The Earth’s upper atmosphere expands and contracts a bit naturally. But a recent record-breaking collapse has scientists confused.
The thermosphere that surrounds the Earth can be anywhere from 55 miles high to 370 miles high, depending largely on solar activity. It is what intercepts extreme ultraviolet light (EUV) headed towards the Earth’s surface.
When sunspots and sunflare activity decline a lot, we are in what scientists call a “solar minimum.” In a solar minimum, the thermosphere cools and contracts a bit. But during a 2008-2009 solar minimum, the thermosphere contracted more than could be explained by the solar minimum, 2-3 times more.
A new research paper, released on June 19 in the journal Geophysical Research Letters, discusses this baffling collapse.
“This is the biggest contraction of the thermosphere in at least 43 years,” said John Emmert of the Naval Research Lab and lead author of the research paper. “It’s a Space Age record. Something is going on that we do not understand.”
Emmert hypothesized that high levels of CO2, which acts as a coolant in the thermosphere (video on how CO2 cools the upper atmosphere but warms the lower atmosphere here), could have been responsible for the collapse. However, incorporating that into the equation still didn’t solve the problem.
“[T]he numbers don’t quite add up,” Emmert says. “Even when we take CO2 into account using our best understanding of how it operates as a coolant, we cannot fully explain the thermosphere’s collapse.”
Low solar EUV could be responsible for about 30% of the collapse and high CO2 for another 10% or so, Emmert and his colleagues say. So, that leaves another 60% that is completely unaccounted for.
With the solar minimum now coming to an end, EUV increasing, and the thermosphere expanding, the researchers are looking for more clues to help them solve this puzzle.
Image Credit: NASA