Getting to Know the Weird Purple Aurora Called STEVE

Last May, the work of citizen scientists helped establish the existence of a weird form of the aurora

STEVE aurora Krista Tinder copyright ESA FEA
Copyright Krista Tinder

STEVE aurora Krista Tinder copyright ESA
Steve and the Milky Way at Childs Lake, Manitoba, Canada. The picture is a composite of 11 images stitched together. Copyright Krista Tinder

Last May, the work of citizen scientists helped establish the existence of a weird form of the aurora nicknamed Steve . Now almost a year later, aurora scientists know a lot more about Steve and happily have kept the name and turned it into a nerdy acronym — STEVE for "Strong Thermal Emission Velocity Enhancement." Apparently, they've known about the phenomenon for some time but never imagined it would be visible from the ground.

I can report that on those rare occasions when Steve does show up, it's very obvious as a narrow arc of pulsing light that extends from northwest to northeast east to west crossing high in the southern sky. It resembles a jet contrail but remains in place with a glow of its own, a sort of line in the sand when all the other auroral forms are dancing about with abandon. In addition to random pulsations, it's occasionally crosshatched by short, picket-fence-type streaks.


Steve aurora sequence of magnetic reconnection S
When a strong blast from the sun blows past Earth, it can peel back part of the planet's magnetic field and stretch it into space on the night side of Earth. When the field lines reconnect or pinch together again, it's like a rubber band that snaps. Solar particles are shot along the field lines into the upper polar atmosphere where they create auroras. NASA's Goddard Space Flight Center

Auroras form when Earth's magnetic field guides atomic particles in the solar wind around Earth and towards the north and south poles. When these particles collide with atoms and molecules in the upper atmosphere, they "relax" a fraction of a second later and beam out tiny bursts of green and red light that create and color the aurora borealis and aurora australis (southern lights). Auroras mostly happen in the polar regions, but during big storms reach much further south and bloom over places like Kentucky and Arizona.

Steve aurora sub aurora region NASA's Goddard Space Flight Center S
In this illustration Steve is the swirly, purple tendril to the south of the main aurora. NASA's Goddard Space Flight Center

While Steve can show up either alone or in the company of other aurora, it stands apart from the main activity as a purple ribbon only about 15 miles (25 km) across.  Cameras show the color beautifully but it generally appears colorless to the naked eye. While Steve is created through the same general process as a normal aurora, it travels along different magnetic field lines and therefore can appear at much lower latitudes.

Most auroras dance around in the northern sky as seen from mid-northern latitudes invisible to observers farther south. But the Steve arc often passes south of the overhead point, placing it in the southern sky and in view of observers who live south of the usual aurora zone.  Steve is an important discovery because of its location in the sub-auroral zone south of where most auroras appear, in an area not well researched.

STEVE aurora forms
Two photographs of the Steve arc show its purple color and the picket-fence aurora. From NASA's Goddard Space Flight Center


At these lower latitudes, scientists say that the alignment of the global electric and magnetic fields makes ions and electrons from the solar wind flow rapidly in the east–west direction, heating them in the process. Measurements made by one of the European Space Agency's  Swarm spacecraft  as it crossed the arc show that Steve is a fast-moving stream of extremely hot atomic particles called a sub-auroral ion drift . In a sense it resembles the glowing filament of an incandescent light bulb, heated to glow by the flow of electricity (electrons). While this all sounds fantastically obscure, this flow or pulsing is actually visible to the patient and discerning naked eye observer.

One of the coolest things about Steve is that all this new research was inspired by ardent aurora watchers and photographers and shows how much ordinary people — citizen scientists — can contribute.

0000017d-94b7-d268-af7d-b7fffd3a0000  If you'd like more information about Steve's charms, check out  the scientific paper just published in Scientific Advances. Speaking of publications, I'm very excited to announce that my new book,  Wonders of the Night Sky You Must See Before You Die, will publish on April 24. You can pre-order a copy on Amazon , BN or Indiebound . As the title suggests, the book is a sort of cosmic bucket list. I'll have more on it in upcoming blogs. Till then, clear skies!

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