There has been a lot of discussion on Twitter about an unusual aurora phenomenon. This phenomenon takes on a deviant appearance from typical aurora so understandably many have asked what is going on. The first time I heard about it on Twitter, it was referred to as a “Proton Arc” and explained to be caused by collision, excitation and relaxation of oxygen and nitrogen atoms in our atmosphere by high energy protons rather than the much more common high energy electrons that make it down into our atmosphere. At least when viewed from the mid-latitudes it tends to be offset to the east or west from the main aurora activity and extend higher in the sky. It also tended to be a pinkish, purplish hue and rather elongated and straight.
I believe I may have seen this once from Rapid City, South Dakota on the night of Sep 8, 2015 (see image below).
This picture was taken about an hour after I arrived home from photographing a mild aurora event in western South Dakota. An hour earlier I captured the following image away from the city lights. However, clouds moved in so I drove home only to see the vertical pink columns from my driveway, which were well west of the aurora I filmed earlier.
This surprisingly not so rare phenomenon is now referred to as “Steve” which comes from the 2006 movie “Over The Hedge” where the characters humorously name an unknown phenomenon “Steve.” There is even a Wiki page summarizing the discovery and ongoing investigation into Steve which now stands for “Strong Thermal Emission Velocity Enhancement” due to some creative backronym efforts.
Last year, before Steve had his official name and was frequently referred to as a proton arc, I met an atmospheric scientist that has studied aurora and other upper atmosphere phenomenon in Antarctica. Dr. John French has worked with the Australian Antarctic Division for over 27 years and studied pulsating aurora while stationed at Macuarie Island (a subantarctic island in the Southern Ocean about halfway between New Zealand and Antarctica). I asked him about this phenomenon, and he gave me a possible explanation. I thought I would share it and hopefully encourage discussion as to whether this might be what is causing “Steve” to appear.
“As for electrons, protons are also channeled down magnetic field lines (but with +ve charge spiral the opposite direction to electrons). However because protons are more massive than electrons it is harder to accelerate them to speeds that penetrate down far enough into the atmosphere and protons comprise only a few percent of the excitation process for oxygen and nitrogen. Protons that do enter the top of the atmosphere with enough energy are very efficient at stripping electrons from neutral atoms (ionizing the O or N atom and releasing its electron) and these secondary electrons then produce aurora in the same way as direct electron impact. In this case the O 557, O 630 or N2 428 nm emissions would be indistinguishable from electron aurora. However…in another twist the proton itself can acquire the electron from another atom during the collision (known as charge exchange) and as an excited hydrogen atom emits lines in the hydrogen Balmer series (410nm, 434nm, 486nm, and 656nm) which produces the reddy-purply colour… . Also, as this is now a neutral atom, it is no longer influenced by the magnetic field so the cloud of protons that undergo this process just continue in the direction they were traveling before the charge exchange – creating the long straight columnar streak.”
In the time that has passed since Dr. French’s shared his possible explanation with me, scientific instrumentation has sampled Steve and initial analysis of the data showed an area of increased temperatures and velocity. The talk given by Dr. Eric Donovan from the University of Calgary is available online and is very interesting (watch from 1:08 through 1:27). He suggests that Steve is not related to proton aurora, but rather something else that has yet to be understood.
I look forward to hearing more from the Swarm satellite data analysis, and hopefully we will soon, through the well established scientific process in collaboration with citizen scientists (or citizen sensors as Dr. Donovan calls them), better understand this interesting sight in the night sky.
Here are some more links to information and stories about Steve and the groups that have photographed him.