The observation could improve space weather forecasts, as well as help improve models of turbulent flow in ionized gas, called plasma.
Turbulence is quite common on Earth, as any frequent airplane passenger can attest. But even physicists get a little queasy when trying to explain the nature of this choppy, swirling flow.
"One cannot predict future behaviours with satisfactory accuracy," says Yasuhito Narita of the Institute of Geophysics and Extraterrestrial Physics in Braunschweig, Germany. "Even a small deviation or uncertainty in the initial state will end up with a completely different state."
Chaos reigns
It's a bit of the butterfly-tornado connection from chaos theory. Without predictive mathematical equations for turbulence, scientists usually resort to statistical descriptions, like how much does the pressure or velocity vary over a certain distance.
Researchers have done such detailed observations of the turbulence in wind tunnels and water pipes. Making similar measurements in space has been harder to do. Still, astrophysicists have inferred the presence of turbulence inside stars, among interstellar clouds, in black hole accretion disks and around Jupiter's red spot.
Single satellites have also studied the solar wind and have detected turbulent signals in the way this plasma flow changes with time. However, to make direct comparisons to models, researchers had to assume something about the size of wind variations.
Data cluster
To avoid this ambiguity, multiple sensors are needed to measure the wind's properties at several points. This is exactly what the Cluster suite of satellites was designed for.
"One needs at least four spacecraft to obtain the spatial resolution in three dimensions," Narita told Space.com. "Cluster spacecraft provide a minimal set of the measurement points for this purpose."
The four identical Cluster satellites orbit the Earth in a pyramid formation, collecting electric and magnetic field data. Of special interest is the Earth's protective magnetosphere, where the planet's magnetic field deflects the ionized solar wind, like air hitting a car's windshield.
A big shock
On Feb. 18 2002, the Cluster quartet ventured out into the leading-edge of the magnetosphere. At this bow shock, reverberating shock waves cause ripples and eddies in the solar wind propagation: a prime place to look for turbulence. Analysing the magnetic field intensities recorded by each satellite, Narita and his colleagues were able to pinpoint changes in the wind speed. From this, they determined how the solar wind's energy varied over distance, as detailed recently in the journal Physical Review Letters.
The results largely matched energy fluctuations seen in Earth-bound fluid turbulence, making this the first "definitive" detection of space turbulence, said Melvyn Goldstein of Goddard Space Flight Center. He has worked on previous studies that gave hints of the same similarity.
That the solar wind behaves like the cream swirling in your coffee is surprising, since the low-density solar wind has almost no viscosity—an important component in fluid turbulence.
"For turbulence to develop in space, there must be some physical processes that can replace the role of viscosity," Narita says.
This viscosity replacement may be some complicated electromagnetic interaction between the solar wind's ionized particles. Goldstein says much of the current work is aimed at understanding how this plasma behaves in relation to the nearby magnetic fields.
Better characterization of solar wind turbulence could help scientists predict space weather, which affects the radiation level for astronauts and spacecraft, Narita says.
Source: Michael Schirber - Special to SPACE.com
While some scientists are predicting a weak cycle, others are predicting a cycle that would be the most intense solar activity yet recorded.
Sunspots are cool, dark patches on the sun’s surface that give rise to solar flares, streams of protons and X-rays that wear down the electricity-generating solar panels on satellites and increase the atmospheric drag on spy satellites, the Hubble Space Telescope and other low-Earth-orbiting spacecraft.
Solar flares also can disrupt communications with airliners traveling on polar routes, degrade the accuracy of GPS satellite signals and cause disruptions in electric power grids on Earth, said Bill Murtagh, a U.S. space weather forecaster.
Satellite manufacturers will review the new prediction, due to be completed in April, to make sure they are planning adequate radiation shielding on electronics and large enough solar arrays to cover the expected wear.
“There’s an optimization that’s done to try to size that appropriately. You don’t want there to be too little, but on the other hand, you don’t want to have too much,” said Barry Noakes, the chief technology officer for Lockheed Martin Commercial Space Systems, in Newtown, Penn.
NASA is funding the work of the 12-person Solar Cycle 24 Prediction Panel, named for the upcoming sunspot cycle, the 24th since accurate records have been kept. Solar physicist Doug Biesecker of the U.S. National Oceanic and Atmospheric Administration’s (NOAA) Space Environment Center in Boulder, Colo., chairs the panel, which met for the first time in October.
