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The Sun's Coronal Rain Puzzle Solved

For years, solar scientists have been confused as to why "coronal rain" falls so slowly through the sun's atmosphere... until now.

Using NASA's brand new Solar Dynamics Observatory (SDO), solar physicists are already beginning to understand some of the sun's greatest mysteries. And last week, the SDO may have provided an answer to one of its most beautiful (yet perplexing) phenomena: coronal rain.

The SDO may have only given us its ‘first-light' images less than two weeks ago, but scientists analyzing the SDO's high-definition movies of the sun are seeing features in the lower corona (the sun's atmosphere) that they have never seen before.

In this case, the SDO spotted an eruption of plasma in the lower corona, spitting huge quantities of hot plasma into space, like water being pumped through a firehose. But this ‘water' has a temperature of over 60,000 Kelvin (not the kind of ‘water' you'd want to get sprayed with, you'd be vaporized in an instant!). However, the plasma couldn't escape the gravitational pull of the sun and fell back to the solar ‘surface' (known as the photosphere).

The erupting plasma was funneled through massive arcs of magnetic field lines known as coronal loops. After being launched, the plasma fell back to the sun, looking like droplets of rain. The scale of these ‘droplets' are gargantuan, each could easily engulf the Earth.

"Blobs of plasma are falling back to the surface of the sun, making bright splashes where they hit," explains Karel Schrijver of Lockheed Martin's Solar and Astrophysics Lab who is currently analyzing the unprecedentedly detailed videos from the SDO. "This is a phenomenon I've been studying for years."

We've known about coronal rain for a long time, but its motion has foxed solar physicists. For some reason, coronal rain falls very slowly, a lot slower than we'd expect for plasma falling toward a gravitational bully like the sun.

"The sun's gravity should be pulling the material down much faster than it actually moves. What's slowing the descent?" he asks.

With thanks to the SDO, Schrijver finally has an answer.

"The rain appears to be buoyed by a ‘cushion' of hot gas," he says. "Previous observatories couldn't see it, but it is there."

The sun's lower corona is a confused forest of magnetic field lines and turbulent plasma of different temperatures. Using its temperature-sensitive instruments, the SDO can see that below the falling coronal rain is plasma at million degree temperatures, many times hotter than the coronal rain itself. The presence of this very hot gas exerts a pressure on the falling "rain," slowing it down.

This is an elegant answer to a beautiful solar phenomena, a very early success for what promises to be a very fruitful mission.

WATCH VIDEO: See the SDO video of coronal rain in action (produced by Jorge Ribas, narration: me):

Source: NASA