadoes to understand more intense storms, so we want to study substorms to understand bigger storms,” said Vassilis Angelopoulos, THEMIS principal investigator.
The earth is surrounded by magnetic field lines that stretch from the north to the south poles, like a giant bar magnet. But charged particles from the sun — called the solar wind — distort that magnetic field, pulling it into a long tail called the “magnetotail.”
The magnetic field lines from the north and the south poles stretch out and out, until finally, at the point about one third of the way to the moon’s orbit, they snap back together and send charged particles flying back towards the earth.
However, scientists weren’t sure whether this “reconnection point” snap triggered the substorms, or whether it was another event, called current disruption, which is a kind of large-scale “short circuit” of currents of charged particles that flow closer to the earth — about one-sixth of the distance to the moon’s orbit.
To find out which of these theories was right, NASA launched five washing machine-size satellites that orbit the earth at different distances. The satellites lined up once every four days, and when scientists were lucky, a substorm occurred while the orbiters were lined up. Sensors on board the satellites recorded electric and magnetic field strength, as well as the charged particle flow.
Using those observations, along with data from ground-based observatories on earth, Angelopoulos and his colleagues figured out that the substorm disturbances begin farther out, at the point where the magnetic fields reconnected, and not at the current disruption location.
Their paper, which was published online this week in the journal Science, analyzed a particular substorm that took place Feb. 26.
The substorm was a good one, says study co-author Stephen Mende, because the satellites were lined up just right — in the “sweet spot” between the northern and southern magnetic field lines, called the neutral sheet, where they could best measure the charged particle flow.
The researchers still have much more work to do, Angelopoulos said; they plan to continue their observations for another year.
For one thing, they want to know what causes reconnection in the first place. Also, one of their observations surprised them — the aurora began only minutes after the reconnection, and before the current disruption. Both of the suggested theories had predicted that the current disruption would come before the aurora.
“This defies both our former models,” Angelopoulos says.
And finally, not every researcher is convinced yet that the debate has been decided. Tony Lui, another THEMIS researcher, says that there were other small electromagnetic disturbances picked up by the satellite near the reconnection point in the hour before the one that this study’s authors believe signaled the beginning of substorm. He thinks that researchers should be looking instead for larger disturbances.
“There were other disturbances […] they look identical in terms of fluctuations — so how can you pick the one?” he says.
Angelopoulos says that the skepticism is natural: “I wouldn’t expect any different from the THEMIS team and the community, this is the nature of scientific discourse.”
He said that although there were other small disturbances before the onset of the substorm, the correlations between the measurements from the different satellites support the conclusion that the substorm started with one particular disturbance at the reconnection point.
Researchers expect to collect dozens more events in the next year, he added, to figure out whether different types of substorms exist under different solar wind conditions.