Can Spicules Explain the Mysteries of Coronal Heating?

Solar spicules as imaged by NASA's Solar Dynamics Observatory (NASA)

Solar spicules as imaged by NASA's Solar Dynamics Observatory (NASA)

There’s one recurring question I’ve been asking for nearly a decade: Why is the Sun’s corona (its atmosphere) so hot?

When asking this out loud I inevitably get the sarcastic “um, because the Sun is… hot?” reply. Yes, the Sun is hot, really hot, but solar physicists have spent the last half-century trying to understand why the corona is millions of degrees hotter than the solar surface.

After all, if the air surrounding a light bulb was a couple of magnitudes hotter than the bulb’s surface, you’d want to know why that’s the case, right? At first glance, the solar atmosphere is breaking all kinds of thermodynamic laws.

The Sun is a strange beast and because of its magnetic dominance, energy travels through the solar body in rather unfamiliar ways. And today, a group of solar physicists have put forward a new theory as to where the coronal energy is coming from. But they’ve only been able to do this with help from NASA’s newest and most advanced solar telescope: the Solar Dynamics Observatory, or SDO.

Using the SDO’s high-definition cameras and imagery from the awesome Japanese Hinode solar observatory, features previously invisible to solar astronomers have been resolved. The features in question are known as “spicules.” These small-scale jets inject solar plasma from the solar surface into the lower corona, but until now they’ve been considered too cool to have any appreciable heating effect.

That was until a new type of hot, high-speed spicule was discovered.

“It’s a little jet, then it takes off,” solar physicist Scott McIntosh, of the National Center for Atmospheric Research’s High Altitude Observatory, told Discovery News’ Larry O’Hanlon. “What we basically find is that the connection is the heated blobs of plasma. It’s kind of a missing link that we’ve been looking for since the 1960s.”

These Type II spicules blast hot multi-million degree Kelvin plasma at speeds of 100 to 150 kilometers per second (62 to 93 miles per second) into the corona and then dissipate. What’s more, these aren’t isolated events, they’ve been observed all over the Sun. “This phenomenon is truly ubiquitous and populates the solar wind,” said McIntosh.

While this research provides more clarity on coronal dynamics, McIntosh is keen to point out that Type II spicules probably don’t tell the whole coronal heating story.

NASA’s coronal physics heavyweight James Klimchuk agrees. “It is very nice work, but it is absolutely not the final story on the origin of hot coronal plasma,” he said.

“Based on some simple calculations I have done, spicules account for only a small fraction of the hot plasma.”

Klimchuk favors coronal heating through magnetic stresses in the lower atmosphere generating small reconnection events. Right at the base of the corona, loops of magnetic flux channeling multi-million degree plasma high above the Sun’s chromosphere become stressed and eventually snap. These reconnection processes produce sub-resolution nanoflare events — akin to small explosions releasing energy into the solar plasma, heating it up.

Another heating mechanism — a mechanism I studied during my solar research days (.pdf) — is that of wave heating, when magnetohydrodynamic waves (I studied high-frequency Alfven waves, or ion cyclotron waves) interact with the lower corona, heating it up.

But which heating mechanism injects the most energy into the corona? For now, although there’s plenty of theorized processes (including these new transient Type II spicules), we don’t really know. We can only observe the solar corona from afar, so getting a true grasp on coronal dynamics is very hard. We really need a probe to dive deep into the solar atmosphere and take a measurement in-situ. Although the planned Solar Probe Plus will provide some answers, it may still be some time before we know why the corona is so hot.

But it is most likely that it’s not one coronal heating mechanism, but a combination of the above and, perhaps, a mechanism we haven’t uncovered yet.

For more on this fascinating research, check out Larry O’Hanlon’s Discovery News article “New Clue May Solve Solar Mystery.”

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Hayabusa Re-Entry Video: Spacecraft Destruction at its Best

There’s not a lot to add to this video, it’s too awesome.

It was captured by NASA’s converted DC-8 jet that was flying over Australia when the Japanese Hayabusa spacecraft broke up during re-entry. I’ll let the video do the rest of the talking:

Oh yes, and that little dot ahead of the falling debris? That’s the sample return capsule before it was found int he Outback safely. Thank goodness its parachute worked (presumably).

For more spacecraft demolition awesomeness, read “NASA Aircraft Videos Hayabusa Re-Entry

Hayabusa Returns to Earth with a Flash

Hayabusa re-enters over the Australian Outback, generating a bright fireball (screen grabs from the JAXA video feed)

Staring hard at the live streaming video of the black Australian skies, I was hoping to see a faint streak of light glide across the camera’s field of view.

But no, it wasn’t that subtle.

Shortly after 9:51 am EDT on Sunday morning (or, for me, a far more civilized 2:51 pm GMT), the Japanese space agency’s (JAXA) Hayabusa’s mission officially came to an end, burning up in the atmosphere. However, a few hours before, the spacecraft released a 40 cm-wide capsule, sending it ahead of the main spacecraft. This sample return capsule would have a very different re-entry than its mothership.

As I watched the small dot of light on the horizon of the streaming video getting brighter and brighter — feverishly hitting the PRTSC button and using some rapid cut&paste-fu in Photoshop — suddenly it erupted, shedding light on the distant clouds that had been invisible in the night.

Far from the re-entry being a faint or dull event, it was dazzling (as seen in the screen grabs to the right).

So, after seven dramatic years in space, the Hayabusa mission has come to an end.

For the full story about how Hayabusa got hit by the largest solar flare in history, limped to visit an asteroid called Itokawa and how its sample-collecting kit malfunctioned, have a read of my main article on Discovery News: Hayabusa Generates Re-Entry Fireball Over Australia

Note: Thanks to everyone who re-tweeted the sequence of re-entry pics. As of this moment it has received over 30,000 views on Twitpic!