SAR Arc 4 Aug 2010 (image) Boston University Medical Center Share Print E-Mail Caption This is an all-sky view of the upper atmosphere as photographed in the red light (630.0 nm) of oxygen from the Mt. John Observatory in New Zealand. This fish-eye lens image displays emissions from a height of 400 km above the Earth's surface (features in upper left are buildings obstructing view to northwest). Just below the lower coast of South Island, the faint emission extending west-to-east is called a Stable Auroral Red (SAR) arc, a manifestation of oxygen atoms heated by hot electrons in the ionosphere. The brightness levels (about 300 Rayleigh units) are ten to twenty times fainter than can be seen by the naked eye. Further to the south, the area colored white is a brighter form of red emission called Diffuse Aurora, produced by in influx of electrons from the magnetosphere; it is still invisible to the unaided eye, but only by factors of two to three. Strong visible aurora, the familiar "curtains of red and green emission" would be still further toward the South Pole, beyond the field of view shown. This Boston University image of sub-visual aurora at mid-latitudes is the first unambiguous case of a SAR arc in the Southern Hemisphere during the new cycle of solar activity. Credit Boston University Center for Space Physics Usage Restrictions None Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.