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  • richardmitnick 2:31 pm on November 23, 2016 Permalink | Reply
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    From Princeton: “An explanation for the mysterious onset of a universal process (Physics of Plasmas)” 

    Princeton University
    Princeton University


    PPPL

    November 23, 2016
    John Greenwald, Princeton Plasma Physics Laboratory Communications

    1
    Magnetic reconnection happens in solar flares on the surface in the sun, as well as in experimental fusion energy reactors here on Earth. Image credit: NASA.

    Scientists have proposed a groundbreaking solution to a mystery that has puzzled physicists for decades. At issue is how magnetic reconnection, a universal process that sets off solar flares, northern lights and cosmic gamma-ray bursts, occurs so much faster than theory says should be possible. The answer, proposed by researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University, could aid forecasts of space storms, explain several high-energy astrophysical phenomena, and improve plasma confinement in doughnut-shaped magnetic devices called tokamaks designed to obtain energy from nuclear fusion.

    Magnetic reconnection takes place when the magnetic field lines embedded in a plasma — the hot, charged gas that makes up 99 percent of the visible universe — converge, break apart and explosively reconnect. This process takes place in thin sheets in which electric current is strongly concentrated.

    According to conventional theory, these sheets can be highly elongated and severely constrain the velocity of the magnetic field lines that join and split apart, making fast reconnection impossible. However, observation shows that rapid reconnection does exist, directly contradicting theoretical predictions.

    Detailed theory for rapid reconnection

    Now, physicists at PPPL and Princeton University have presented a detailed theory for the mechanism that leads to fast reconnection. Their paper, published in the journal Physics of Plasmas in October, focuses on a phenomenon called “plasmoid instability” to explain the onset of the rapid reconnection process. Support for this research comes from the National Science Foundation and the DOE Office of Science.

    Plasmoid instability, which breaks up plasma current sheets into small magnetic islands called plasmoids, has generated considerable interest in recent years as a possible mechanism for fast reconnection. However, correct identification of the properties of the instability has been elusive.

    The Physics of Plasmas paper addresses this crucial issue. It presents “a quantitative theory for the development of the plasmoid instability in plasma current sheets that can evolve in time” said Luca Comisso, lead author of the study. Co-authors are Manasvi Lingam and Yi-Ming Huang of PPPL and Princeton, and Amitava Bhattacharjee, head of the Theory Department at PPPL and Princeton professor of astrophysical sciences.

    Pierre de Fermat’s principle

    The paper describes how the plasmoid instability begins in a slow linear phase that goes through a period of quiescence before accelerating into an explosive phase that triggers a dramatic increase in the speed of magnetic reconnection. To determine the most important features of this instability, the researchers adapted a variant of the 17th century “principle of least time” originated by the mathematician Pierre de Fermat.

    Use of this principle enabled the researchers to derive equations for the duration of the linear phase, and for computing the growth rate and number of plasmoids created. Hence, this least-time approach led to a quantitative formula for the onset time of fast magnetic reconnection and the physics behind it.

    The paper also produced a surprise. The authors found that such relationships do not reflect traditional power laws, in which one quantity varies as a power of another. “It is common in all realms of science to seek the existence of power laws,” the researchers wrote. “In contrast, we find that the scaling relations of the plasmoid instability are not true power laws – a result that has never been derived or predicted before.”

    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. The Laboratory is managed by Princeton 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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    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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  • richardmitnick 6:40 pm on May 16, 2016 Permalink | Reply
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    From Goddard via AGU: “Swept Up in the Solar Wind” 

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    NASA Goddard Space Flight Center

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    AGU

    May 10, 2016
    Sarah Schlieder
    NASA’s Goddard Space Flight Center, Greenbelt, Md.

