Melbourne: Researchers are wondering why part of the Earth’s atmosphere recently suffered its biggest ever collapse known to us.
The collapse occurred in the thermosphere, a rarefied layer of the Earth’s upper atmosphere between 90 and 600 kilometres above the surface, which protects us from the Sun’s extreme ultra violet (EUV) radiation.
A team led by Dr John Emmert from the US Naval Research Laboratory found that the thermosphere went through its biggest contraction in 43 years.
The thermosphere typically expands and contracts in line with the Sun’s 11-year solar cycle. During solar maximum when solar activity increases, the thermosphere heats up to 1100°C and expands. The opposite happens during solar minimum.
Currently, the Sun is experiencing its longest solar minimum on record, with little sunspot activity and few solar flares or coronal mass ejections.
To see how the solar minimum is affecting the thermosphere, Emmert and colleagues monitored the impact of atmospheric drag on satellites in low Earth orbit (LEO). These satellites fly through the thermosphere, so the thicker the thermosphere the more drag it puts on spacecraft.
The researchers expected to see a contraction in line with solar minimum, but the level of collapse was up to three times greater than solar activity alone can explain.
They believe an increase in atmospheric CO2 may explain the contraction. CO2 has a cooling effect in the thermosphere, which would then amplify the impact of the extended solar minimum.
But the researchers found low levels of EUV radiation only account for about 30 per cent of the collapse, while the increase in CO2 levels account for another 10 per cent at most.
That still leaves some 60 per cent, which can’t be explained by current modelling.
Moreover, the current anomaly appears to have started in 2005, well before the current solar minimum began.
According to Emmert and colleagues, there may be an as yet unidentified climatological tipping point involving both energy and chemical feedbacks.
"They are suggesting that the whole composition and chemistry of the thermosphere might have changed and they way we come out of this solar minimum will tell us how it’s changed," ABC Science quoted Dr Phil Wilkinson, of the Ionospheric Prediction Service, with the Australian Bureau of Meteorology, as saying.
Wilkinson added: "Or it could be that minor constituents in the thermosphere play a far more important role than we thought and we’re only realizing that now. If that’s the case then the thermosphere will eventually return to normal conditions."
Wilkinson said the only real impact of the thermosphere collapse is in space, where less atmospheric drag will keep spacecraft in orbit longer.
He said: "That’s good news if you want to keep your satellite flying, bad news if you’re trying to de-orbit space junk."
Emmert`s study appears in Geophysical Research Letters.