Solar Flare Surprise

Solar flares are the most powerful explosions in the solar system. Packing a punch equal to a hundred million hydrogen bombs, they obliterate everything in their immediate vicinity. Not a single atom should remain intact.

At least that’s how it’s supposed to work.

"We’ve detected a stream of perfectly intact hydrogen atoms shooting out of an X-class solar flare," says Richard Mewaldt of the California Institute of Technology. "What a surprise! If we can understand how these atoms were produced, we'll be that much closer to understanding solar flares."

The event occurred on Dec. 5, 2006. A large sunspot rounded the sun’s eastern limb and with little warning it exploded. On the "Richter scale" of flares, which ranks X1 as a big event, the blast registered X9, making it one of the strongest flares of the past 30 years.

NASA managers braced themselves. Such a ferocious blast usually produces a blizzard of high-energy particles dangerous to both satellites and astronauts. An hour later they arrived, but they were not the particles researchers expected.

NASA’s twin Solar TErrestrial RElations Observatory (STEREO) spacecraft made the discovery: "It was a burst of hydrogen atoms," says Mewaldt. "No other elements were present, not even helium (the sun’s second-most abundant atomic species). Pure hydrogen streamed past the spacecraft for a full 90 minutes."

Next came 30 minutes of quiet. The burst subsided and STEREO’s particle counters returned to low levels. The event seemed to be over when a second wave of particles enveloped the spacecraft. These were the "broken atoms" flares are supposed to produce—protons and heavier ions such as helium, oxygen and iron. "Better late than never," he says.

At first, this unprecedented sequence of events baffled scientists, but now Mewaldt and colleagues believe they’re getting to the bottom of the mystery.

First, how did the hydrogen atoms resist destruction?

"They didn’t," says Mewaldt. "We believe they began their journey to Earth in pieces, as protons and electrons. Before they escaped the sun’s atmosphere, however, some of the protons captured an electron, forming intact hydrogen atoms. The atoms left the sun in a fast, straight shot before they could be broken apart again." (For experts: The team believes the electrons were recaptured by some combination of radiative recombination and charge exchange.)

Second, what delayed the ions?

"Simple," says Mewaldt. "Ions are electrically charged and they feel the sun’s magnetic field. Solar magnetism deflects ions and slows their progress to Earth. Hydrogen atoms, on the other hand, are electrically neutral. They can shoot straight out of the sun without magnetic interference."

Imagine two runners dashing for the finish line. One (the ion) is forced to run in a zig-zag pattern with zigs and zags as wide as the orbit of Mars. The other (the hydrogen atom) runs in a straight line. Who’s going to win?

"The hydrogen atoms reached Earth almost two hours before the ions," says Mewaldt.

Mewaldt believes that all strong flares might emit hydrogen bursts, but they simply haven’t been noticed before. He’s looking forward to more X-flares now that the two STEREO spacecraft are widely separated on nearly opposite sides of the Sun. (In 2006 they were still together near Earth.) STEREO-A and –B may be able to triangulate future bursts and pinpoint the source of the hydrogen. This would allow the team to test their ideas about the surprising phenomenon.

"All we need now," he says, "is some solar activity."

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Sunlight May Cut Risk of Advanced Breast Cancer by half

A research team from the Northern California Cancer Center, the University of Southern California, and Wake Forest University School of Medicine has found that increased exposure to sunlight – which increases levels of vitamin D in the body -- may decrease the risk of advanced breast cancer.

In a study reported online this week in the American Journal of Epidemiology, the researchers found that women with high sun exposure had half the risk of developing advanced breast cancer, which is cancer that has spread beyond the breast, compared to women with low sun exposure. These findings were observed only for women with naturally light skin color. The study defined high sun exposure as having dark skin on the forehead, an area that is usually exposed to sunlight.

