Aurora Australis is featured in this image photographed by an Expedition 23 crew member on the International Space Station. Among the views of Earth afforded crewmembers aboard the ISS, surely one of the most spectacular is of the aurora. These ever-shifting displays of coloured ribbons, curtains, rays and spots are most visible near the North (Aurora Borealis) and South (Aurora Australis) Poles as charged particles streaming from the Sun (the solar wind) interact with Earth's magnetic field, resulting in collisions with atoms of oxygen and nitrogen in the upper atmosphere. The atoms are excited by these collisions, and typically emit photons as a means of returning to their original energy state. The photons form the aurora that we see. The most commonly observed colour of aurora is green, caused by photons (light) emitted by excited oxygen atoms at wavelengths centered at 0.558 micrometres, or millionths of a metre. Visible light is reflected from healthy (green) plant leaves at approximately the same wavelength. Red auroras are generated by light emitted at a longer wavelength (0.630 micrometres), and other colours such as blue and purple are also sometimes observed. While auroras are generally only visible close to the poles, severe magnetic storms impacting Earth's magnetic field can shift them towards the equator. This striking aurora image was taken during a geomagnetic storm that was most likely caused by a coronal mass ejection from the Sun on 24 May 2010. The ISS was located over the Southern Indian Ocean at an altitude of 350 kilometres, with the observer most likely looking towards Antarctica (not visible) and the South Pole. The aurora has a sinuous ribbon shape that separates into discrete spots near the lower right corner of the image. While the dominant colouration of the aurora is green, there are faint suggestions of red photon emission as well (light fuscia tones at center left). Dense cloud cover is dimly visible below the aurora. The curvature of Earth's horizon is clearly visible as is the faint blue line of the upper atmosphere directly above at top centre. Several stars appear as bright pinpoints against the blackness of space at top right. NASA photo ISS023-E-058455