Tuesday, July 29, 2008

Secrets of the Aurora

Anyone who has ever witnessed the northern or southern lights would certainly attest to their beauty, but few would imagine that the mysteries of the aurora are still being uncovered. Last week, scientists announced a fundamental discovery about the physical mechanism that converts a stream of charged particles from the Sun into a stunning display of color and light in the Earth's upper atmosphere. The work represents a significant step forward in efforts to predict such events, which can damage satellites in orbit and disrupt power grids on the ground.

The basic mechanism that drives the aurora has been known for a long time. Our Sun is constantly shedding a "wind" of charged particles, primarily electrons and protons, which slams into the Earth's magnetic field. Since charged particles in motion create their own magnetism (the underlying concept of an electromagnet), the solar wind spirals towards the Earth's north and south magnetic poles. There, it slams into the upper atmosphere and interacts with the oxygen and nitrogen atoms to produce shimmering curtains of green and red light in the sky.

The researchers used observations of the aurora from detectors on the ground, as well as measurements from a network of five NASA satellites that orbit at different heights within the Earth's magnetic field. By combining the data from these different sources, the scientists determined the detailed series of electro-magnetic events that precede and follow an auroral display. During times of relatively mild solar wind, the Earth's magnetic field works a bit like a capacitor -- accumulating charge for several hours and then quickly releasing it. The process is accompanied by a sudden disruption of the current in the upper atmosphere (something like blowing an electric fuse), as well as a snapping of the Earth's magnetic field lines far above (a process known as magnetic reconnection). The standard model for aurora suggested that the magnetic reconnection caused a disruption in the current, which then produced the aurora. The new observations show that the aurora are produced directly by the snapping field lines, and the disruption of the current occurs later.

Although the discovery is unlikely to change the way most people view the northern and southern lights, it may improve scientist's ability to predict the potentially harmful consequences of the storms. This could ultimately allow us to avoid regional electric blackouts and service outages from satellites that provide navigation services and communications -- the dark side of the auroral lights.