Tag Archives: Aurora Science

Did You Just See A Proton Arc?

A proton arc is oftentimes described as a broad band of diffuse aurora. If you do a Google Image search for “Proton Arc” a plethora of beautiful images depicting a purple, red, green, or pale band of aurora will greet your eyes. Go ahead, really, search it, I can wait. Or, you can visit this website at Spaceweathergallery.com.

I had the pleasure of seeing this pale phenomenon in Juneau on September 20th, 2016 for the first time ever. In the scene, the aurora swirled to the north in front of me over mountains.  However, a  pale, confined, band of aurora ran perpendicular to the northern display, and stretched far to the south past a large, brilliant moon. In my camera it was cool blue/white in color and was in stark contrast to the green aurora that played on the northern horizon over the mountains of Juneau.  I posted the image to an aurora group on Facebook and labeled it a “proton arc” as so many before me had done. However, I received an interesting response from renowned aurora researcher Neal Brown – a true “proton aurora” is nearly undetectable by the human eye and the concept of a “proton arc” is a widespread misconception. The disagreement between the science and the public perception set my wheels turning, and even though I am not an aurora scientist, I would like to dissect why proton arcs are not truly visible.

Proton Arc, Hoonah, Alaska, Aurora Borealis
On September 20th, 2016 I thought I saw a “proton arc” in Juneau, however, it seems my misunderstanding of this auroral phenomenon is the same of many non-scientists.

There are two ways that auroras may be formed. Most auroras are formed when excited electrons collide with oxygen or nitrogen or if protons collide with nitrogen or oxygen. Electrons which are lighter and have a lot of energy result in the traditional, dancing auroras. Electron auroras emit light at many wavelengths including 630nm (red) and 427.9nm (blue). The second way that auroras can form is when protons collide with nitrogen and oxygen. The proton collisions result in emissions of 656.3nm (red) and 486.1nm (blue) (Lummerzheim et al. 2001).  Separation of these light bands are difficult because at 656.3 the emissions require a precise instrument to differentiate them from the electron aurora. The same can be said of the emissions at 486.1 which are nearly indiscernible from the electron emissions.  To quote Neal Brown’s response in the aurora group, “To prove it is a true proton arc one would have to use some sort of spectral discrimination to see if it contained only 656.3 and 486.1 nm emissions”. Aurora researcher Jason Ahrms had this to say in a detailed Facebook post – “We don’t use color, location in the sky, how long it’s been there, or anything like that to identify a proton aurora.”. This means that simply looking at an aurora with your eyes is not enough to determine if it is a proton arc – so why is it so commonly mislabeled. The mistake is likely an innocent use of scientific jargon; those posting the images (like me) simply did a brief search to confirm what they saw before spreading the lie themselves.

A chart of the light spectrum. Copyright : http://techlib.com/images/optical.jpg
A chart of the light spectrum. Copyright : http://techlib.com/images/optical.jpg

 

The Aurora Borealis shows off a pale display in Hoonah, Alaska which is often identified a "Proton Arc"
The Aurora Borealis shows off a pale display in Hoonah, Alaska which is often identified a “Proton Arc”

Although it is impossible to detect a proton aurora with your eyes, they have been successfully photographed once identified with instrumentation. Tony Phillips of Spaceweather.com discussed the phenomena with University of Alaska Fairbanks Researcher Jason Arhns.  His image below shows how difficult true differentiation between electron and proton aurora is. Where the proton arc has been identified is barely discernible from the aurora.

This proton arc was captured by Jason Ahrns of the University of Alaska, Fairbanks. The region fo the proton arc was determined from spectral instruments, but as you can see it is very similiar in form to electron auroras. Image copy right to Spaceweather.com

It was interesting to realize that my perception of what a proton arc was had been so wrongfully influenced by what I saw online. However, if the pale auroras being captured by photographers (like the photos below) are not truly proton arcs, what are they? Incredibly, as Jason Ahrns explains, to date there are is no known explanation for these pale, elusive aurora displays! They are a new opportunity for scientific exploration in the aurora research arena. I hope they keep us posted.

 

Citations:

http://pluto.space.swri.edu/image/glossary/aurora2.html

Lummerzheim, D., M. Galand, and M. Kubota. “Optical emissions from proton aurora.” Proc. of Atmospheric Studies by Optical Methods 1 (2001): 6.

news.spaceweather.com/protonarc/

 

February 16th, 2015 : The “Coronal Hole” Red Aurora

Traditionally I think of the aurora being generated from a sun event. Often C-class flares, M-class flares, and X-class flares (the largest) hurtle plasma towards at the earth resulting in brilliant auroras. I have dug into the science of auroras during previous posts, and wrote about some of the science of the auroral colors and why the aurora can go from a nice show to a great show.  However, last nights aurora event was generated by a “Coronal Hole” in the sun. That term was new to me, and although it sounds like a headline from an end of days article, it’s really not that bad!

