Auroras - Living with a Star
Premieres Tuesday February 11, 2003


Close-ups of the Sun and its
magnetically-shaped surface:
courtesy NASA and Lockheed Martin Solar and Astrophysics Lab.

Instead of the featureless, never-changing golden disk that ancient philosophers thought the Sun to be, we now know it's an ever-changing, dramatically varying star. Looking through the telescopes of NASA's TRACE and other spacecraft, we see the Sun's surface in churning turmoil. The shapes we see - loops of cooler gas against the Sun's hot surface, vast sunspots many times the size of Earth - are in fact all caused by the same fundamental physics that gives our planet its own magnetic field. On the Sun it's electrically charged gas - called "plasma", the fourth state of matter - in rapid rotation, which ultimately generates intense magnetic fields.

Within the Earth it's molten metal in the core which creates our planetary magnetic field - different materials, to be sure, but the same underlying forces and physics. Like rubber-bands twisted past their breaking point, magnetic field lines on the Sun are constantly snapping, releasing vast amounts of energy. As we see during an eclipse, the Sun's outer atmosphere is constantly boiling off in what's known as the "solar wind." But sometimes, especially during the high points of the Sun's 11-year sunspot cycle, there are seasons when the "solar wind" blows with especially powerful "gales" and "hurricanes", the period we call "Solar Maximum."


Close-ups of the Sun and its
magnetically-shaped surface:
courtesy NASA and Lockheed Martin Solar and Astrophysics Lab.


"Halo CME" imaged by the SOHO satellite,
courtesy ESA & NASA

Seen through the silicon eyes of the ESA-NASA SOHO satellite, there's a tell-tale halo shape when the Sun's magnetic energy creates what are known as "Coronal Mass Ejections" - huge outbursts of matter and energy that sometimes are headed in Earth's direction. Like a vast, rotating garden sprinkler, the Sun sprays out clouds of plasma which carry along with them some of our star's magnetic field. Most of the time, Earth's field and that of the Sun are oriented so that energetic particles are directed harmlessly away from Earth, just like water flowing around a rock. But when the field in the solar wind is aligned southward relative to Earth's northerly field, "opposites attract" - as every schoolgirl knows - and the Sun's energy can more easily affect Earth's magnetosphere. Sub-atomic particles already resident in Earth's vicinity are now accelerated to great speeds, and spiral back down magnetic field lines towards the polar regions.

In great ovals centered on Earth's north and south geomagnetic poles, the particles collide with molecules of gas in Earth's atmosphere. NASA Goddard's Nicky Fox describes the science behind the aurora. Depending on how fast they're traveling, the particles impact gases higher or lower in the atmosphere. Low in the atmosphere are molecules of oxygen. Hit by energetic particles they give off light our eyes see as green.


Nicky Fox, NASA and Johns Hopkins Applied Physics Lab, and the Polar Satellite
Auroral oval as seen by the POLAR satellite,
courtesy NASA & University of Iowa

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