Even with all the effort to build and deploy a five-spacecraft fleet, it isn't enough to identify the substorm trigger location. The problem? When the spacecraft align over the two possible trigger locations, they are, well, in a line. Since the substorm region is so huge, the substorm trigger could be somewhere to the east or west of that line.
"It would take 100 satellites to resolve this east – west ambiguity, which would make the mission hopelessly expensive," said Eric Donovan of the University of Calgary, THEMIS Co-Investigator. The mission is possible because of the only part of the substorm process that is visible – the aurora, also known as the northern or southern lights. Most times, the aurora resembles a slow-moving, greenish-white ribbon hanging in the sky.
During a substorm, the auroral ribbon may split into several ribbons and break into clusters that race north and south. "Since substorms are magnetically connected to the aurora, we can take advantage of signatures in the aurora to pin down where the substorm trigger is relative to the THEMIS satellite line," said Donovan. "This makes the mission possible with just five satellites."
The substorm auroral display is huge, covering an entire continent, so a network of observatories is needed. The THEMIS team built 20 ground-based observatories (GBOs), networked across Alaska and Canada. Each station includes a digital camera with a fish-eye lens to capture images of the aurora every three seconds, and a magnetometer to measure changes in Earth's magnetic field due to electric currents surging through the upper atmosphere. "The speed and quality of these images, 10 times faster and at least 50 times more detailed than any previous space-borne global auroral imager, is unprecedented," said Stephen Mende, GBO lead, at the University of California, Berkeley.
Building a network of 20 observatories is hard enough, but because the aurora is usually only visible in far northern regions of North America, they had to be placed in the wilderness. "There's not a lot of options up north in Canada, and we can't spend a lot of money," said Donovan. "Just putting in a power pole can cost $10,000. We've got competing requirements - we need it to be close enough to a town so we can hook into power, but we also need it to be dark enough to photograph the aurora."
"It was a real challenge. In 2002, the team sat in a room in Berkeley with a map of Canada and Alaska, and an understanding of the latitudes where the aurora is visible. We just started picking towns. We rented a truck and started driving around to check them out. It was a big job. We discovered that one of our towns no longer existed – the mine closed and it just went out of business."
"But it's been manageable so far. We've managed to find volunteer custodians in the towns, people who are ham radio operators, weather forecasters, or teachers. These people have technical skills and are drawn to projects like THEMIS. They can help with equipment maintenance and troubleshooting."
But the THEMIS team went beyond the auroral imagers: The current surge produced in space at the time of the auroral origin is also a means to navigate the satellite fleet to ensure it captures explosive magnetic field reconfigurations during substorms. Those currents deflect your ordinary compass, and with super-sensitive magnetometers they can pinpoint the longitude of the explosive energy release even on a cloudy night.
In fact, these space currents can be seen from low latitudes - as low as in Southern California and Florida - if far away from moving cars, trains and elevators. The THEMIS team decided to take advantage of this to engage elementary and high school students to promote space science in the classroom: the THEMIS UCLA team, which developed the magnetometers for all the GBOs, installed 10 ground magnetometers in competitively selected rural schools around the country. Teachers use the data to teach about the aurora not only in math and science, but also in history and art. "These students are our ambassadors to rural America," said THEMIS Principal Investigator Vassilis Angelopoulos, of the University of California, Berkeley. "Their excitement about space and THEMIS is just contagious."