Earth's Magnetosphere: On the Ropes

Few spectacles in nature top a really good aurora. Sure, total solar eclipses are rare treats, but during totality the corona just sits there and glows. Auroras glow and dance. If you've never seen nature's greatest light show, you're missing out!

Scientists have understood the basics of auroras — namely, that electrons rain down onto the upper atmosphere and cause the gases way up there to fluoresce — for decades. But just exactly how these sky shows are triggered isn't completely understood. Somehow ions and electrons from the solar wind leak into Earth's magnetosphere, collect "downwind" in the magnetotail, and then cascade Earthward in a torrent.

We're a little closer to knowing what that "somehow" is, thanks to observations made by a fleet of five spacecraft collectively called THEMIS. Not to be confused with the Mars-orbiting experiment with the same acronym, the "Time History of Events and Macroscale Interactions during Substorms" mission seeks to learn how and why intense geomagnetic outbursts occur.

At a huge meeting of geoscientists now taking place in San Francisco, THEMIS investigators described how the solar wind manages to get around Earth's magnetic defenses. It had been thought that, when aligned just so, the solar magnetic field "handshakes" with Earth's along the magnetosphere's dayside boundary (the magnetopause) and draws the field lines downwind — like peeling back an onion's layers.

That might still be occurring, says David Sibeck, project scientist for the mission at NASA's Goddard Space Flight Center. But THEMIS has found that the colliding fields often tangle, creating magnetic "ropes" that allow bursts of solar-wind particles to slip into the magnetosphere. Other spacecraft had detected hints of these ropes before, but it took THEMIS's multi-spacecraft approach to map their 3D structure.

And the discovery was serendipitous, notes Vassilis Angelopoulos (University of California, Los Angeles), the mission's principal investigator. When the THEMIS spacecraft detected their first magnetic rope on May 20th, they were still an 8,000-mile-long "string of pearls," roughly the diameter of the rope itself. Since then, Angelopoulos told me, they've spread about 10 times farther apart — and are now too widely spaced to be able to detect the ropes.

The THEMIS team reported other key discoveries at the meeting; you can read about them (and this one) here.