Coronal holes are a simple concept. The sun normally has a stable magnetic field that controls solar winds and energy from the sun. During a coronal hole event, magnetic field lines extend far away from the sun and allow high speed solar winds to escape. Solar wind speeds may exceed 10,000,000 km/hr! Translating that to terms I understand more, solar winds can travel at 500 miles/second. That’s a quick commute to work, or in this case the earth!  If the coronal hole is ‘geo-effective’ it means that those solar winds are headed towards our planet. These events can lead to a lot of high energy resulting in red and multicolored auroras even during times of low solar activity. (http://www.spaceweatherlive.com/en/help/what-is-a-coronal-hole, http://www.exploratorium.edu/spaceweather/holes.html)

Last night’s show was stopped at 10:30 due to clouds over Fairbanks. From 8:30 – 10:30 it remained subtle, but beautiful. The high energy from the coronal hole produced a quickly changing, but not well defined red aurora. I hope you enjoy!

Why Does the Aurora Flare Up?

The aurora last night was a prime example of what I want to illustrate – why does the aurora flare up? In the timelapse below the aurora dances on the horizon before exploding into one of the best shows I have seen overhead. During my time in Alaska I have tried to glean scientific information on the aurora. Last week I attended a talk by Dr. Akasofu who has been studying the aurora for 50 years, and his talk was focused on the very question I pose here.

So first, the setting. You are on top of a large hill in Alaska and it’s 11:00 PM. As  you stare into the inky darkness of the moonless night a green band of light plays in front of your eyes, and it is OK, but it’s not a jaw-dropper. Often time that is the form of the aurora. But suddenly as you watch the green smudge it goes super-nova expanding rapidly in size, color, and intensity. In fact, it’s so intense that the snow is lit up green and even your coat might be. Over your head and on all sides, the aurora builds in greens and reds. Pulses of light can be seen on the far horizon which flow towards you like a wave over your head breaking in unpredictable patterns. Green light shoots in all directions.

Why did that happen? I always assumed the high intensity auroral moments were created by extra energy (solar wind) entering the system. In contrary to that, the research conducted by Dr. Akasofu and other suggests the aurora is a circuit. Incoming solar wind is pushed against the earths magnetosphere where it reacts in an auroral sub-storm. If more energy is input into the system than can be output it starts to build up in a ‘secondary circuit’. The extra energy is stored and builds up within the atmosphere. When the conditions are right the energy is released in ONE pulse of energy causing the aurora to erupt suddenly. It also explains why eruptions last roughly the same amount of time (1 hour) since a finite amount of energy can be built up.

Based on this model, the aurora goes through three phases. Growth which is aurora formed directly by solar winds and is often manifested by low-grade auroras. Expansion which is the unloading of the secondary circuit and direct solar wind. And finally recovery, which is just driven by solar winds.

I hope you’ve enjoyed some of the science of the aurora! I’ll put my disclaimer on the end that I disseminated the information of the talk to you the best I could, and I hope I got it right!

I added this so you could see something - snow in Fairbanks already! It snows during the afternoon and was heavy enough to stick around in the hills.
I added this so you could see something – snow in Fairbanks already! It snowed during the afternoon and was heavy enough to stick around in the hills.
This is a prime example of auroral eruption when the 'secondary circuit' overloads. This power-ful and fast over head display was remarkable!
This is a prime example of auroral eruption when the ‘secondary circuit’ overloads. This power-ful and fast over head display was remarkable!
I love the split of the lights here and their intensity. The 'white' aurora on the left is actually starting to become over exposed. During this time the snow was lit up green. This was during the eruption phase.
I love the split of the lights here and their intensity. The ‘white’ aurora on the left is actually starting to become over exposed. During this time the snow was lit up green. This was during the eruption phase.
This shot was taken as the aurora started to wind down. However, it still had some color, and was still very active.
This shot was taken as the aurora started to wind down. However, it still had some color, and was still very active.
I love to frame spruce trees into my shots - it's almost as if they're pointing in wonder, too!
I love to frame spruce trees into my shots – it’s almost as if they’re pointing in wonder, too!
A moody green auroral sky. Not unlike a scene from a Ghostbusters movie :)
A moody green auroral sky. Not unlike a scene from a Ghostbusters movie 🙂