The panel’s prediction will be the official solar-cycle forecast for NASA, NOAA and the International Space Environment Service, which operates 11 regional space weather warning centers around the world.
The task of forging a “single, clear voice for the user community” will be challenging because of the wide array of predictions that need to be reconciled this time around, Biesecker acknowledged, noting the contrast between those predicting a weak cycle and those predicting a cycle that would nearly rival the record setting Cycle 19 observed in the 1950s.
A similar prediction panel met 10 years ago, before the current solar cycle, and reached a unanimous consensus from predictions that spanned a narrower range. “In the previous cycle there was a tendency to believe it would be a big cycle. It was just a question of how big,” Biesecker said.
At the meeting in San Francisco, Biesecker brought together two members of the panel who were scientists representing the two leading techniques for predicting sunspot activity.
Ten years ago, scientists using these techniques arrived at similar predictions. Now, their results are diametrically opposed, and it will be up to Biesecker’s panel to reconcile them.
Solar physicist Dean Pesnell of Goddard Space Flight Center in Greenbelt, Md., believes that the best predictor of solar activity is the characteristics of the Sun’s polar magnetic field.
“If we look at the current value of the polar fields, they’re way down here, about half of the field strength that was measured in the previous solar minimum. This leads us to produce a prediction that the next solar cycle will be relatively weak,” he said.
Solar physicist David Hathaway of NASA’s Marshall Space Flight Center in Huntsville, Ala., believes a better approach is to study fluctuations or vibrations in the intensity of Earth’s own magnetic field as it reacts to streams of particles from the Sun during the current solar cycle.
“It’s like listening to a freight train in the distance and trying to estimate the size of the train, but what we’re listening to is the Earth’s magnetic field,” he said. “Listening to what the Earth’s magnetic field was doing back in 2003, as it turns out, we find that it was a strong peak that suggests that the next cycle ought to be a big cycle,” Hathaway said.
Hathaway and Pesnell said the situation facing the panel is interesting because in the past their techniques have pointed to similar results. “The surprising thing is they disagree so fundamentally this time,” Pesnell said.
“As scientists, we need to get to the bottom of this as far as understanding how the sunspot cycle works,” Hathaway said.
Biesecker said the panel would issue a prediction in April and update it after that, similar to the way hurricane forecasters issue updates.
“A year from April, we will revise our prediction… We may have discovered we were wrong spectacularly or we are headed in the right direction,” Biesecker said.
Source: Ben Iannotta - Space News Correspondent Space.com
Unlike our Sun, many stars in the universe have at least one stellar companion. How such multiple star systems form is still an open question, but there are two popular theories.
According to one idea, a large disk of dust, gas and other material swirls around so quickly that it breaks apart to form two or more young stars, called "protostars."
Each protostar is surrounded by its own disk that, in time, can coalesce to form individual solar systems, complete with planets, moons, comets and asteroids. According to this so-called "competitive accretion" hypothesis, stars can form in time spans as brief as 1 million years or less.
An alternative idea is that stars are born alone, by a slow accretion of gas and dust that can take 10 million years or more. According to this scenario, multiple star systems form when a star gravitationally ropes in another star as it passes by.
Scientists led by Jeremy Lim of the Institute of Astronomy & Astrophysics, Academia Sinica, in Taiwan, examined the protostars in L1551 IRS5, a still-forming star system located 450 light-years from Earth. Using the Very Large Array (VLA) radio telescope, the team discovered a third star that was previously hidden. The close snapshots also revealed features that support both formation theories.
"Our new study shows that the disks of the two main protostars are aligned with each other, and also are aligned with the larger, surrounding disk. In addition, their orbital motion resembles the rotation of the larger disk," Lim said. "This is a 'smoking gun' supporting the fragmentation model."
Alyssa Goodman, an astronomer at Harvard-Smithsonian Center for Astrophysics who was not involved in the study, said the findings suggest L1551 IRS5, now a relatively calm region of the sky, must have once been a much more turbulent place for two protostars to form together.
