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  • richardmitnick 9:12 am on November 9, 2017 Permalink | Reply
    Tags: , Alfvén waves, , , , Carrying Energy to the Corona with Waves, , DKIST under construction by the National Solar Observatory atop the Haleakala volcano on the Pacific island of Maui Hawaii USA at an altitude of 3084 m (10118 ft) with a planned completion date of 201, Photosphere- the surface of the Sun,   

    From AAS NOVA: “Carrying Energy to the Corona with Waves” 

    AASNOVA

    AAS NOVA

    8 November 2017
    Susanna Kohler

    1
    How does the solar corona, the Sun’s outer atmosphere visible in this image, get so hot? [Luc Viatour]

    The solar corona has a problem: it’s weirdly hot! A new study explores how magnetic waves might solve the mystery of the unusually hot corona by transporting energy to the outer atmosphere of the Sun.

    The Problem with the Corona

    2
    The temperatures of different layers of the Sun. [ISAS/JAXA]

    The corona, the outer layer of the Sun’s atmosphere, has typical temperatures of 1–3 million K — significantly hotter than the cool 5,800 K of the photosphere, the surface of the Sun far below it. Since temperatures ordinarily drop the further you get from the heat source (in this case, the Sun’s atom-fusing center), this so-called “coronal heating problem” poses a definite puzzle.

    As is the case for many astronomical mysteries, the answer may have something to do with magnetic fields. Alfvén waves, magnetohydrodynamic waves that travel through magnetized plasma, could potentially carry energy from the convective zone beneath the Sun’s photosphere up into the solar atmosphere. There, the Alfvén waves could turn into shock waves that dissipate their energy as heat, causing the increased temperature of the corona.

    Daniel K. Inouye Solar Telescope, DKIST under construction by the National Solar Observatory atop the Haleakala volcano on the Pacific island of Maui, Hawaii, USA, at an altitude of 3,084 m (10,118 ft), with a planned completion date of 2018

    Predicting Observations

    Alfvén waves as a means of delivering heat to the corona makes for a nice picture, but there’s a lot of work to be done before we can be certain that this is the correct model. Observational evidence of Alfvén waves has thus far been limited to specific conditions — and the observations have not yet been enough to convince us that Alfvén waves can deliver enough energy to explain the corona’s temperature.

    Lucas Tarr, a scientist at the Naval Research Laboratory, argues that upcoming solar telescopes may make it easier to detect these waves — but first we need to know what to look for! In a recent study, Tarr uses a simplified analytic model to show which frequencies of waves are likely to carry power when magnetic field lines in the corona are pertubed.

    3
    The power carried by Alfvén waves as a function of frequency, as a result of an initial perturbation, plotted for several different initial conditions (such as the size of the perturbation or the length of the loop on which it is introduced). [Tarr 2017]

    A Promising Future

    Tarr modeled the effects of a minor perturbation — like a local magnetic reconnection event in the corona — on a coronal arcade, a common structure of magnetic field loops found in the corona. Tarr determined that such a disturbance would peak in power at a low frequency (maybe tens of millihertz, or oscillations on scales of minutes), but a substantial portion of the power is carried by waves of higher frequencies (0.5–4 Hz, or oscillations on scales of seconds).

    Tarr’s findings confirm that with the cadence and sensitivity of current instrumentation, we would not expect to be able to detect these Alfvén waves. The results do indicate, however, that high-cadence observations with future telescope technology — like the instrumentation at the upcoming Daniel K. Inouye Solar Telescope, which should be completed in 2018 — may have the ability to reveal the presence of these waves and confirm the model of Alfvén waves as the means by which the Sun achieves its mysteriously hot corona.
    Citation

    Lucas A. Tarr 2017 ApJ 847 1. doi:10.3847/1538-4357/aa880a

    Related Journal Articles
    See the full article for further references with links.

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    1

    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 11:54 am on March 31, 2017 Permalink | Reply
    Tags: Alfvén waves, , ,   

    From Goddard: “NASA Observations Reshape Basic Plasma Wave Physics” 

    NASA Goddard Banner
    NASA Goddard Space Flight Center

    March 31, 2017
    Mara Johnson-Groh
    mara.johnson-groh@nasa.gov
    NASA’s Goddard Space Flight Center, Greenbelt, Md.

    When NASA’s Magnetospheric Multiscale — or MMS — mission was launched, the scientists knew it would answer questions fundamental to the nature of our universe — and MMS hasn’t disappointed.


    MMS stacked


    MMS in flight

    A new finding, presented in a paper in Nature Communications, provides observational proof of a 50-year-old theory and reshapes the basic understanding of a type of wave in space known as a kinetic Alfvén wave. The results, which reveal unexpected, small-scale complexities in the wave, are also applicable to nuclear fusion techniques, which rely on minimizing the existence of such waves inside the equipment to trap heat efficiently.


    Credits: NASA’s Goddard Space Flight Center/Genna Duberstein
    Access mp4 video here .

    Kinetic Alfvén waves have long been suspected to be energy transporters in plasmas — a fundamental state of matter composed of charged particles — throughout the universe. But it wasn’t until now, with the help of MMS, that scientists have been able to take a closer look at the microphysics of the waves on the relatively small scales where the energy transfer actually happens.

    “This is the first time we’ve been able to see this energy transfer directly,” said Dan Gershman, lead author and MMS scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland in College Park. “We’re seeing a more detailed picture of Alfvén waves than anyone’s been able to get before.”

