Tracking the Solar Wind by the Tail

The supersonic outflow of electrically charged gas from the Sun’s atmosphere, also known as the solar wind, is a bit of a mystery. Sure, scientists know a lot about its properties and behavior, but they’re still learning about the physics that accelerates the solar wind, especially at its fastest speeds. Also, the solar wind varies considerably, and it is very, very hot—hotter than it should be at such great distances from the Sun’s corona. So what gives?

To find out, researchers from the heliophysics group at Southwest Research Institute (SwRI) used NASA’s Solar and Terrestrial Relations Observatory, or STEREO-A, to take a closer look at the tail of Comet Encke. Why? Unlike most comets, Comet Encke has a “compact” tail. Rather than feathering out loosely, creating a wide spray of ions, Comet Encke’s ion tail streams out in a tight, bright ribbon of glowing gas with well-defined (one might say “compact”) features. Perhaps the comet’s tail could reveal some of the mysterious of the invisible solar wind.

The team observed the movements of hundreds of dense clumps of glowing, ionized gas within the ribbon of Comet Encke’s tail and discovered that fluctuations in the solar wind were mirrored in changes in the tail. By tracking these clumps, they were able to reconstruct the motion of the solar wind, catching an unprecedented look at its turbulence.

Turbulence in the solar wind could provide one of the answers to the mystery of the solar wind heating. Based on analysis of the comet tail movement, the researchers calculated that large-scale turbulence provides sufficient kinetic energy to drive the high-temperatures observed in the solar wind.

“The solar wind at Earth is about 70 times hotter than one might expect from the temperature of the solar corona and from how much it expands as it crosses the void,” says SwRI physicist Craig DeForest, lead author of a study published this week in the Astrophysical Journal. “The source of this extra heat has been a mystery of solar wind physics for several decades.”

Turbulence may also explain solar wind variability. “Turbulent motion mixes up the solar wind, leading to the rapid variation that we see at Earth,” says DeForest.

Who knew a tail could reveal so much? –Molly Michelson

Is Jupiter’s Great Red Spot Shrinking?

It’s been called the biggest hurricane in the solar system—a colossal vortex towering five miles (eight kilometers) above most of the cloud deck around it and whose outer edge spins at a speed of about 300 miles (480 kilometers) per hour. First sighted in 1665 by Domenico Cassini, Jupiter’s Great Red Spot was described as a permanent feature in observations that Cassini made through 1713. A 118-year long gap in records from 1713–1830 is as yet unexplained, so it is not known whether the visibility of the Red Spot itself changed or if record-keeping was just poor during that period. Although it’s often stated that the Red Spot has been observed for 350 years, the truth of the matter is that astronomers aren’t positive whether Cassini and Pritchett observed the same thing. The Red Spot was visually reacquired by Heinrich Schwabe in 1831, who made the first detailed drawings of it. Then, Reverend Carr Walter Pritchett observed it in 1878, starting an era of continuous monitoring ever since.

However, the Red Spot has not been quite as permanent a feature as some would think. It has changed in ellipticity, longitude, and in the intensity of its color, gradually turning from a once-deep, brick red to a pale salmon hue. Long-time amateur astronomers—including this writer—can recall when the Red Spot stood out more prominently in contrast with the clouds around it, making it easier to see even in small telescopes. New high-resolution imagery from the Hubble Space Telescope, taken as part of the Outer Planet Atmospheres Legacy (OPAL) program, has confirmed that the Great Red Spot is also growing smaller, shrinking from what was once estimated to be three times Earth’s diameter, or about 24,000 miles (38,000 kilometers) to less than half that. In the past year alone, NASA scientists say that its diameter had decreased by 150 miles. At the same time, the Spot is becoming more circular. Will it stop shrinking? Or will it continue shrinking until nothing is left? In an e-mail to USA Today, Hubble Science Team member Amy Simon said, “The most likely case is that it will stabilize at a smaller size than it is now,” she said. “It could reach that point in the next decade.”

Also captured in the new images is a subtle and elusive wave-like structure that has been observed only once—in 1979—and which was thought to be a fluke. Now, appearing somewhat similar to a baroclinic wave in Earth’s atmosphere, which is formed in rapidly rotating layered fluid and which is involved in the formation of cyclonic circulation patterns.

Hubble’s OPAL series will provide annual portraits of the giant planets Jupiter, Saturn, Uranus, and Neptune that scientists can use to monitor changes in their atmospheres and gain a better understanding of the meteorological phenomena taking place there and perhaps on other giant exoplanets that have been discovered orbiting distant stars. –Bing Quock

Eleven Years at Saturn and Still Going Strong

On October 14, NASA’s Cassini spacecraft, which has been orbiting Saturn since 2004, flew 1142 miles (1837 kilometers) over the icy moon Enceladus, one of the most intriguing of the ringed planet’s 63 known satellites. Measuring only 310 miles (500 kilometers) in diameter, Enceladus is thought to have a body of liquid water beneath its frozen crust that fuels spectacular geysers in the moon’s south polar region, ejecting plumes of water vapor about 450 miles (720 kilometers) into space. This area, streaked with four long, roughly-parallel cracks dubbed “tiger stripes,” is the general location of approximately 100 geysers, or roughly 10 percent of all the known geysers in the solar system (about 1,000 of them being on Earth). Interestingly, with all the interest in the geysers at the south pole of Enceladus, this was Cassini’s first opportunity to examine the other side of the icy moon.

When it arrived in 2004, it was winter in Saturn’s northern hemisphere, with the Sun shining from below Saturn’s equatorial plane and illuminating mainly the southern hemispheres of the planet and its major moons. The Saturnian equinox occurred in 2009, when the angle of illumination, gradually moving northward, finally started shining on the north polar regions. This also cast the shadows of Saturn’s magnificent rings across the planet’s northern hemisphere, making for some spectacular images. With the approach of the northern summer solstice, which occurs in 2017, Cassini has the opportunity to observe the north poles of the planet and its larger moons for the first time are fully-illuminated.

What it saw in striking detail were a network of cracks extending across the crater-cluttered north pole. These are a continuation of thin fractures that stretch across the Saturnian moon’s entire surface and which open and close daily in response to tidal forces exerted by Saturn’s gravity. Images also revealed a row of three adjoining craters that has whimsically been named the “Saturnian Snowman.”

Two more close flybys of Enceladus are scheduled before the end of Cassini’s mission in 2017, which will be marked by a series of perilous “Grand Finale” plunges between the planet and the innermost edge of its ring system. Cassini’s next flyby will take place on October 28, when it will fly a figurative hair’s-width from Enceladus, passing only 30 miles (50 kilometers) above its surface and through its plumes of water vapor. –Bing Quock

Odd, But Probably Not Aliens

There has been buzz building this week over a publication first printed in late September that notes some admittedly bizarre starlight patterns coming from the star KIC 8462852. Proof of alien megastructures? Well, let’s take a look.

KIC 8462852 is a big star—brighter, hotter, and more massive than the Sun—about 1,500 light years away in the constellation Lyra. Too far to see with the naked eye, KIC 8462852 was one of the 150,000 stars monitored by NASA’s Kepler Mission. Kepler searched for exoplanets around the distant stars via the transit method, tracking the amount of light emitted by a star and looking for dips in its brightness. Planets transiting, or passing in front of, their star cause regular periodic dips, every few days, weeks, or months depending on the length of the planet’s orbit. The decrease in starlight is small—even giant Jupiter would represent a dip of less than one percent of the Sun’s light.

And that is what makes KIC 8462852 strange. Kepler detected multiple—hundreds even—of non-periodic dips that went as deep as 15 to 22 percent of the star’s light! Far too big and irregular to be planets, and if it were another star, we would detect it. “We’d never seen anything like this star,” says Tabetha Boyajian of Yale University. “It was really weird. We thought it might be bad data or movement on the spacecraft, but everything checked out.” So what is it?

A few potential explanations have been put forward. One is that we are seeing a debris cloud from a large planetary collision, similar to the Earth-Theia collision that produced our moon. Another idea is that we are seeing a large field of comets and surrounding dust orbiting the star much further out and that a gravitational influence of a very nearby—perhaps 80 miles (130 kilometers) distant—red dwarf star might be breaking and knocking around the orbits of these comets to produce the irregularity. However, in both of these explanations, especially the former, we’d expect to see greater infrared emission from surrounding dust, but we don’t.

So because it’s weird and we don’t have a ready explanation for it, some cry aliens. What’s specifically gaining attention is the idea we might be seeing evidence of a “megastructure” built by an alien civilization around the star that resembles something once proposed here on Earth—the so-called Dyson Sphere. Theorizing that a civilization’s technological advances would lead to increased energy needs surpassing the natural resources of a planet, physicist and mathematician Freeman Dyson proposed a system of orbiting structures orbiting a star to collect stellar energy. This would block a significant amount of starlight in irregular ways, especially in visible wavelengths that would be shifted to infrared after being “used.”

One way we might be able to learn more is to point big, ground-based radio spheres in the direction of the star to look for signatures of technology, such as leaked infrared or radio wavelengths. SETI already does broad surveys for radio signals, but this would require a far more sensitive, very narrowly directed look.

Whether it turns out to be a natural phenomenon or not, KIC 8462852 has certainly captured the attention and imagination of both the scientific and lay communities. –Elise Ricard

Enceladus Image: NASA/JPL-Caltech/Space Science Institute

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