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Beautiful sub-auroral arcs across the sky: what we know so far about the phenomenon

October 14, 2017

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G4 solar storm over Denmark!

October 27, 2016

Earlier this week the Earth got under the influence of a severe solar storm and the coronal hole it was facing sent charged particles that hit our planet at an unexpected time and with an underestimated strength. 

 

 

To understand fully what a solar storm is, and what consequences it can have on our planet, we have to start our journey on the Sun. The star, center of our solar system, is the host of a powerful radioactive activity, where nuclei of atoms bind together by fusion. This happens at an unbelievably high temperature, and the energy delivered by the reaction is unprecedented. This process also gives out plasma, that is to say matter in its highest energy state (i.e. more energetic than gas). The energy state of these particles also makes them very dangerous for life as we know it and electro-magnetic equipment (in other words everything!), and exposure to them will result in immediate death and probable apocalypse scenarii. Unfortunately for us our so beloved and life-powering Sun also sends the deadly plasma our way when a hole (called coronal hole or CH, see picture below) appears like a dark spot on the surface of the star. The darker color of the hole is a hint that the sun's corona is losing gas, plasma and energy, and decreases in temperature and density.

 

 

The activity underneath the hole is though so powerful that it pushes the matter outwards and ejects plasma to outer space with colossal outbursts. These are called CME or coronal mass ejections, and are violent charged-particle storms that would eradicate every form of life it encounters. However life has been thriving on Earth for billions of years because of the planet's special inner composition. At the very center of the blue planet lies a solid core made of iron, that acts like a giant magnet and offers the Earth a compromising protection against the nasty particles. It is the Earth's magnetosphere (imaginary lines that act like a shield) that is able to counter the solar winds and deviate the course of the charged particles from the sun. As you can see on the picture below, there is one area where we are less protected: the poles. It is where the lines of force rejoin the Earth's core, and the particles directly follow them down to the atmosphere where they collide with its the molecules and atoms. This collision gives out energy in the form of light or glow: the Aurorae (Borealis at the North pole, Australis at the South pole). As a consequence particles have lost so much energy in the collision that they have much less effect on life and electronic/communication equipments. Northern or southern lights produce different colors because of the different atmospheric molecules they collide with. For example green tends to be oxygen while blues and reds, nitrogen. If you want to read more on them, here are two fascinating articles from BBC and Gizmodo.com.

 

On October 25th and some days preceding, scientists and forecasters had emitted a G1-G2 storm (moderate storm) watch for several days. The forecast had been issued for late Oct. 26th but it hit the Earth earlier than expected. And for once I was sitting in wait on the beach, with my Sony and newly-bought Canon 6D and Sony a7s, ready for a medium-sized show. In theory the lights would appear between 0 and 45 degrees above the horizon, and my cameras were pointing accordingly. At 7:30 pm (20:30 UTC), something rather weird and unexpected happened. I was under the impression that someone was turning on a light right above my head. As I look up to examine the strange light, I am still questioning what it can be. It looked like a bright band floating in the twilight sky. I was convincing myself that it couldn't possibly be the northern lights. I point one of the cameras at it anyway and got completely astounded by the picture: It was indeed lady aurora showing up above my head! I had never seen it happening at more than 45 degrees before. I quickly set both my cameras for a time-lapse, and start shooting real-time with my new a7s. I was also out to test its low-light capabilities, and I have to say that I am overly impressed for a first time and with a cheap lens (Canon 50mm f/1.8). You can watch the result of the real-time movie here:

 

 

In reality overhead aurora does not occur at so low latitudes (under 60 deg. N) unless the storm is extraordinarily powerful. I checked the magnetometer situated in Kiruna out of curiosity, and it turned out that the Kp index actually reached 8 on a scale of 9 (severe storm: G4)! 

 

The display continued the whole night but the clouds stole the show after 01:00 am, so I headed home. I had shot about 3000 pictures during the course of the night, and was kind of exhausted, but it was well worth being there in the cold October night to witness a rare and surprising sky phenomenon. You can watch the full length of the event in my time-lapse video at the end of the article. The solar storm is still on-going because the apparently biggest coronal hole of the solar cycle so far is continuously shooting plasma-filled winds at the Earth. Make sure to check my facebook page for updates on recent aurora sightings. In the mean time, good hunting!

 

 

THE VIDEO: