NGC 891. Credit: R. Jay Gabany

 

Corona Discharges
Jan 26, 2011

From galaxies to the laboratory, transverse electric currents accompany a main discharge channel.

That "main discharge channel" experienced by most people comes in the form of a terrestrial lighting bolt. Slow-motion studies reveal that each flash is a complex episode. Multiple leader strokes descend from the clouds, while similar, less visible potential contacts rise from the ground. Once two points of contact are made, the electrical energy stored in the cloud-to-ground capacitor discharges, drawing current from several square kilometers.

On Earth, a surface lightning stroke is accompanied by transverse or "corona discharges" at right angles to the main channel. They appear to be “tributaries” joining the primary discharge. Surface lightning forces close tributaries to be parallel because of electromagnetic forces between them. Brachiated burns or cuts in various materials at different scales are sometimes called "Lichtenberg figures."

Lichtenberg figures are named after Georg Christoph Lichtenberg. They form when lightning bolts strike some material on Earth – it can be soil or even human tissue. They have also been artificially generated in blocks of acrylic plastic. Many Lichtenberg figures have been discovered in the south polar region of Mars and have been discussed in previous Pictures of the Day as Martian "spiders."

To understand the connection that plasma formations in space have with laboratory experiments the scalability of plasma phenomena must be considered. Plasma discharges can produce the same formations irrespective of size. The same patterns appear in laboratories, on planets, around stars, and inside galaxies.

What have been called "streamers" were seen inside the shell of gas and dust that makes up the haze around the nucleus of Comet Holmes 17P. They remain in the same alignment as they had since their initial formation. Despite several months of travel, they did not rotate to stay aligned with the Sun. As has been discussed in previous Picture of the Day articles, the braided filaments are the sign of helical Birkeland currents.

A few years ago, scientists analyzing data from the Subaru Telescope announced the discovery of a "threadlike structure" emerging from RB199, one of the larger elliptical galaxies inhabiting the Coma cluster. The filament extends approximately 260,000 light years and contains groups of stars enclosed by glowing, ionized shells. Researchers refer to these rapidly moving star cluster knots as "fireballs," because they look like flaming projectiles shooting out from the galaxy.

Duration is also directly proportional to size. Sparks lasting two or three microseconds in the laboratory might scale up to decade-long events at stellar scales, or millions of years at the galactic scale. Filamentary shapes appear to exist no matter where investigators search.

Forces exerted by electrified plasma contained in the twisting filaments of Birkeland currents dominate the Universe. They flow in a cosmic circuit that comes into our field of view and then goes back out into the void with long-range attraction between them

In the image at the top of the page, NGC 891 also displays streamers of material moving away from its equatorial plane at right angles. Could it be that corona arc discharges, an electrical phenomenon associated with terrestrial lightning, are occurring on a thousands of light-years scale?

Stephen Smith

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