Newly-Launched ICON Observatory Sees the December 2019 Eclipse

What happens when airglow is temporarily “turned off”?

Just over six weeks after launch and early calibrations, NASA’s Ionospheric Connection Explorer (ICON) mission was presented with a unique opportunity. ICON flew very close to the December 26 solar eclipse track that extended across Asia, and observed the major changes in upper atmospheric airglow that naturally occurred. ICON’s four instruments, primed to look at the ionosphere, the dynamic region where Earth meets space, were in position to observe the effects. What happens when airglow –the natural glow of Earth’s atmosphere caused by solar radiation – is temporarily “turned off” when the sun is blocked by the moon’s shadow for a few minutes? Preliminary data shows that all four instruments – MIGHTI, EUV, FUV and IVM – were able to see changes the eclipse wrought.

MIGHTI

The Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) tracks red and green airglow 100 km above the Earth’s surface to measure how the neutral atmosphere moves. MIGHTI looked for the cold spot caused by the moon's shadow that puts the atmosphere in motion and generates a planetary-scale wave. Such a wave was observed during the 2017 Great American Eclipse (below).

A simple view of MIGHTI’s data shows the effect on the airglow brightness. Further analysis will provide associated changes in thermospheric winds and temperatures.

Effect of eclipse on the airglow brightness as seen by ICON’s MIGHTI instrument – Brian Harding

EUV

ICON was observing Earth’s extreme ultraviolet dayglow, itself produced by solar ultraviolet radiation, as it was affected by moon’s shadow during the eclipse. Shown below are data from successive orbits of EUV, the Extreme Ultraviolet instrument. The plot shows that the brightness dropped very rapidly from maximum down to 40% in just nine minutes. ICON EUV measures several different wavelengths in the extreme-UV range, and these behaved a bit differently because of the physics of the atmosphere at different altitudes where the emission is produced.

Light curves (brightness vs. time) for four emission lines seen by EUV – Martin Sirk

FUV

ICON’s Far Ultraviolet instrument is looking at the height and density of the daytime upper atmosphere. FUV also measured the daytime far ultraviolet emissions during the eclipse as they were affected in a way similar to EUV. During the normal midday peak in the eclipse, the brightness dropped very rapidly to 30% of its normal daytime value, and the nitrogen emission dropped even more, to 22% of the daytime value.

FUV observed a drop in the dayglow emissions - Harald Frey

The brightness dropped rapidly during the eclipse - Harald Frey

Below, the movie of the six altitude profiles from FUV shows the increasing brightness when the spacecraft moves from the night side to the dayside where the solar extreme ultraviolet light ultimately produces the airglow emission from oxygen (left) and nitrogen (right). Around 5:44, the observed area comes to the shadow region from the eclipse and the airglow brightness drops to its minimum at 5:51. When the viewed area leaves the shadow region the brightness increases again until the instrument sees the evening region and moves into the night.

IVM

IVM temperatures - Kealan Hennesy

The ion velocity meter (IVM) measures the temperature of the charged particles around the Earth. These charged particles are produced when sunlight hits the atmosphere. However, IVM’s data from the orbit with the eclipse does not show a major change in the temperature compared to the previous or next orbits, suggesting that the ionosphere at 585 km is not rapidly affected like the dayglow seen nearer 200 km altitude.

It is likely that the ionospheric effects far from the eclipse are quite different and possibly much larger than the effects at the eclipse location. This is because of the connection between the dynamical response to the eclipse, carrying the effect across the planet in atmospheric waves.

Map of ICON's path(s) near the maximum eclipse. When cross-checked with the plot above, they generally match up quite well.

More observations to come

ICON will be making observations over the next two years, greatly increasing our understanding of this relatively unexplored region between Earth and space, and of its potential for impact on communications and GPS, space weather prediction, and protection of our space assets from radiation. Look for more remarkable results from the ICON science team here soon!