Davis Skies Light Up

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This week at Davis: 6 May 2011

Thanks to the diligent efforts of David Hosken (Laser Dave), the LIDAR (LIght Detection and Ranging) once again provides a stunning green beam in the Davis sky. This marvel of optical engineering gathers data for the Climate Processes and Change theme of the new Science Strategy. The laser is now performing at the expected output power level (28 Watts), and after realignment to the transmitter optics the beam was back up in the sky on May 1. Alignment of the receiver optics was also completed, and data was once again able to be recorded. It is now expected that the green laser beam will be back in its place above Davis whenever the weather permits – to probe the whole-of-atmosphere region as a sentinel for observing climate processes in the polar atmosphere.

See the video below.

Majestic Celestial Light Show

For our family and friends back home, colleagues at Kingston and expeditioners down south: as the maximum in solar activity approaches, more and more beautiful photographs of the ‘aurora australis’ will populate Icy News editions. A basic explanation of the majestic southern lights follows - written by Gary Burns and Ray Morris for school children, members of the public and the Lonely Planet edition on Antarctica. The photographs were taken at Davis earlier this week and the excellent time lapse video was recorded by Tony ‘Boj’ Bojkovski.

What is an Aurora?

Aurora is the collective name (taken from the Roman goddess of the dawn) given to the photons (light) emitted by atoms, molecules and ions that have been excited by energetic charged particles (principally electrons) travelling along magnetic field lines into the Earth's upper atmosphere. Aurora results from the interaction of the solar wind with the Earth's magnetic field.

Colours in Aurora

The colours in the aurora result from photons from specific energy level transitions in excited atoms, molecules and ions of the upper atmosphere returning to their lowest energy state. There are hundreds of individual colours in an auroral display, but three are dominant in the visible spectrum. The brightest auroral line is generally a green line emitted by excited oxygen atoms. A red diffuse glow results from another oxygen atom transition. A purple colour results from a transition in a nitrogen molecular ion. The mixture of the major green, red and purple emissions may combine to give aurora a general 'whitish' appearance.

Shape of Auroral forms

Most commonly, auroral glows form a band aligned in a magnetic east-west direction. If sufficient numbers of energetic electrons are impacting the upper atmosphere, bands may have shimmering rays extending upwards from them. These rays define magnetic field lines along which the auroral electrons travel into the atmosphere. The twisting of auroral rays and bands results from the dynamic interaction of electric currents and magnetic fields in the upper atmosphere. In active displays, multiple bands may be visible. These may break into small arcs. If rapid horizontal motion of the auroral form is apparent, the form may appear more violet on its leading edge and greenish on its trailing edge. This results from a small delay (less than a second) between the peak intensity of the nitrogen molecular ion emission and the green oxygen atom emission. The active phase of an auroral display will last of the order of 15 to 40 minutes and may recur in 2 to 3 hours. Auroral band features may persist all night. A red dominated auroral glow will be very diffuse. It will vary in location and intensity very slowly (time scales of half a minute or so). This results from a significant time delay in the emission of light by the atomic oxygen state which smooths out any rapid variation in where the auroral electrons are impacting on the atmosphere.

Height of Aurora

Generally, if an auroral band has an easily discernible lower border, this will be at around 100 to 110 km altitude. Auroral rays may extend above the lower border for hundreds of kilometers. If the lower border has a pinkish edge to it (resulting from an emission of molecular nitrogen), the altitude may be around 90 to 100 km. A diffuse red aurora occurs above 240 km.

Intensity variations

During an active auroral display, the intensity variations will be rapid and spectacular. A most dramatic intensity variation is an increase in intensity moving up or along an auroral form. If the intensity variation is moving upward from the lower border, this has resulted from velocity dispersion of the electrons from an equatorial acceleration region. The faster electrons reach the atmosphere first and deposit their energy lower in the atmosphere. This is followed by lower energy electrons which are stopped at progressively higher altitudes, giving an intensity variation moving upward from the lower border. These variations may recur in rapid succession. Movement along the auroral form may result from a rapid variation in the longitude of the acceleration region at the auroral equator. Strong moonlight and station lights reduce the ability to discern auroral features.

Auroral conjugacy and effect of the sunspot cycle

The global distribution of auroral activity is an oval around the magnetic poles in both hemispheres. Indeed the ‘aurora australis’ in the Southern Hemisphere is linked by magnetic field lines to the ‘aurora borealis’ in the Northern Hemisphere – which are said to be conjugate and develop simultaneously as almost mirror images. As the level of magnetic disturbance of the Earth's magnetic field increases, the oval of auroral activity expands equatorward. Auroral displays are seen more often equatorward of their normal locations at times of high sunspot activity, and the next peak in the 11-year sunspot cycle is expected during the 2011–2012 interval. We anticipate a spectacular celestial ‘auroral’ light show to help celebrate the centenary of Australian Antarctic Expeditions. Over this time ANARE and Australian scientists have made important contributions to the scientific knowledge and understanding of the processes causing the ‘aurora australis’. So keep your cameras pointing to the heavens when the night sky lights up!

Lidar and Aurora
Davis Skies Light up with LIDAR beam and Aurora
(Photo: Ray Morris)
David Hosken
David gets the beam going again.
(Photo: Ray Morris)
Aurora Davis
Station Machinery under auroral sky.
(Photo: Shane Mitchell)
Auroras everywhere
Auroras Everywhere
(Photo: Jim Dunnett)
More Auroras
(Photo: Jim Dunnett)
Jim Dunnett Aurora
Jimmy the Flame , flash Dancing
(Photo: Jim Dunnett)
Tony Bojkovski
Boj with his name up in lights.
(Photo: Jim Dunnett)
Ray Morris
Ray, an Atmospheric scientist in Dream world
(Photo: Jim Dunnett)
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