Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2003 December 17
See Explanation.  Clicking on the picture will download
 the highest resolution version available.

A Proton Aurora
Credit: IMAGE Satellite, NASA

Explanation: What are aurorae made out of? Triggered by solar activity, normal aurorae are caused by collisions between fast-moving electrons and the oxygen and nitrogen in Earth's upper atmosphere. The electrons come from the magnetosphere, the region of space controlled by Earth's magnetic field. As the excited oxygen and nitrogen molecules return to their low energy state, they emit light, seen as the auroral glow. Sometimes, however, aurorae can be caused by collisions with heavier protons, causing a more energetic display with strong ultraviolet emission. In addition, protons can temporarily capture an electron and emit light for themselves. Such a proton aurora is seen above, recorded by the IMAGE satellite. A special feature is the bright spot near picture centre, embedded in a ring of auroral emission around the north magnetic pole of planet Earth. Most solar wind protons never reach the Earth to cause aurorae because they are completely deflected away at a great distance by the Earth's magnetic field. The bright spot in the auroral ring indicates a particularly deep crack in the Earth's magnetic field where protons were able to flow along a temporarily connected region between the Sun and the Earth, relatively undeflected, until they impacted the Earth's ionosphere.

Tomorrow's picture: Express to Mars

< | Archive | Index | Search | Calendar | Glossary | Education | About APOD | >

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Web Site Statements, Warnings, and Disclaimers
NASA Official: Jay Norris. Specific rights apply.
A service of: LHEA at NASA / GSFC
& Michigan Tech. U.