The total eclipse of the sun on 21 August 2017 lasted less than three minutes for observers on the ground. But eclipse-watching scientists using a variety of sophisticated tools are working to patch together observations from dozens of locations to compile a continuous record as the path of totality swept 3,500 kilometres across the United States.
Final results from most of these studies are still pending, because nobody before has ever attempted such to collect this type of data, let along analyse it.
“We’re inventing the analysis,” said Matt Penn of the National Solar Observatory in Tucson, Arizona, US, at a meeting of the American Geophysical Union early this week in New Orleans, Louisiana. “It’s going to take time.”
Penn’s project, called the Citizen Continental-America Telescopic Eclipse (CATE) experiment used 68 small telescopes distributed to students and citizen scientists along the path of totality.
Of these, 61 were able to obtain good observations. “At all times, at least one telescope was looking at the corona,” Penn says. “At some times, we had up to five simultaneously.”
In combination, he says, these instruments produced 45,000 images that are now being merged into a high-resolution movie spanning 83 minutes of the activity in the sun’s corona.
That’s important because a total eclipse is the only time at which telescopes can view important inner sections of the corona. At other times, even if a device called a coronagraph is used, in an attempt to mimic an eclipse by blocking out the brightness of the solar disc, the laws of optics do not permit the super-crisp images of the inner corona that are possible during an actual eclipse. And learning more about the inner corona is important, says NASA solar astrophysicist Lika Guhathakurta, because “this is the missing link where space weather gets formed.”
Already, Penn says, his team’s images have revealed material flowing out from the sun at speeds of 100 kilometres per second — something that has never been seen before. They also show interactions between the sun’s visible surface and the inner edge of its super-hot corona. “We are hoping to analyse this in more detail,” he says.
Other ground-based telescopes observed the corona in infrared radiation, which can help reveal its magnetic field. {%recommended 5617%}
“Infrared is very sensitive to the magnetic field,” Guhathakurta says. And she adds, “at the end of the day it’s the magnetic field that shapes the corona and is at the heart of everything that goes on in the solar wind.”
Another team, led by Amir Caspi of the Southwest Research Institute in Boulder, Colorado, used two high-flying jets to chase the eclipse, in tandem. This allowed the scientists to obtain an uninterrupted view, lasting for nearly eight minutes.
Already, the images are extraordinary, but more image processing will be required to remove all the jitter and pull out additional details. “What we are looking for are dim features and waves,” Caspi says. “We can’t see them yet, but we will when we are finished.”
Other scientists used the eclipse to study phenomena closer to home. Greg Earle, of Virginia Tech, Blacksburg, Virginia, looked at the way the sudden dimming of sunlight altered the ionosphere, an upper layer in the earth’s atmosphere that, among other things, affects long-distance radio transmissions.
“When an eclipse happens, it creates a hole in the ionosphere that affects radio waves,” he says.
What his team found by beaming out radar waves during the eclipse wasn’t groundshaking, but it did verify current models of ionospheric behavior. “If you can validate your model using an eclipse,” he says, “then you can have high confidence you can use it to predict other things,” including how ionospheric events might impact communications, navigation signals, and even military warning systems for approaching missiles.
Meanwhile, Jay Herman of NASA Goddard Space Flight Centre, Greenbelt, Maryland, used satellite images to calculate that the eclipse produced a 9.6% decrease in the total amount of solar energy reaching the earth.
Again, the find wasn’t earthshaking, but it could be used to help calculate how sunlight-blocking from clouds (as opposed to the moon) might affect the earth’s energy budget — a difficult problem for current climate models.
Still another team, led by Angela Des Jardins of Montana State University, recruited 55 groups of college and high school students to launch weather balloons during the eclipse. Most of these balloons were for high-altitude studies, but twelve were designed to study the earth’s lower atmosphere, seeking to see how it reacted to the rapid passage of the Moon’s shadow.
Studying eclipse weather is important, says Guhathakurta, because during the eclipse, solar radiation can decrease three times faster than during a normal sunset. “This allows calibration of our models,” she says.
One of the goals was to see how the eclipse affected the planetary boundary level — the height at which surface features interact with the atmosphere to create temperature changes.
Normally, this layer, about 1500 metres above the surface, rises during the day and falls at night. And that was exactly what Des Jardins’s data showed, even revealing the effects of big mountains, such as Oregon’s Mt Hood or Wyoming’s Teton Range, both traversed by the eclipse. “This was anticipated, but has never been measured in this level of detail,” she says.
