Secrets of Auroras · 4:55pm Dec 10th, 2023
Secrets of Starlight was a lovely final flare to Make Your Mark, with a glittering new pony tribe: the Auroricorns, and a sparkling new location at Starlight Ridge. Plenty of fan fiction opportunities. And I get to write a blog on the physics of auroras. It actually is a bit of a surprise that I haven’t done this one yet. Of course, the aurora flares of Starlight Ridge have as much in common with the Northern Lights of our world as the Cloudsdale weather factory production line does with real rainbows. More on this later.
I have never seen an auroral display. I like travel adventures, so I have briefly toyed with the idea of taking a winter trip to the North with this aim. However, I’m not super-enthusiastic about cold dark places, so I’ve never actually done it. My aurora discovery journeys have so far just been armchair travelling. For this, I would recommend Melanie Windridge’s book: Aurora. In Search of the Northern Lights. She also has a nice website.
What is an aurora? These shimmering curtains of coloured light, visible in the long polar nights, are, by all accounts, one of the most spectacular atmospheric optical phenomena, but not the easiest to explain. There are a lot of confusing, incomplete, and sometimes outright wrong explanations out there.
The key ingredients are the solar wind—electrons and protons streaming out from the sun, especially after big solar flare; the earth’s magnetic field—with its north and south poles; and the ionosphere—the upper low-pressure part of our atmosphere, where radiation strips electrons off atoms.
Shoot charged particles in a low-pressure gas and it can glow and emit some cool colours. Electronics in atoms get knocked out of place and are excited into higher energy levels. When they relax back to the lower energy state, they emit light, with a wavelength or colour set by the spacing of the energy levels. Green is the most common colour, due the energy structure of oxygen atoms, but other colours can be produced by transitions from other energy levels in oxygen, nitrogen, or other elements in the atmosphere. If you want to enjoy a similar electroluminescent display at home, ask Santa to bring you a plasma ball for Christmas.
How do we excite such a natural light show? One explanation of the aurora says that electrons are “accelerated along magnetic field lines towards the poles of the Earth”. That didn’t sound quite right to me as a particle physicist, I tell students that it is electric fields that accelerate a particle beam, and magnetic fields that bend it into a circle. Schoolbook physics tells you that the force acting on a moving charged particle in a magnetic field always acts perpendicular to its motion, so it doesn’t speed it up, but just keeps nudging it to the side so, causing its path to spiral around a magnetic field line.
Better explanations talk about electrons being ‘channelled’ or ‘funnelled’ by the magnetic field towards the poles. That makes more sense to me. When an electron reaches the Earth, it will be deflected by the magnetic field, moving in circles around the field lines and drift towards the north or south pole. However, Dr Windridge is keen to stress that there is acceleration: “Solar particles… simply do not have enough energy to cause the vibrant colourful lights, so they must be accelerated somehow.”
The acceleration mechanism that is powering the aurora is still not clear to me, and I don’t think it is entirely understood by the physicists studying it. The messy plasma of inner space is more complicated that the controlled environment in the Large Hadron Collider. The acceleration is directed by the geomagnetic field, but driven by the energy in the solar wind. Around the poles, the sheer number of electrons and protons mean they interact with each other, and make electric currents that shape the field. This is space weather, and like ordinary weather, it is chaotic and difficult to predict.
All this leads to the question of how the space weather is managed in Equestria. The G5 pegasi have not yet rediscovered the lost art of active weather management. But it seems that the auroricorns have some control of the ionosphere with their magic. In Starlight Ridge, the shimmering green auroral curtain is just the background for the brighter flare, which have a symmetry that suggests ice crystals are involved (except it’s not hexagonal).
Insert your theory of auroricorn physics here.
Its an 8 pointed Star?
The Mark Of Chaos.
As for acceleration along a converging set of magnetic field lines causing increase in energy?
Hydrocyclonic field. also, vortex flow rocket motors, gives a really accurate match to extremely powerful linear exhaust jets from an extremely compact source.
Its like thunderstorms.. Natures own Crookes tubes. which are like Cathode Ray tubes, but accelerate particles in opposite directions depending on the opposite charges?
I've only seen the Northern Lights once in my life, in Alaska. Pictures really don't do them justice.
I'm betting it's witches.
5758733
Giving them a good whack with a broomstick is one way to accelerate particles.