Black aurorae are a mysterious phenomena, creating dark rings, shadowy waves, and black blobs that contrast with the glowing colors of the Northern and Southern Lights. Look closely or you might miss them. Nevertheless, powerful cosmic and earthly forces are needed to bring this celestial show to life.
The phenomenon is created under conditions that are the exact opposite of those in the normal aurora, hence their other nickname “anti-aurora”.
To rewind a bit, typical aurorae appear in the night sky when charged solar particles are sneezed out of the Sun and crash into Earth’s magnetic field. Here, they interact with gases in the ionosphere, a layer of the upper atmosphere, causing them to emit light that can culminate in stunning displays of green, red, purple, or blue lights in night skies.
Aside from bouts of intense solar activity, they are mainly seen near the Earth’s poles because the planet’s magnetic field directs charged solar particles toward the polar regions. These regions can be associated with negatively charged electric potential structures, which attract the incoming electrons.
Illustration of the black aurora mechanism proposed in the 2001 study.
Image credit: ESA 2001 / Illustration by Medialab
A 2001 study in the journal Nature used data from the European Space Agency’s Cluster spacecraft to show that black aurorae occur in parts of the ionosphere that have patches with relatively few negatively charged particles.
In these “holes”, there are positively charged electric potential structures, which repel electrons away from the ionosphere into space, as opposed to attracting the incoming electrons. So, instead of colorful lights, we simply see odd blots of darkness.
“The black aurora isn’t actually an aurora at all; it’s a lack of auroral activity in a region where electrons are ‘sucked’ from the ionosphere,” Professor Göran Marklund, lead author of the Nature study from the Alfvén Laboratory in Sweden, said in 2001.
Another study in 2021 dived deeper into the mystery by explaining anti-black aurora, tiny bits of extremely bright light that can be found within around 10 percent of black aurora patches.
The researchers write: “A patch or arc segment of enhanced luminosity, distinctly brighter than the diffuse background, which we name the anti-black aurora, may appear adjacent to the black aurora. The anti-black aurora is of similar shape and size, and always moves in parallel to the drifting black aurora, although it may suddenly switch sides for no apparent reason.”
Source Link: Black Aurora: A Mysterious Phenomenon Caused By Cosmic And Earthly Forces Colliding
