by Dr. Tony Phillips
The TIMED satellite
monitoring the temperature
of the upper atmosphere
The sun is entering one of the deepest
of the Space Age. Sunspots have been absent for most of
2018, and the sun's ultraviolet output has sharply dropped.
New research shows that
Earth's upper atmosphere is responding.
"We see a cooling
trend," says Martin Mlynczak of NASA's Langley Research Center.
"High above Earth's
surface, near the edge of space, our atmosphere is losing heat
energy. If current trends continue, it could soon set a Space
record for cold."
These results come from
SABER instrument onboard NASA's
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
at the very top of the atmosphere - a layer researchers call "the
always cools off during Solar Minimum. It's one of the most
important ways the solar cycle affects our planet," explains
Mlynczak, who is the associate principal investigator for SABER.
When the thermosphere
cools, it shrinks, literally decreasing the radius of Earth's
This shrinkage decreases
aerodynamic drag on satellites in low-Earth orbit, extending their
lifetimes. That's the good news.
The bad news is, it also delays the
natural decay of
space junk, resulting in a more cluttered
environment around Earth.
Layers of the atmosphere.
To help keep track of what's happening in the thermosphere,
Martin Mlynczak and colleagues recently introduced the
"Thermosphere Climate Index" (TCI) - a number expressed in Watts
that tells how much heat NO (nitric oxide) molecules are dumping
Solar Maximum, TCI is high
Solar Minimum, it is low
"Right now, it is
very low indeed," says Mlynczak.
"SABER is currently
measuring 33 billion Watts of infrared power from NO. That's 10
times smaller than we see during more active phases of the solar
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.
A 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.
As 2018 comes to an end, the Thermosphere Climate Index is on
the verge of setting a Space Age record
"We're not there
quite yet," says Mlynczak, "but it could happen in a matter of
"We are especially pleased that SABER is gathering information
so important for tracking the effect of the Sun on our
atmosphere," says James Russell, SABER's Principal Investigator
at Hampton University.
"A more than 16-year
record of long-term changes in the thermal condition of the
atmosphere more than 70 miles above the surface is something we
did not expect for an instrument designed to last only 3-years
Soon, the Thermosphere
Climate Index will be added to Spaceweather.com as a regular
data feed, so our readers can monitor the state of the upper
atmosphere just as researchers do.
Mlynczak, Linda A. Hunt, James M. Russell, B. Thomas
Thermosphere climate indexes:
Percentile ranges and adjectival descriptors -
ournal of Atmospheric and Solar-Terrestrial Physics.
Mlynczak, M. G.,
L. A. Hunt, B. T. Marshall, J. M. RussellIII, C. J. Mertens,
R. E. Thompson, and L. L. Gordley (2015) -
A combined solar and geomagnetic index
for thermospheric climate - Geophys. Res. Lett.,
42, 3677 - 3682.
Mlynczak, M. G.,
L. A. Hunt, J. M. Russell III, B. T. Marshall, C. J. Mertens,
and R. E. Thompson (2016) -
The global infrared energy budget of
the thermosphere from 1947 to 2016 and implications for
solar variability - Geophys. Res. Lett., 43,
11,934 - 11,940.