Whether a swirling disk gives birth to one star or many depends on how vigorous the interactions are between its parts. If there is a lot of mixing occurring within the disk, it is more likely to fragment and give birth to multiple stars. Goodman likens the chances of interactions between stellar material to the probability of people living in rural, suburban and urban areas coming across one another.
People living in the city are more likely to run into one another than those in the countryside. The same is true for star-forming disks, Goodman said.
"The reason L1551 IRS5 is interesting for me is because it says that even this region that looks rural, or possibly even suburban, that something which is thought to happen only in urban regions must have happened," Goodman told SPACE.com.
The third protostar, Lim's team discovered, is aligned along a different plane than its two neighbors, suggesting it might have formed elsewhere. "The misalignment of the third protostar and its disk leaves open the possibility that it could have formed elsewhere and been captured," Lim said.
The misaligned protostar is not conclusive evidence of the capture scenario, however, since gravitational interactions with its two larger neighbors could have skewed the protostar’s alignment. Lim’s team is planning further studies to test the two hypotheses.
The study is detailed in the Dec. 10 issue of Astrophysical Journal.
Source: Ker Than - Staff Writer Space.com
The discoveries, detailed in four separate studies in the Dec. 21 issue of the journal Nature, could reveal a new way stars exit the universal stage.
Until now, scientists had thought stars expire in only one of two ways. When stars up to eight times more massive than our Sun run out of fuel, their outer layers slough away to leave behind a smoldering stellar corpse, called a white dwarf.
Heftier stars above this mass-threshold die more violently. When these stars run out of fuel, their cores collapse, triggering titanic explosions called supernovae that can fling several solar masses worth of material into space. After the blast, what's left in place of the star is either a neutron star or a black hole.
Recent studies have linked supernova explosions to luminous, energetic events called gamma-ray bursts (GRBs), particularly those lasting longer than two seconds. These so-called "long" GRBs are a kind of death knoll for massive stars and are emitted shortly before the stars explode.
In June, NASA's Swift telescope detected a long GRB in a dwarf galaxy 1.6 billion light-years away in the constellation Indus. Dubbed GRB 060614, it lasted for 102 seconds. Astronomers tipped off to the event quickly aimed their ground telescopes at GRB 060614 expecting to see a supernova.
But nothing happened.
In one case, astronomers using the European Southern Observatory's Very Large Telescope kept vigil for 50 days. "Despite our deep monitoring, no rebrightening due to a supernova was seen," said Gianpiero Tagliaferri, an astronomer at the Observatory of Brera in Italy who was involved in one of the studies.
The massive star's "dark" death puzzled astronomers. "It is a bit like not hearing the thunder from a nearby storm when one could see a very long-lasting flash," said Johan Fynbo of the DARK Cosmology Center at the Niels Bohr Institute of Copenhagen University in Denmark.
Fynbo's team had observed another long GRB one month before, with similar results. Called GRB 060505, it occurred in a small spiral galaxy and lasted only four seconds.
Together, GRB 060614 and GRB 060505 suggest there is another road to oblivion that massive stars can take, an exit that does not involve a supernova explosion.
"It's easy to explain away one of these anomalous events as a fluke, but two strange events give our claim some oomph," said Joshua Bloom, an astronomer at the University of California, Berkeley involved in one of the studies. "These events are observational threats to the one-to-one association between long bursts and supernovae."
One possible explanation is that some massive stars bypass the supernova phase completely in death, letting out only a long gamma ray sigh before collapsing immediately into a black hole. In this scenario, "all the material that is usually ejected in a supernova explosion would then fall back and be swallowed," explained Guido Chincarini, an astronomer at the University of Milano-Bicocca in Italy, who was on one of the teams studying GRB 060614.
Another possibility is that the new type of long GRB is produced by a cosmic merger of some sort. For example, collisions between two neutron stars or a neutron star and a black hole also produce GRBs.
However, these GRBs are typically much more fleeting, lasting less than 2 seconds, and they also tend to be much less energetic.
"Some unknown process must be at play, which we have presently no clue," said study team member Massimo Della Valle of the Osservatorio Astrofisico di Arcetri in Firenze, Italy. "Either it is a new kind of merger which is able to produce long bursts, or a new kind of stellar explosion in which matter can't escape the black hole."
Source: Ker Than - Staff Writer Space.com