Though much of the far southern hemisphere is bathed in winter darkness, there was a bit of light last week. The aurora australis, or southern lights, fluttered north and south of the Antarctic Circle on several nights.
The image above was acquired by the Visible Infrared Imaging Radiometer Suite (VIIRS) on the NASA-NOAA Suomi NPP satellite in the early morning hours of July 18, 2022. The aurora and the light of the waning gibbous Moon faintly illuminated the icy coast of eastern Antarctica. VIIRS has a day-night band that detects nighttime light in a range of wavelengths from green to near-infrared and uses filtering techniques to observe signals such as city lights, auroras, and reflected moonlight.
In the image above, the sensor was actually detecting the light emissions as energetic particles rained down from Earth’s magnetosphere into the upper atmosphere. Fast-moving electrons from space collide with oxygen and nitrogen molecules in the ionosphere and chemically excite them. As the gases return to their normal state, they emit small bursts of energy in the form of light (photons); oxygen molecules and atoms tend to glow green, white, or red, while nitrogen tends to be blue or purple. This ghostly light originates at altitudes of 100 to 400 kilometers (60 to 250 miles).
Astronomer Ian Griffin, director of New Zealand’s Otago Museum, was part of a team that also captured time-lapse photographic imagery (above) of the aurora late on July 17 (a few hours before VIIRS passed over). As reported on Twitter and Spaceweather.com, scientists used a new auroral camera system deployed on the Stratospheric Observatory for Infrared Astronomy (SOFIA), a Boeing 747 outfitted for astronomy by NASA and the German Aerospace Center. The plane was flying south of New Zealand at roughly 62 degrees latitude at the time.
Solar Cycle 25 is now underway and more sunspots and solar storms are starting to emerge, which should bring more opportunities to see auroras in both hemispheres. Solar cycles are traditionally measured by the rise and fall in the number of sunspots, but they also coincide with increases in solar flares, coronal mass ejections (CMEs), radio emissions, and other forms of space weather. Scientists have forecasted that the Sun should reach its next peak of activity (solar maximum) in mid-2025.
NASA Earth Observatory image by Joshua Stevens, using VIIRS day-night band data from the Suomi National Polar-orbiting Partnership. Story by Michael Carlowicz.