Aurora Australis has its moment

It is award season for our astrophotographers, and like every year there are some spectacular inspiring images that make you want to get outside and take in the night sky.  Last week one of the big competitions, the Astronomy Photography of the Year, announced its winners.  Organised by the Royal Observatory Greenwich (of Greenwich Mean Time fame) it is one that both sky objects and photographers from our Southern hemisphere often do well in.

But until this year, a picture of the Aurora Australis - the Southern Lights -  counterpart of the Aurora Borealis, Northern Lights - had never been a winner of the Aurora category of the competition. Larryn Rae’s spectacular shot of the red aurora crowing the hills from the foreshore of Queenstown, NZ took honours in what is probably one of the toughest categories, where every one of the shortlist is likely to take your breath away.  

Photos of the Aurora Australis are inevitably rarer, there are a lot fewer people and landmass in the Southern Hemisphere.  Crucially though, most of the landmass does not extend as close to the poles as that in the Northern Hemisphere, as this is where the Earth’s magnetic field lines open up to the solar wind. 

A pas de deux of physics and chemistry, the Aurora dances across our sky when the solar wind interacts with the Earth’s magnetic field.  It is the shape of our magnetic field that governs where the aurora displays can be seen.  

Recalling the iron filings we messily poured onto bar magnets in high school, the Earth’s field has similar features.  Viewed as a cross-section through the Earth, the butterfly-like shape means that close to the equator, the field lines are more of a barrier, so that the solar wind buffets off and around our planet staying many thousands of kilometres from our atmosphere.  But at the poles, the magnetic field lines are open and can carry some of the charged particles of the solar wind down towards the atmosphere, where oxygen atoms and nitrogen molecules are sitting peacefully.

The Aurora are the result of the interaction of the channelled solar wind with the oxygen and nitrogen in our upper atmosphere.   The charged particles (the solar wind is made up of electrons, protons and alpha particles - all have some charge) collide with the oxygen and nitrogen and send some of the electrons in these atmospheric species into a variety of excited states.  As these calm down, this process releases photons of particular wavelengths.  Though one of the magical coincidences of nature (like the Moon and Sun being about the same size in our sky), these wavelengths are in the narrow range of visible colour that our eyes can observe.  

There are a variety of colours that can be seen, relating to the height of the atmosphere that you are seeing the interaction from.  Red being the highest and from the oxygen atoms upwards of 240 km altitude.  Then green (the most common colour observed in the Aurora) is also from oxygen but at lower altitudes around 100 -240 km above the surface.  Blue aurora can be seen if the interaction is happening right overhead and requires the solar wind to interact with molecular nitrogen.  

The colours displayed are very variable (and can include pinks, purples and yellows too) and the Aurora itself is dynamic where it appears on the Earth to some extent.  Very usually an oval around the pole, but how this pans out in latitude has a lot to do with the strength of the solar wind, which is far from constant.  In fact, 2024 is turning out to be quite the year for strong solar winds.

Before you book that flight to Iceland or fly straight into the Aurora, there is some good news for the antipodean Aurora hunters. Like the surface winds of our metrological cycle, the solar wind can have blustery days and there have been more than normal of these in 2024.    Stronger solar wind activity leads to a larger influx of particles channelling down those field lines - this means that the aurora ovals can push further towards the equator, making Aurora potentially more visible to us even on the island North of Tassie.  A fantastic example of this happened in May this year - when a large solar storm meant that the Auroral oval burgeoned so that Aurora was spotted in Darwin (though thanks to La Nina, it was pretty cloudy over most of the populated areas of Australia).  

The source of these winds and the storms is always the same, our Sun.  The sun is always an angry beastie, which is why low-level aurora can be seen at any time.  However, 2024 is the peak of the solar activity cycle - our sun is going through its most spotty and irritable period - which can lead to phenomena such as the coronal mass ejections pictured.  If one of these events happens and the Earth is in the path of this maelstrom of particles then aurora activity can light up our skies.  

So keep a weather eye on the space weather, there are numerous forum and website that will alert you to the potential of a coroal mass ejection heading our way.  If you get clear dark skies, keep a watch for dancing colours to your South!