Tamworth Regional Astronomy Club: What is an Aurora?

This image was taken at Waipapa Point, near Invercargill, New Zealand on May 28 2017. Photos by Garry Bott.

This image was taken at Waipapa Point, near Invercargill, New Zealand on May 28 2017. Photos by Garry Bott.

In 1989, I was excited to see the Aurora Australis from my farm, North of Moonbi, NSW.  It was 2.30am and I was checking levels from the top of a water tank having just driven back from a meeting in Sydney.  In the Southern sky, at about 25 degrees elevation I saw a pink glow.

I concluded it to be an aurora (I had seen these as a child in Melbourne and while bushwalking in Tasmania).  My family was disappointed that I didn’t wake them to see the phenomenon. So, the next night, we went out to have a look…and there it was again! 

Tamworth Regional Astronomy Club’s member, Dr Garry Bott, recently gave an interesting presentation to members at their monthly viewing meeting in Victoria Park and explained some details about Aurora. Dr Bott suggested that if we can get a good clear southern horizon and all the predictors line up, further observations of the Southern Aurora (Aurora Australis) from Tamworth should be possible. – Warwick Schofield

The following, written by Dr Bott and his wife Merri is a summary of Dr Bott’s presentation.

Aurora are caused by electrons coming from the Sun via coronal mass ejection (CME) or solar flares associated with sun spots. These electrons and protons are carried by the interplanetary magnetic field, sometimes crossing the Earth’s orbit. The Earth’s magnetic field deflects this “blast” and protects us. 

This aurora image was taken at Jindabyne NSW early on New Year’s morning, 2017.

This aurora image was taken at Jindabyne NSW early on New Year’s morning, 2017.

The upper atmosphere doesn’t experience the full impact of the solar wind, but the interplanetary magnetic field, originating from the sun, can align itself with Earth’s field which directs electrons onto the polar upper atmosphere. 

An example of magnetic field graphs, used to predict the occurrence of Aurora (from aurora-service.net).

An example of magnetic field graphs, used to predict the occurrence of Aurora (from aurora-service.net).

This produces atomic oxygen – electron collisions cause excited states which cannot relax by any other means than by emitting a photon by a “forbidden” process…hence the green, and less commonly the reds.  We say “forbidden” because atoms cannot normally exist alone but are combined as molecules.

The green and red colours are from “forbidden” transitions of oxygen atoms (not molecules). 

The green transition occurs at approximately 100 km altitude. The reds will occur above 150 km. The blue colours may be due to nitrogen molecules at lower altitudes.

Since 1975, each of the National Oceanic and Atmospheric Administration’s (NOAA) Geostationary Operational Environmental Satellites  (GOES), located in Earth’s geographic equatorial plane, approximately 6.6 Earth radii from the centre of Earth, have carried magnetometers to monitor the geomagnetic field and its variations.  Typically, there are two GOES operational satellites:  GOES East, located over the east coast of the US, and GOES West, located over the Pacific, just west of the US mainland.

The green transition occurs at approximately 100 km altitude. The reds will occur above 150 km. The blue colours may be due to nitrogen molecules at lower altitudes.

Auroras are best photographed using a DSLR camera on a tripod and 3-4 second time exposure.  Cameras are more sensitive to colour and light than the human eye.

A good app for Aurora forecasts can be found at the not for profit website www.aurora-service.net