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.

March 17, 2022: The March 13th CME did more than spark bright auroras. It also wiped out a lot of cosmic rays. Neutron monitors at the Sodankyla Geophysical Observatory in Oulu, Finland, recorded a sharp drop in cosmic radiation just after the CME arrived:

This is called a “Forbush decrease,” named after American physicist Scott Forbush who studied cosmic rays in the early 20th century.  It happens when a coronal mass ejection (CME) sweeps past Earth and pushes galactic cosmic rays away from our planet. Radiation from deep space that would normally pepper Earth’s upper atmosphere is briefly wiped out.

There’s something odd about this Forbush decrease. It’s a double dip decrease. Cosmic rays dropped precipitously on March 13th–then they surged midday on March 14th–then they dropped precipitously again. The up-and-down may be a sign of structure inside the CME.

As Solar Cycle 25 intensifies, more and more CMEs will sweep past Earth. Forbush decreases will become increasingly common and may even begin to overlap. This will cause a persistent decline in cosmic rays around our planet.

A recent paper in the Astrophysical Journal looked at the last two solar cycles and compared the daily rate of CMEs to the strength of cosmic rays. The plot, above, shows the results. At the peak of Solar Cycle 24, the sun was spitting out more than 5 CMEs per day; at the same time, galactic cosmic rays (GCRs) dropped more than 60%.

Evidence is mounting that new Solar Cycle 25 will be stronger than Solar Cycle 24. If so, CMEs will be more abundant and cosmic rays even more depressed–a welcome reduction for astronauts, air travelers, and even some mountain climbers.

Meanwhile, the March 13th Forbush decrease is still underway. Cosmic rays remain depressed 4 full days after the CME arrived.

March 4, 2022: A recent display of auroras over Canada has experts scratching their heads. The mystery? They were orange. Pilot Matt Melnyk was flying 36,000 feet over Canada on Feb. 23rd when he saw the strangely-colored lights from the cockpit window:

“I have been chasing and photographing auroras for more than 13 years (often from airplanes) and this is the first time I have ever seen orange,” says Melnyk.

What’s so strange about orange? Joe Minow of NASA’s Marshall Space Flight Center explains: “Theoretically, nitrogen and oxygen (N2, N2+, and O2+) can produce emissions at orange wavelengths, but these are typically weak compared to stronger emissions from the same molecules at the red end of the spectrum. It is hard to understand how orange could dominate in an auroral display.”

Even so, Melnyk says “these appeared to be real auroras.” The orange fringe danced in sync with regular red and green auroras overhead. It did not appear to be an artifact of city lights or distant twilight. Moreover, Melnyk saw the orange color with his naked eye, and his camera recorded it, too.

Kjellmar Oksavik, a space physicist at the University Center in Svalbard (UNIS), has an idea: “Normally, auroras are produced by electrons with energies less than 10 keV. Raining down from space, they stop an an altitude of 100 km where the dominant color is green (caused by electrons hitting oxygen). During strong activity, however, electrons can reach energies of 20 keV and even higher. These electrons penetrate deeper, all the way down to 80-100 km. Here nitrogen molecules dominate, with multiple emission lines in blue, purple, orange, red and magenta.”

“I think this is what is happening in the picture,” says Oksavik. “On this particular day the precipitating electrons were so energized that they reached deeper into the atmosphere (probably 80-90 km) where nitrogen molecules emitted a wide range of colors, that combines into what looks like an orange glow.”

Oksavik’s colleague Fred Sigernes, chief of the UNIS Aurora Observatory, agrees with Oksavik, but also wonders “why have we never observed this up here with our cameras in Svalbard?” It’s a mystery, indeed.

Have you observed orange auroras? Submit your photos here.