Scientists have discovered yet another enigma about our planet: the thermosphere has undergone serious shrinkage. The thermosphere is the largest portion of the Earth's atmosphere and is the next-to-last region before you reach the vacuum of outer space. The fact that it has contracted is not surprising; the thermosphere absorbs extreme ultraviolet (EUV) photons from the sun and warms and cools—expanding and contracting—in a pattern that follows the 11-year solar cycle. While we are coming out of one of the longer periods of low solar activity in a century, scientists have found that the thermosphere has shrunk some 28 percent. That's the largest drop in recorded history, and they cannot explain why.

Solar cycle 23 (the previous one) was unusually long—12.4 years—and the minimum between it and cycle 24 had the most days without sunspots since 1933, both of which will result in a cooled thermosphere. CO 2 in the lower thermosphere is the dominant cooling agent, since it traps heat in the lower regions of the atmosphere, so increased concentration of CO 2 will lead to a cooler, more contracted thermosphere. The cooling process is accelerated during a solar minimum as well, causing the entire system to be very complex and difficult to fully describe.

To understand what is going on, scientists from Naval Research Lab in Washington, DC and George Mason University have taken a look at how the incoming solar irradiance has affected the mass density of the outer reaches of our atmosphere.

We have a long history of accurate measurements of atomic density at a variety of altitudes in the thermosphere. This data is derived from measuring the drag on various spacecraft and satellites, and that gives us over four decades of detailed measurements. EUV photons have been directly measured only since the launch of the TIMED/SEE instrument in 2002, so a proxy for EUV irradiance must be used. The only reliable proxy that has been around for long enough is the continuous observation of the 10.7 cm solar radio flux. While it is not a perfect indicator, it is stable and well calibrated.

Using the global-average density data at 400km between 1967 and January 2010, the researchers found a low in 2008 where the mass density was "unequivocally lower than at any time in this historical record." The authors add that the 2008 minimum was the lowest since the beginning of the space age, 1957, when measurements of this type became possible.

The thermosphere density was a full 28 percent lower during the cycle 23/24 minimum than it was during the cycle 22/23 minimum. This is much larger than the decrease expected from the long-term trend seen over the past few cycles (that's six percent). In contrast, the solar radio flux was down only 3.7 percent between cycles.

What could drive such a large change? The authors examined the temperature profile of an arbitrary atmospheric column. Using a model that describes the temperature of the thermosphere as a function of altitude, they tweaked parameters in order to fit the data that has been seen in the recent contraction. They found that the temperature of the exosphere must be 14K lower, and that the levels of atomic oxygen at 120km must be 12 percent lower and other atomic species must be three percent lower than normal.

These results still don't explain why any of this happened, only what is needed for the model to fit.

So, the authors look at known factors and how strongly they can influence the thermosphere's mass density. The decrease in the solar radio flux is capable of explaining about one-third of the observed contraction. Another sixth or so can be explained by the elevated levels of CO 2 in the atmosphere, which radiatively cooled the thermosphere. However, known mechanisms stop there, leaving over half of the decrease in mass density unaccounted for.

The authors suggest a few possibilities. First, the relationship between the observed solar radio flux and the actual amount of EUV radiation reaching Earth may have changed drastically in the past few years. This would make the proxy measurement invalid, but there is no experimental support for it—it would have to reflect some undescribed solar phenomenon.

The other possibility they consider is that changes in the chemical makeup and dynamical processes in mesosphere and lower thermosphere affect the concentration of atomic oxygen at the lower boundary. The authors point out that such internal processes, coupled with known anthropogenic changes, could produce the missing 50 percent of the change that's unaccounted for.

The latter could represent an ominous change. As the authors themselves put it, "If changes in the radiative properties of the MLT [mesosphere and lower thermosphere] are responsible for the temperature and composition changes of the upper thermosphere, then the density anomalies may signify that an as yet unidentified climatological tipping point, involving energy balance and chemistry feedbacks, has been reached."

Before any hard conclusions are reached, the authors point out that the thermosphere's recovery as we climb out of this solar minimum needs to be monitored carefully. And we'll want to see what happens in 11 years, during the next solar minimum.

Geophysical Research Letters, 2010. DOI: 10.1029/2010GL043671