The scientists used a portable reflectometer to measure skin color on the underarm, an area that is usually not directly exposed to sunlight. Based on these measurements, they classified the women as having light, medium or dark natural skin color. Researchers then compared sun exposure between women with breast cancer and those without breast cancer. Sun exposure was measured as the difference in skin color between the underarm and the forehead.

In women with naturally light skin pigmentation, the group without breast cancer had significantly more sun exposure than the group with breast cancer. The fact that this difference occurred only in one group suggests that the effect was due to differences in vitamin D production – and wasn’t just because the women were sick and unable to go outdoors. In addition, the effect held true regardless of whether the cancer was diagnosed in the summer or in the winter. The difference was seen only in women with advanced disease, suggesting that vitamin D may be important in slowing the growth of breast cancer cells.

“We believe that sunlight helps to reduce women’s risk of breast cancer because the body manufactures the active form of vitamin D from exposure to sunlight,” says Esther John, lead researcher on the study from the Northern California Cancer Center. “It is possible that these effects were observed only among light- skinned women because sun exposure produces less vitamin D among women with naturally darker pigmentation.”

These new findings about breast cancer risk and sun exposure based on skin color measurements are consistent with previous research by John and colleagues that had shown that women who reported frequent sun exposure had a lower risk of developing breast cancer than women with infrequent sun exposure.

The researchers stressed that sunlight is not the only source of vitamin D, which can be obtained from multivitamins, fatty fish and fortified foods such as milk, certain cereals and fruit juices. Women should not try to reduce their risk of breast cancer by sunbathing because of the risks of sun-induced skin cancer, they said.

“If future studies continue to show reductions in breast cancer risk associated with sun exposure, increasing vitamin D intake from diet and supplements may be the safest solution to achieve adequate levels of vitamin D,” says Gary Schwartz, a co-researcher from the Comprehensive Cancer Center at Wake Forest University School of Medicine.

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NASA Spacecraft Make First 3-D Images of Sun

NASA's twin Solar Terrestrial Relations Observatory (STEREO) spacecraft have made the first three-dimensional images of the sun. The new view will greatly aidscientists' ability to understand solar physics and thereby improve space weather forecasting.

"The improvement with STEREO's 3D view is like going from a regular X-ray to a 3D CAT scan in the medical field," says Michael Kaiser, STEREO project scientist at NASA's Goddard Space Flight Center, Greenbelt, Md.

The STEREO spacecraft were launched October 25, 2006. On January 21 they completed a series of complex maneuvers, including flying by the moon, to position the spacecraft in their mission orbits. The two observatories are now orbiting the sun, one slightly ahead of Earth and one slightlybehind, separating from each other by approximately 45 degrees per year. Just as the slight offset between a person's eyes provides depth perception, the separation of spacecraft allow 3-D images of the sun.

Violent solar weather originates in the sun's atmosphere, or corona, and can disrupt satellites, radio communication, and power grids on Earth. The corona resembles wispy smoke plumes, which flow outward along the sun'stangled magnetic fields. It's difficult for scientists to tell which structures are in front and which are behind.

"In the solar atmosphere, there are no clues to help us judge distance. Everything appears flat in the 2D plane of the sky. Having a stereoperspective just makes it so much easier," says Russell Howard of the Naval Research Laboratory, Washington, the principal investigator for the SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) suite of telescopes on the spacecraft.

"With STEREO's 3D imagery, we'll be able to discern where matter andenergy flows in the solar atmosphere much more precisely than with the 2D views available before. This will really help us understand the complex physics going on," says Howard.

STEREO's depth perception also will help improve space weather forecasts. Of particular concern is a destructive type of solar eruptioncalled a Coronal Mass Ejection (CME). CMEs are eruptions of electrically charged gas, called plasma, from the sun's atmosphere. A CME cloud can contain billions of tons of plasma and move at a million miles per hour.

The CME cloud is laced with magnetic fields, and CMEs directed toward Earth smash into the planet's magnetic field. If the CME magnetic fieldshave the proper orientation, they dump energy and particles into Earth'smagnetic field, causing magnetic storms that can overload power lineequipment and radiation storms that disrupt satellites.

