NASA's TIMED mission - short for Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics - yielded a batch of new discoveries to end its 15th year in orbit. From a more precise categorization of the upper atmosphere's response to solar storms, to pinpointing the signatures of a fundamental behavior of carbon dioxide, TIMED's unique position and instruments, along with its decade-plus data record, continue to give scientists an unparalleled look at Earth's upper atmosphere, our interface to space. Indeed, the very length of the data set has provided unprecedented opportunities to analyze near-Earth space.

Launched Dec. 7, 2001, NASA's TIMED spacecraft observes the chemistry and dynamics of the upper regions of Earth's atmosphere - composed of the mesosphere, thermosphere and ionosphere. The critical region that TIMED studies spans altitudes of about 40 to 110 miles above Earth's surface. Here, the atmosphere is just a tenuous wash of particles that reacts both to energy inputs from above - from changes in the space environment largely due to the sun - and forcing from below, including terrestrial winds.

TIMED's 15 years of data provides scientists an unrivaled perspective on changes in the upper atmosphere. The long lifespan allows scientists to track the upper atmosphere's response to both quick-changing conditions - like individual solar storms - throughout the sun's 11-year activity cycle, as well as longer-term trends, such as those related to climate change.

"By being a longer-term research mission, TIMED naturally became a brand-new mission," said Diego Janches, TIMED project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Now we can address completely new science that we couldn't with observations performed over shorter periods of time." For example, a 2015 study based on TIMED data revealed that carbon dioxide was increasing unexpectedly fast in the upper atmosphere, a trend that had persisted over TIMED's long data record.

All of TIMED's instruments are still producing data, enabling continuing studies of the upper atmosphere. Recently, researchers from the University of Colorado at Boulder used TIMED measurements to evaluate the presence of a trace chemical called nitric oxide in the upper atmosphere. Produced by the changing chemistry triggered by different types of space weather, such as solar flares and geomagnetic storms, nitric oxide can actually counteract some of the effects of space weather.

When energy is transferred into Earth's atmosphere during a geomagnetic storm, some of that energy manifests as heat - and that heat causes the upper atmosphere to swell, as individual molecules fight for more room.

"This swelling means there's more stuff at higher altitudes than we would otherwise expect," said Delores Knipp, a space scientist at the University of Colorado at Boulder. "That extra stuff can drag on satellites, disrupting their orbits and making them harder to track."

This is called satellite drag. The most intuitive understanding of the upper atmosphere's response would have the most powerful eruptions from the sun ultimately creating the largest amount of swelling, and therefore, more powerful satellite drag.

However, something counteracts that swelling: nitric oxide. During some geomagnetic storms, the energy input triggers a chemical reaction that produces larger amounts of this nitric oxide. Nitric oxide acts as a cooling agent at very high altitudes, radiating energy to space, so a significant increase in this compound sometimes causes something called overcooling.

"Overcooling causes the atmosphere to quickly shed energy from the geomagnetic storm much quicker than anticipated," said Knipp. "It's like the thermostat for the upper atmosphere got stuck on the 'cool' setting."

That quick energy loss counteracts the previous expansion, causing the upper atmosphere to collapse back down - sometimes to an even smaller state than it started in, leaving satellites to orbit through lower-density regions than anticipated.

One of TIMED's instruments tracks the quantity of nitric oxide in the upper atmosphere. Researchers compared this TIMED data with data on geomagnetic storms to pin down what types of coronal mass ejections, or CMEs - giant clouds of ejected solar material - lead to an overproduction of nitric oxide and rapid collapse of the upper atmosphere.

"Overcooling is most likely to happen when very fast and magnetically-organized ejecta from the sun rattle Earth's magnetic field," said Knipp.

This means that, counterintuitively, the most energetic CMEs are likely to trigger the geomagnetic storms that provide a net cooling and shrinking effect on the upper atmosphere, rather than heating and expanding it as had been previously understood.

TIMED data has also been key to understanding some of the fundamental physics of the upper atmosphere - including those that govern atmospheric loss, a phenomenon that shapes the habitability of planets. For example, Mars' ongoing loss of hydrogen atoms from its upper atmosphere may be partially responsible for the planet's lack of water.

In another TIMED study, published in Nature Communications on Dec. 6, 2016, TIMED data indicated the presence of a significant population of hot hydrogen atoms at altitudes as low as 170 miles, much lower than previously expected.

"This result suggests that current atmospheric models are missing some key physics that impacts many different studies, ranging from atmospheric escape to the thermal structure of the upper atmosphere," said Lara Waldrop, a space scientist at the University of Illinois at Urbana-Champaign and co-author on the study.

A comprehensive analysis of the TIMED data showed that the temperature of this atomic hydrogen rises when solar activity falls, counter to the behavior of most other neutral molecules in the atmosphere.

"The hydrogen density distribution is critical to the investigation of our atmospheric system as well as its response to space weather," said Jianqi Qin, a space scientist at the University of Illinois at Urbana-Champaign and lead author on the study.

Another recent work by researchers at NASA Goddard; George Mason University, Fairfax, Virginia; and SRI International, Arlington, Virginia, has helped scientists clarify a fundamental behavior of carbon dioxide molecules. The new understanding of the complex chemical reactions in the upper atmosphere allows researchers to better track this critical compound in the 15 years of measurements TIMED gathered while flying over Earth's night side.

"This new finding is particularly key for the polar night regions, where the long-lasting lack of solar radiation is supposed to significantly influence the chemistry and dynamics," said Peter Panka, a doctoral student at George Mason University and lead author on the study.

At night, a recently discovered energy transfer mechanism for carbon dioxide leads to the observed infrared light emission. Now that scientists understand this phenomenon, they can better track the densities of carbon dioxide in the upper and middle atmosphere at night.