One of the most prized among the rare privileges that taking part in an Australian Antarctic Expedition affords is the opportunity to view the Aurora Australis. This breathtaking sight evokes a profound awe that has the power even to suspend our cares and worldly concerns — at least temporarily — with a display of spectral splendour.
Auroras are the result of disturbances in Earth’s magnetic field (the geomagnetic field) caused by the interaction with the solar wind and other bursts of electrically-charged material from the Sun. The solar wind is a stream of charged particles released from the upper atmosphere of the Sun. The interaction causes a geomagnetic storm that may disrupt Earth’s magnetic field, compressing it on the near side and extending the far side into a magnetic tail. The distorted magnetic field lines on the far side violently reconnect, releasing energy and accelerating charged particles back toward Earth’s upper atmosphere. These charged particles in turn electrically-excite the atmospheric gases which then emit the coloured light that results in an aurora.
The Aurora of 19th March
To view an aurora, one needs either to be standing somewhere in the high-latitude regions, such as in the Arctic or Antarctic (check!), be out of doors on a clear, dark night and far enough removed from the obscuring effects of light pollution, and finally one needs fortune to be smiling. Alternatively, one can avail oneself of the Bureau of Meteorology’s Space Weather Services web site. This great service provides a very useful contrivance named the Auroral Oval Prediction Tool. The Auroral Oval tool displays estimates of the equatorward and poleward boundaries of the auroral ovals around the north and south geomagnetic poles. It uses a model that predicts the location of the auroral ovals as a function of the K index (more about this later). The tool also displays an estimate of the equatorward visibility line, assuming a clear night sky and an aurora height of 150km. Looking at the equatorward boundary for 19th of March at about 11:30 PM (which is not long after I knocked-off work), the following auroral viewing extents were predicted:
Not bad! The green line in the image is the Auroral Oval (which is flattened out to a curve on the 2 dimensional map); it sweeps out north of Casey towards the equator. Accordingly, had we braved the cold and ventured outdoors that night (and many of us did) and cast our gaze northwards, we should’ve been able to see an aurora (and, by golly, we did!)
But what about this K-index, and indeed, what devilry is this? Without getting too technical, the K-index is a quantity that’s derived from the measurement of Earth’s magnetic field. This quantity varies from location to location over (and under, and above) the Earth. The range of the variation over the 3 hour interval determines K, which is assigned a value from 0 to 9. In plain terms, the K-index is a quantity that indicates the likelihood of seeing an aurora.
The K-index for the day (or rather, the night) of interest can also be obtained from the Bureau of Meteorology’s Space Weather Services web site. So what was the K-index for 19th March at, say, 11:30pm?
Less than 4?! If a K-index value of 0 means there’s no chance of seeing an aurora, and 9 means whoa! Then a value of about 2 as it was for the 19th of March hardly presented a thrilling prospect. And yet it turned out that despite all of the homework we did, fortune was indeed smiling that night because when we cast our keen little starry orbs heavenwards, the picture below is the scene we were rewarded with.