Satellite and utility operators can take precautions to minimize CME damage, but they need an accurate forecast of when the CME will arrive. To do this, forecasters need to know the location of the front of the CME cloud. STEREO will allow scientists to accurately locate the CME cloud front. "Knowing where the front of the CME cloud is will improve estimates of the arrival time from within a day or so to just a few hours," says Howard. "STEREO also will help forecasters estimate how severe the resulting magnetic storm will be."

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Light Gives Asteroids Spin

Astronomers have observed an asteroid change the rate at which it spins for the first time, and shown that this is due to a theoretical effect predicted but never before seen. The international team of scientists from Europe and the United States used a range of telescopes to find that the asteroid is rotating faster by 1 millisecond every year.

The acceleration in the rate of rotation is due to heating of the asteroid’s surface by the Sun. Eventually it may spin faster than any known asteroid in the solar system. The Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect is believed to alter the way small bodies in the solar system rotate. YORP is a torque due to sunlight shining on the surfaces of asteroids and meteoroids and warming their surfaces, leading to a gentle recoil effect as the heat is emitted. By analogy, if one were to shine light on a propeller over a long enough period, it would start spinning.

Although this is an almost immeasurably weak force, astronomers believe it may be responsible for spinning some asteroids up so fast that they break apart, perhaps leading to the formation of binary asteroids. Others may be slowed down so that they take many days to rotate once. The YORP effect also plays an important role in changing the orbits of asteroids between Mars and Jupiter, including their delivery to planet-crossing orbits.

Despite its importance, the effect has never been seen acting on a solar system body, until now. Using extensive optical and radar imaging from powerful Earth-based observatories, astronomers have directly observed the YORP effect in action on a small near-Earth asteroid, known as (54509) 2000 PH5. This work is reported in two companion papers, in the March edition of Science Express, by Stephen Lowry et al. (Queens University Belfast, UK) and Patrick Taylor et al. (Cornell University, Ithaca, NY, USA).

Shortly after its discovery in 2000, it was realized that this asteroid would be the ideal candidate for such a YORP detection. At just 114m in diameter, it is relatively small and so more susceptible to the effect. Also, it rotates very fast, with one day on the asteroid lasting just over 12 Earth minutes, implying that the YORP effect may have been acting on it for some time. With this in mind, the team of radar and optical astronomers undertook a long term monitoring campaign of the asteroid with the aim of detecting any tiny changes in the spin-rate.

Over a 4 year time span, Stephen Lowry, Alan Fitzsimmons and colleagues took images of the asteroid at a range of telescope sites including the 8.2m Very Large Telescope array and the 3.5m New Technology Telescope of the European Southern Observatory in Chile, the 3.5m telescope at Calar Alto, Spain, along with a suite of other telescopes from the Czech Republic, the Canary Islands, Hawaii, Spain and Chile. With these facilities the astronomers measured the slight brightness variations as the asteroid rotated.

Over the same time period, the radar team led by Patrick Taylor and Jean-Luc Margot of Cornell University employed the unique capabilities of the Arecibo Observatory in Puerto Rico and the Goldstone radar facility in California to observe the asteroid by ‘bouncing’ a radar pulse off the asteroid and analyzing its echo. With this technique astronomers can reconstruct a 3-D model of the asteroid’s shape, with the necessary detail to allow a theoretical YORP value to be derived and compared with the actual observed spin-rate change seen at optical wavelengths. With careful analysis of the optical data, the asteroid’s spin rate was seen to steadily increase with time, at a rate that can be explained by YORP theory. Most significantly, the effect was observed year after year.

Furthermore, this number was elegantly supported via analysis of the combined radar and optical data, as it was required that the asteroid increase its spin-rate at exactly this rate in order for a satisfactory 3-D shape model to be determined.

