The sun having long set, and the progressive twilight faded, the night is now continuous down here.
In one of the most spectacular natural phenomenon’s able to be seen on earth, we have been graced by some fantastic Aurora Australis displays over the last month, with many more still to come.
The spectacle of Auroras are created by solar particles interacting with our atmosphere. In the land of no direct sunlight for six months, instead, we get a proxy of sunlight in the form of the Aurora Australis, the Southern Lights
The generation of these is created by an interaction between charged particles in the solar wind originating from the sun, the earth’s magnetosphere and our atmosphere. The process is analogous to the old school Cathode-Ray tube Televisions of yesteryear, of electrons interacting with a magnet and being projected onto a screen.
Solar wind, consisting predominantly of electrons and protons, and constantly being ejected by the Sun, to the tune of 1,000,000 tons a second. That is distributed out in all directions, radiating away from the sun, with only a small proportion being directed towards us.
The ejection of material varies significantly, depending on the solar weather. Solar weather. A bizarre concept. Yes, you can be a weather forecaster for space. I wonder if they’ll be an unreliable as terrestrial weather forecasters? If its like the mix of horoscopes and normal weather, then surely it will be incredibly vague. Along the lines of ‘the sun will most likely still be shining, and the world will probably turn’ with possibly an interpretation that is completely irrelevant such as ‘change is coming to your life, make hay whilst the sun shines.’
Anyway, you can get a good idea for what might happen depending on what is happening with the sun. The sun is a ball of predominantly hydrogen continuously fusing in to helium. Given the sun is gaseous rather than solid but has angular momentum (ie it is spinning), the matter in the central portion spins faster, doing a complete revolution in less time than the poles. Within the sun lies a magnetic field, but this is being wound up more in the middle given the faster revolutions. This is similar to holding a rubber band in two fingers, and twisting the middle segment to wind that up more. Just as the rubber band will start to buckle out of a straight line between the two poles, the sun’s magnetic field also buckles, at times creating pockets of the magnetic field outside of the surface of the sun. This causes the convecting gas fury to occasionally follow these lines of magnetic field that are outside of the normal sphere of gas. The end result is the sun’s matter straying out in long wisps, getting further away from the sun’s gravity and therefore more likely to escape its bounds and continue out into the solar system. We see these as solar flares, and to optical instruments (our eyes included), they can be seen as darker patches or ‘sun spots’. This sun spots persist for several weeks, so can be used to predict when more energy dense solar wind will be coming. Occasionally major episodes of this nature are seen, and visualised as coronal mass ejections.
Particularly at temperate latitudes such as southern New Zealand, Tasmania and the bottom of South America, where the Aurora are difficult to see, this relative increases in solar activity can mean the difference between seeing something and nothing.
The earth has a magnetic field, due to our liquid iron core. The axis of the field is similar to the axis of rotation, but is off in alignment by around 25 degrees, meaning that the magnetic south pole lies on the edge of Antarctica, on the section closest to Tasmania. The flux lines of the field are complex, and not the same as a simple dipole magnet. Therefore there is two entities when it comes to the earth’s magnetic field. One, the magnetic pole is where your compass will point to, is where the pole of the asymmetrical field is sensed when on the surface of the earth.
The geomagnetic south pole on the other hand, is theoretically where the main axis of the pole runs through, when distant from the surface of the earth. This therefore is what the electrons ‘see’ as they interact with the magnetosphere. It is over this point that the ‘aurora oval’ is centred. Similar to an apple, which has the stalk coming out along the axis of the apple, the surface of the skin is where the particles can follow. Where they reach the top is not at the stalk, but instead in a oval centred around the stalk. In a similar way, the auroras are most intense not at the actual geomagnetic pole itself, but instead in an oval centred around the geomagnetic pole. At the South Pole, we are on the inside of the oval, so although not the theoretically most prime location, it is still essentially still one the best sites in the world for the Aurora Australis (Aurora Borealis’ oval overlies a lot of Alaska, Canada, Russia and Scandinavia, so there are plenty of inhabited places within the oval), as the rest of the oval overlies uninhabited polar plateau. Furthermore, we have pretty consistently clear skies, few storms, and a very long night to enjoy the auroras over.
As the electrons and a proportionally fewer protons (as some of the protons have combined with electrons and become magnetically neutral) follow the magnetosphere, they enter the atmosphere, and start hitting the ions and molecules in the atmosphere. It can happen from anywhere between 60 and 500 km from the earth’s surface, but typically between 200-300km (space theoretically starts at 100km). The most common element to hit in the outer layers of the atmosphere is elemental oxygen (O, not O2), and one electron can hit several oxygen’s during its descent. The energy imparted to the oxygen molecule, pushes one of its outer valence electrons to a higher energy state where it is unstable and prefers to return to its more inert form. Typically the oxygen will go to its second excited state (an electron two valence rings out from its inert state). It is unstable there, and within a second, normally its has released energy in the form of light, to return to its first excited state. The wavelength released by going from 2nd to 1st is 557.7nm which is green light. One it is in its first excited state, the urge to get back to the inert state is lower, and therefore takes a couple of minutes. Given that the time period for this transition is much longer, the oxygen may instead bump into another molecule in the atmosphere transferring its excess energy. Therefore, the release of light from the 1st excited state to the inert state is much less common in the lower, denser atmosphere. The wavelength released is 636.4 nm corresponding to red light. Therefore green light predominates over red light in lower auroras. In order to produce visualisable auroras, you need around a millions and millions of electrons hitting a square cm! That is quite a few. Lucky our sun is such a source of spewing electrons.