    The waves could be studied on a small scale for the first time because of the unique design of the MMS spacecraft. MMS’s four spacecraft fly in a compact 3-D pyramid formation, with just four miles between them — closer than ever achieved before and small enough to fit between two wave peaks. Having multiple spacecraft allowed the scientists to measure precise details about the wave, such as how fast it moved and in what direction it travelled.


    In a typical Alfvén wave, the particles (yellow) move freely along the magnetic field lines (blue).
    Credits: NASA Goddard’s Scientific Visualization Studio/Tom Bridgman, data visualizer
    Access mp4 video here .

    Previous multi-spacecraft missions flew at much larger separations, which didn’t allow them to see the small scales — much like trying to measure the thickness of a piece of paper with a yardstick. MMS’s tight flying formation, however, allowed the spacecraft to investigate the shorter wavelengths of kinetic Alfvén waves, instead of glossing over the small-scale effects.

    “It’s only at these small scales that the waves are able to transfer energy, which is why it’s so important to study them,” Gershman said.

    As kinetic Alfvén waves move through a plasma, electrons traveling at the right speed get trapped in the weak spots of the wave’s magnetic field. Because the field is stronger on either side of such spots, the electrons bounce back and forth as if bordered by two walls, in what is known as a magnetic mirror in the wave. As a result, the electrons aren’t distributed evenly throughout: Some areas have a higher density of electrons, and other pockets are left with fewer electrons. Other electrons, which travel too fast or too slow to ride the wave, end up passing energy back and forth with the wave as they jockey to keep up.


    In a kinetic Alfvén wave, some particles become trapped in the weak spots of the wave’s magnetic field and ride along with the wave as it moves through space.
    Credits: NASA Goddard’s Scientific Visualization Studio/Tom Bridgman, data visualizer
    Access mp4 video here .

    The wave’s ability to trap particles was predicted more than 50 years ago but hadn’t been directly captured with such comprehensive measurements until now. The new results also showed a much higher rate of trapping than expected.

    This method of trapping particles also has applications in nuclear fusion technology. Nuclear reactors use magnetic fields to confine plasma in order to extract energy. Current methods are highly inefficient as they require large amounts of energy to power the magnetic field and keep the plasma hot. The new results may offer a better understanding of one process that transports energy through a plasma.

    “We can produce, with some effort, these waves in the laboratory to study, but the wave is much smaller than it is in space,” said Stewart Prager, plasma scientist at the Princeton Plasma Physics Laboratory in Princeton, New Jersey. “In space, they can measure finer properties that are hard to measure in the laboratory.”

    This work may also teach us more about our sun. Some scientists think kinetic Alfvén waves are key to how the solar wind — the constant outpouring of solar particles that sweeps out into space — is heated to extreme temperatures. The new results provide insight on how that process might work.

    Throughout the universe, kinetic Alfvén waves are ubiquitous across magnetic environments, and are even expected to be in the extra-galactic jets of quasars. By studying our near-Earth environment, NASA missions like MMS can make use of a unique, nearby laboratory to understand the physics of magnetic fields across the universe.

    Related Link

    Learn more about NASA’s MMS Mission

    See the full article here.

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    NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

    Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.


    NASA/Goddard Campus

     
  • richardmitnick 7:25 pm on August 15, 2016 Permalink | Reply
    Tags: Alfvén waves, , , ,   

    From PPPL: “Simulations by PPPL physicists suggest that external magnetic fields can calm plasma instabilities” 


    PPPL

    August 15, 2016
    Raphael Rosen

    1
    Magnetic Perturbations. (Photo by Gerrit Kramer.)

    Physicists led by Gerrit Kramer at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have conducted simulations that suggest that applying magnetic fields to fusion plasmas can control instabilities known as Alfvén waves that can reduce the efficiency of fusion reactions. Such instabilities can cause quickly moving charged particles called “fast ions” to escape from the core of the plasma, which is corralled within machines known as tokamaks.

    Controlling these instabilities leads to higher temperatures within tokamaks and thus more efficient fusion processes. The research was published in the August issue of Plasma Physics and Controlled Fusion and funded by the DOE Office of Science (Fusion Energy Sciences).

    “Controlling and suppressing the instabilities helps improve the fast-ion confinement and plasma performance,” said Kramer, a research physicist at the Laboratory. “You want to suppress the Alfvén waves as much as possible so the fast ions stay in the plasma and help heat it.”

    The team gathered data from experiments conducted on the National Spherical Torus Experiment (NSTX) at PPPL before the tokamak was recently upgraded.

    PPPL NSTXII
    NSTX

    Then they conducted plasma simulations on a PPPL computer cluster.

    The simulations showed that externally applied magnetic perturbations can block the growth of Alfvén waves. The perturbations reduce the gradient, or difference in velocity, of the ions as they zoom around the tokamak. This process calms disturbances within the plasma. “If you reduce the velocity gradient, you can prevent the waves from getting excited,” notes Kramer.

    The simulations also showed that magnetic perturbations can calm Alfvén waves that have already formed. The perturbations alter the frequency of the plasma vibration so that it matches the frequency of the wave. “The plasma absorbs all the energy of the wave, and the wave stops vibrating,” said Kramer.

    In addition, the simulations indicated that when applied to tokamaks with relatively weak magnetic fields, the external magnetic perturbations could dislodge fast ions from the plasma directly, causing the plasma to cool.

    Along with Kramer, the research team included scientists from General Atomics, Oak Ridge National Laboratory, the University of California, Los Angeles, and the University of California, Irvine.

    See the full article here .

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    Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University. PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

     
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