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Chasing the southern lights

Posted at 10:00am Thursday 29 Apr, 2021

The green curtain became brighter and brighter until it occupied most of the field of view.
The green curtain became brighter and brighter until it occupied most of the field of view.

With travel to sunnier places ruled out this winter due to Covid, Kiwis will be looking at travel options closer to home for a getaway. One option is a flight heading towards Antartica to see the Southern Lights (aurora australis), which are best viewed during the winter months. Mahurangi Matters science columnist Professor Ralph Cooney made the trip …  

One of my earliest memories as a small child was observing the brilliant range of primary colours refracted by tiny droplets of water on the tips of grass leaves exposed to bright morning sunlight. I thought the reds and blues looked like brilliant small jewels. This contributed to a life-long obsession with light and colour in my scientific research career, which has now covered more than 50 years.

It is not surprising then, that my personal interests over the years have included the photography of coral reefs, native wild flowers and especially our galaxy, the Milky Way. One long-term item on my bucket list was to observe the auroras in the zones of the earth's two planetary magnetic poles: aurora borealis (northern pole region) and aurora australis (southern pole region). Living most of my early life in sub-tropical latitudes was therefore unhelpful.

But recently I had the opportunity to take a 10-hour round-trip chartered flight from Christchurch on an Air New Zealand  Dreamliner 787, which has unusually large viewing windows. The flight with 250 fellow obsessives on board was an aurora-chasing expedition.  

The preparation for taking photos on the aurora flight was not simple, but fortunately the organisers provided detailed prior advice on the choice of cameras, lenses, settings and mountings. The aurora is not especially visible to human eyes, which are thousands of times less sensitive than modern digital cameras. Good fortune was on my side as my two digital cameras were both suitable. I also had an astro-lens (wide angle, 12mm, f/2.5) which seemed a good fit for aurora imaging, and which I have used frequently in taking Milky Way images from my Kaipara Flats home.

Anticipating that cabin reflections would be a problem, I also acquired a soft flexible hood fitting on the end of the lens, which bridged to the viewing window. I mounted the camera using a soft suction mount on the window and recorded images using a remote control device. I purchased a cheap black cloth from a local emporium to further exclude cabin reflections. The cabin lights and the entertainment system were extinguished to reduce cabin reflections once the aurora was detected.

We lifted off from Christchurch at about 8pm under twilight conditions and before long we were flying in darkness, with a brilliant starry sky evident. The passengers on the previous night flight observed an aurora only 20 mins out of Christchurch, but on our flight we had no aurora sightings during the first two hours of the flight. My heart was sinking fast and I was  tempted to simply doze off and dream about another hypothetical future flight when luck would provide me with a real view of the aurora. Then I noticed a faint green haze aligned with the airplane wing. My spirits picked up as the green curtain began to become brighter and brighter,  until within 20 minutes it occupied most of the field of view.

We were in the midst of the aurora! Like a massive green curtain swaying in a breeze, it changed minute by minute. Sometimes white or brick red tones appeared amongst the dominant green curtain.

The shapes changed in seemingly turbulent fashion and sometimes the slowly waving curtain gave way to a green whirlpool, with smaller green tornados of light or great sweeps of colour, rather like an artist's brush stroke sweeping across the entire sky. In the background and shining through the green curtains were the recognisable star constellations of the southern sky, including Scorpio and Orion. I recorded 500 images during the flight and only stopped when my overwhelming enthusiasm faded from fatigue and my hand on the shutter remote control started to experience cramp. I declared at last that I was satisfied.

My scientific insights into the aurora were both new and old. Our Sun – the furnace and light source for our planetary system – generates a solar wind, which radiates out to the planets. When there is a localised solar storm on the Sun's surface, intensely hot plasma with associated charged particles (mainly electrons) is ejected into the solar wind. The wind carries these charged particles into the declining polar magnetic field and the planetary atmosphere, comprising nitrogen and oxygen gases.

The charged particles “excite” the two gases. When the excited gases fall back to their normal stable state they emit light – green or blue for oxygen, and red or violet for nitrogen. The green emissions tend to be slightly more dominant at higher altitudes and red emissions are likely to be more evident at lower altitudes. Hence, when we view the aurora from a plane window at 13,000 metres we expect to see mainly green and when we view from the earth's surface we might expect to see more red tones. We observed mainly green with a little red on this flight. It was the trip of my lifetime.


Ralph Cooney travelled with Viva Expeditions https://vivaexpeditions.com/

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