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Physicists Test the Physics of Star Formation

The formation of stars and planets remains one of the big questions in astrophysical science. Currently, scientists do not understand the required conditions and the accretion, or matter collection process, involved in star and planet formation. There are contentious debates about whether hydrodynamic turbulence is responsible. The Magnetorotational Instability (MRI) experiment at the Princeton Plasma Physics Laboratory (PPPL) in collaboration with the Astrophysical Sciences Department of Princeton University is shedding light on this mystery.

Results published in Nature show that it is virtually impossible for hydrodynamic turbulence to generate sufficiently effective accretion to form stars and planets. The U.S. Department of Energy, NASA, and National Science Foundation are funding the work jointly.

"The Earth must have sufficient angular momentum so that it does not fall into the Sun under the influence of gravity. We also know that galaxies and solar systems have a preferred direction of rotation. Consequently, matter forming these systems must also have had net angular momentum, which must have been overcome by gravity for the matter to coalesce," says Hantao Ji, the lead author of the Nature paper. "The angular momentum prevents matter from falling into the star directly, so an accretion disk is formed, which consists of matter losing its angular momentum and swirling into the core of the star. For example, when our Sun was formed, the accretion process must have been very efficient in casting off angular momentum because most of the material comprising our solar system ended up in the Sun."

Since angular momentum cannot be created or destroyed, it must flow outward through the disk as the accreting mass flows inward. But how does this happen? Star formation occurs in deep space and therefore, while accretion disks are seen, the details of the accretion process cannot be discerned except in theoretical models and computer simulations. The Princeton project's primary mission is to test the plausibility of a 1991 theory that indicates the magnetorotational instability, a disruptive plasma process, plays a major role in accretion.

The elimination of hydrodynamic turbulence as a mechanism for accretion, makes it much more likely that magnetorotational instability is responsible. Matter in an accretion disk is composed of plasma, dust, and other materials. However, the MRI experiment does not use these materials. Ji and Jeremy Goodman, the primary collaborator from the Princeton University Astrophysics Department and also a co-author of the Nature paper, came up with a way to physically simulate an accretion disk with material "standing in" for the plasma, dust, and other materials. The system consists of two concentric cylinders, each 28 centimeters in length, free to rotate independently about a common axis.

The inner cylinder has a radius of 7.1 centimeters and is made of steel, and the outer cylinder has a radius of 20.3 centimeters and is made of plastic to allow visual inspection. The inner and outer cylinders rotate independently in the same direction, but at significantly different speeds, 1200 rpm and 160 rpm, respectively, as reported in the paper. What made this project a significant engineering challenge is the requirement to have two rotating disks at each end of the cylinders. The disks must be driven at different speeds by separate motors through six concentric pipes in order to achieve the required rotation patterns of the fluid.

For the experiments reported in Nature, the space between the cylinders was filled with water. Water cannot carry a significant electrical current or interact with a magnetic field and therefore cannot display magnetorotational instability, but according to nonmagnetic theories of accretion disks, should have become turbulent anyway with fast enough spinning. Future experiments are planned in which the space between the cylinders will be filled with a liquid metal chosen because it is easy to maintain and interacts with the magnetic field in ways similar to plasma. The researchers have chosen a mixture of 67 percent gallium, 20.5 percent indium and 12.5 percent tin. Future experiments will be conducted with and without a magnetic field parallel to the axis of the cylinders.

Computer simulations of the experiment predict that when a strong magnetic field is applied to the rotating liquid metal, magnetorotational instability will cause angular momentum to be transferred from the inner cylinder toward the outer cylinder, resulting in an increase in measured torque between the cylinders. This result would further support the hypothesis that magnetorotational instability is responsible for the transport of angular momentum in accretion disks, and hence for star formation. Accretion disks also form around massive black holes in the center of many galaxies and in binary star systems.

Results from the PPPL experiments will help astrophysicists better understand these phenomena. Understanding transport phenomena in plasmas is important for basic plasma physics in general, and for fusion plasmas in particular.

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