Comet explores sun!

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What the comet taught

Lovejoy no killjoy: Comet cracks corona question!


Comet passed by the Sun, crossing open and closed magnetic field lines.

Modified from original graphic by Cooper Downs
When Comet Lovejoy streaked past the sun, its tail formed a series of rapid squiggles as it was driven by the sun’s complex magnetic fields. Satellite images converted a chunk of ancient ice into a natural scientific instrument that reached where no artificial instrument will ever go. Those open magnetic fields drive the solar wind throughout the solar system.

To explore the swirling magnetic fields surrounding our neighborhood star, scientists have press-ganged a “dirty snowball” into an instrument of science.

As comet Lovejoy brushed perilously close to the sun, it was observed by instruments sensitive to extreme ultraviolet light. “EUV” is a short-wavelength, high-energy electromagnetic radiation that acts as the language of the corona, the turbulent, million-degree region surrounding our sun.

Lovejoy’s suicidal sojourn to within 140,00 kilometers of the sun occurred in December, 2011. As it passed through a fiery-hot region where artificial satellites may never visit, the comet was under the watchful eye of instruments on three satellites.

Although this was only the second comet ever observed in such high-energy ultraviolet light, “Sun grazing comets have been around for a long time,” says Cooper Downs, a solar physicist at Predictive Sciences in San Diego who was first author of the paper in this week’s Science. “This family of comets is thought to have come from something that broke apart about fifteen hundred years ago.”

Before its solar roasting, Lovejoy was probably a few hundred meters across, composed mainly of ice.

Video courtesy Cooper Downs
Author Cooper Downs discusses how comet Lovejoy may offer new clues to the sun’s outer layer, called the corona.

As you can see from the video, the comet’s tail seems to be buffeted by an invisible wind. In reality, that wind is the sun’s intricate magnetic field.

Solar magnetism

As the icy comet cruised through the super-hot corona, it sublimated into gaseous water, which three seconds later separated into oxygen and hydrogen under intense radiation. The oxygen atoms immediately lost electrons and became ions, forming the state of matter called plasma.

Because plasma is electrically charged, it responds to magnetic fields.

The hot plasma also creates the EUV radiation that was “seen” by the satellite instruments dedicated to observing the sun. These detectors are tuned to EUV because the star’s intense magnetism and radiation elevate conditions to 1 million degrees C, creating a bountiful source of this high-energy radiation.

EUV, says Downs, “is the natural ballpark for observing the corona, plasma, and the arching magnetic structures” near the sun. “The Solar Dynamic Observatory sits and stares, making a continuous image, at these wavelengths.”

When a July, 2011 comet cruised by the sun, it was “a happenstance that it showed up in EUV,” says Downs. “This led the operators of SDO to off-point the satellite to catch Lovejoy’s ingress in December 2011, which was a big deal because it was an interruption of the standard SDO observing program.”

The pay dirt in the Science study, the fact that the tail would respond to the magnetic field, “was unexpected,” Downs says. But by responding to the magnetic field and sending out ultraviolet messages, plasma in the comet’s tail became a magnetic-field detector.

Diagram shows solar prominence: A filament of gas shaped by magnetic fields; solar flare: Sudden release of mass of ions and atoms; photosphere: visible surface of sun; Core: generates energy by fusion; Radiation zone: energy leaves by radiative diffusion; Convection zone: energy leaves by convection

Modified from original graphic by Kelvinsong
The sun isn’t just a ball of hydrogen and helium.
Solar prominence: A filament of dense, relatively cool gas shaped by closed magnetic fields.
Solar flare: Sudden release of energetic mass of ions and atoms, often leading to a coronal mass ejection that can spit this material past Earth’s orbit.
Photosphere: Visible surface of sun, source of most of the visible light.
Corona: Super-hot gas and plasma surrounding the star.
Core: generates almost all of Sun’s energy by fusion
Radiation zone: energy is transferred out mainly by radiative diffusion
Convection zone: energy is transferred out mainly by convection

Sun: Hottest act in town!


Comet points down toward Earth's horizon with its two tails behind.

Comet Lovejoy above Earth’s horizon on Dec. 22, 2011. Why the twin tails? The curved tail is made of dust and usually follows the comet’s orbit. The gas tail is strongly influenced by the solar wind and thus points away from the Sun. Both tails are made largely of water molecules.

Earth has its strong points, but it’s the sun that dominates our tiny corner of the universe. Beyond the torrent of visible and invisible radiation, the sun also emits a solar wind of charged particles, spits out the occasional mass of hot atoms, and is surrounded by a phenomenal, changing magnetic field.

The magnetic field originates in a dynamo — rotation of magnetic materials — inside the sun.

The comet’s tail responds to both open and closed magnetic fields, Downs says. The complex, closed fields loop back to the sun, while the open fields are the energy driving the solar wind. “As the plasma streams out, the field is carried out with it, and the wind buffets Earth.”

The wind can interfere with electric lines, radio transmissions and satellites.

The outer extent of the solar wind, far beyond Pluto, defines the boundary between the heliosphere, dominated by the sun, and galactic space, says Downs.

So what?

The study was the first observation of the wiggle in a comet’s tail this far down in the solar atmosphere, says Downs. “It tells us about what happens to the comet as it begins to feel the magnetic field, becomes part of the magnetic field.”

And so Lovejoy was converted into a disposable scientific instrument, yielding data from a zone that spacecraft almost certainly will never enter.

None of this will help poor old Lovejoy. The little comet nearly survived its attempted suicide, but instead gave its all in the name of science. “Everyone expected it to disappear, because it came so close.” The comet was seen leaving the sun, says Downs, “which was surprising, although it soon fragmented as the thermal stress worked its way into the core.”

By Dec. 19, Lovejoy’s corpse had disappeared, Downs adds. Mission accomplished!

— David J. Tenenbaum

Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer; Amy Toburen, content development executive