The other colour seen is purple, which is generated by electrons hitting primed nitrogen. The nitrogen first has to be primed by sunlight, so down here, typically only happens closer to the periods of twilight, where although the sun is well below the horizon and not reaching us on the surface of the earth, its rays are still hitting the upper atmosphere, priming the nitrogen. The purple colours tend to be in curtain like displays, as different to the more dynamic greens.
The auroras vary in their intensity and movement, with typically the brighter ones also being the most variable. With the magnetic field shifting, the patterns include shapes such as spirals, arcs and veils. It can be so short lived, that if someone is outside, and radios to others that they are looking good, by the time you’re out there within a few minutes, things have lapsed and you’re back to a clear sky. On our radios that we use for communication around the base, interested aurora spotters have a dedicated channel to alert others about what is happening outside. All of our windows are boarded up, to prevent light pollution disrupting or damaging light sensitive science experiments, so what is happening outside is often an unknown unless your want to peak out one of the fridge doors that insulate the station.
In low light conditions, our eyes are pretty rubbish at picking up colour. Instead of using the cones in the retinas of our eyes, which detect colour and give us our central fields of vision, we use the rods that make up the most of our peripheral vision, which are much more sensitive in low light, but only detect black and white. It is for this reason, that out of the corner of your eyes, you can’t see colour, and similarly, in dark situations, you will often see movement in your peripheries, but when you turn to look, you can’t detect it any more. As the germans say “all cats at night are grey”. For aurora gazing, you can often see the auroras, which are clearly visible against the black starlight background, but the they appear as greyscale clouds. When captured on camera, they take up their vivid colours, as cameras are detecting the true colour that they are. We have had fleeting glimpses of the super bright auroras which light up the sky and ice beneath to such a degree that our cones can pick them up, and they are seen actual visible colour, but I haven’t been able to time it to be out for them. Our eyes are six times more sensitive to green light than red and blue, so if we are to see any colour, green is by far the most likely.
Photographing the auroras is a bit of a learned art. A combination of shutter speed, aperture, ISO settings, mixed with ambient light (is the moon up) and the intensity of the auroras varies the quality of the images. Too long a shutter speed, and things will lose definition and get blurred. Too wide an aperture, and peripheral stars on a wide angle lens get distorted with comatic aberration. Too much ISO and the photo will look grainy or too little, and the photo won’t be bright enough. Too short a shutter speed, low and ISO or narrow an aperture and not enough light will be captured to see the auroras. Taking photos outside if full complement of Extreme Cold Weather gear (ECW) is challenging especially if you want to change camera settings. With the help of resident experts, I have rigged up a remote with a nut (nut and bolt, not cashew…) epoxy’d on to the button to aid in dexterity through thick multi layer mittens. Unwarmed, my camera with a specific cold weather battery tends to last 15 minutes on timelapse before giving up the ghost. After being outside for just a couple of minutes, touching the camera or tripod without gloves tends to burn your fingertips at -60C, if you try to adjust something not amenable to fat finger mitts. Plastic remote cables don’t like -60 temps, and tend to become brittle and snap, so several of us keen novices had a soldering session replacing the plastic coated wire with cold-tolerant teflon wiring. Furthermore, we had to change the grease in our tripods as normal grease freezes at the temperatures we experience, making tripods much less helpful.
I have recently constructed a warmed camera box, out of a polystyrene box used to ship temperature dependent medication and lab equipment down here. Warmed with two drink bottles of boiling water, and with a hole to poke the lens out of, hopefully it will be suitable to capture some long duration time lapses over the course of half a day.
Many thanks to resident scientist/astronomer/aurora guru Robert Schwarz for his knowledge imparted about auroras and space during out weekly astronomy classes, his advice about aurora photography and skills and equipment in fabricating camera boxes, regreasing tripods and rewiring remotes. If you want to see the products of a master aurora chaser who is down here for his 12th winter, check out his collection of videos and photos on https://vimeo.com/polarlights, www.antarctic-adventures.de, and https://www.facebook.com/southpoleskies/
It has been a real highlight to see some of the most intense auroras around. In a place that doesn’t see direct sunlight for six months, having another sun-produced substitute that is much more beautiful is a awesome way to accept the long long night.