TCI is the “Thermosphere Climate Index”, a number NASA publishes every day to keep track of the temperature at the top of Earth’s atmosphere–a layer of gas researchers call “the thermosphere.”

“The thermosphere always cools off during Solar Minimum–and it warms up again during Solar Maximum,” explains Martin Mlynczak of NASA’s Langley Research Center. “It’s one of the most important ways the solar cycle affects our planet.”

When the thermosphere warms, it expands, literally increasing the radius of Earth’s atmosphere. This expansion increases aerodynamic drag on satellites in low-Earth orbit, which can bring them down prematurely. When the thermosphere cools, it shrinks; satellites get a reprieve.

Mlynczak and colleagues recently introduced the “Thermosphere Climate Index” (TCI)–a number expressed in Watts that tells how much heat nitrogen oxide (NO) molecules in the thermosphere are dumping into space. During Solar Maximum, TCI is high (“Hot”); during Solar Minimum, it is low (“Cold”).

Above: An historical record of the Thermosphere Climate Index. Mlynczak and colleagues recently published a paper on the TCI showing that the state of the thermosphere can be discussed using a set of five plain language terms: Cold, Cool, Neutral, Warm, and Hot.

TCI is based on measurements from the SABER instrument onboard NASA’s TIMED satellite. SABER monitors infrared emissions from carbon dioxide (CO2) and nitric oxide (NO), two substances that play a key role in the energy balance of air 100 to 300 kilometers above our planet’s surface. By measuring the infrared glow of these molecules, SABER can assess the thermal state of gas up there.

Although SABER has been in orbit for only 17 years, Mlynczak and colleagues recently calculated TCI going all the way back to the 1940s. “SABER taught us to do this by revealing how TCI depends on other variables such as geomagnetic activity and the sun’s UV output–things that have been measured for decades,” he explains.