    1
    This image from the ESA/NASA Solar and Heliospheric Observatory on June 15, 1999, shows streaks of bright light. This represents material streaming out from the sun (which is obscured in this picture by the central red disk so that it cannot overwhelm the image of the fainter material around it). Two other NASA spacecraft measured this material closer to Earth to better understand what causes this regular outflow, known as the solar wind, from the sun. Credits: NASA/SOHO

    ESA/SOHO
    ESA/SOHO

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    A constant outflow of solar material streams out from the sun, depicted here in an artist’s rendering. This solar wind is always passing by Earth. Credits: NASA Goddard’s Conceptual Image Lab/Greg Shirah

    From our vantage point on the ground, the sun seems like a still ball of light, but in reality, it teems with activity. Eruptions called solar flares and coronal mass ejections explode in the sun’s hot atmosphere, the corona, sending light and high energy particles out into space. The corona is also constantly releasing a stream of charged particles known as the solar wind.

    But this isn’t the kind of wind you can fly a kite in.

    Even the slowest moving solar wind can reach speeds of around 700,000 mph. And while scientists know a great deal about solar wind, the source of the slow wind remains a mystery. Now, a team at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has explored a detailed case study of the slow solar wind, using newly processed observations close to Earth to determine what in fact seeded that wind 93 million miles away, back on the sun. The team spotted tell-tale signs in the wind sweeping by Earth showing that it originated from a magnetic phenomenon known as magnetic reconnection. A paper* on these results was published April 22, 2016, in the journal Geophysical Research Letters.

    Knowing the source of the slow solar wind is important for understanding the space environment around Earth, as near-Earth space spends most of its time bathed in this wind. Just as it is important to know the source of cold fronts and warm fronts to predict terrestrial weather, understanding the source of the solar wind can help tease out space weather around Earth — where changes can sometimes interfere with our radio communications or GPS, which can be detrimental to guiding airline and naval traffic.

    Slow and Fast Solar Wind

    Fast solar wind — not surprisingly — can travel much faster than the slow wind at up to 1.7 million mph, but the most definitive difference between fast and slow solar wind is their composition. Solar wind is what’s known as a plasma, a heated gas made up of charged particles — primarily protons and electrons, with trace amounts of heavier elements such as helium and oxygen. The amount of heavy elements and their charge state, or number of electrons, differ between the two types of wind.

    “The composition and charge state of the slow solar wind is very different from that of fast solar wind,” said Nicholeen Viall, a solar scientist at Goddard. “These differences imply that they came from different places on the sun.”

    By studying its composition, scientists know that fast solar wind emanates from the interior of coronal holes — areas of the solar atmosphere where the corona is darker and colder. The slow solar wind, on the other hand, is associated with hotter regions around the equator, but just how the slow solar wind is released has not been clear.

    But the new results may have provided an answer.

    Tracking Down the Source: Magnetic Reconnection

    Magnetic reconnection can occur anywhere there are powerful magnetic fields, such as in the sun’s magnetic environment. Imagine a magnetic field line pointing in one direction and another field line nearby moving toward it pointing in the opposite direction. As they come together, the field lines will cancel and re-form, each performing a sort of U-turn and curving to move off in a perpendicular direction. The resulting new magnetic field lines create a large force — like a taut rubber band being released — that flings out plasma. This plasma is the slow solar wind.

    The team studied an interval of 90-minute periodic structures in the slow wind, and identified magnetic structures that are the telltale fingerprints of magnetic reconnection. They also found that each 90-minute parcel of slow wind showed an intriguing and repeating variability that could only be remnants of magnetic reconnection back at the sun.

    “We found that the density and charge state composition of the slow solar wind rises and falls every 90 minutes, varying from what is normally slow wind to what is considered fast,” Viall said. “But the speed was constant at a slow wind speed. This could only be created by magnetic reconnection at the sun, tapping into both fast and slow wind source regions.”

    Researchers first discovered the periodic density structures about 15 years ago using the Wind spacecraft — a satellite launched in 1994 to observe the space environment surrounding Earth. The scientists observed oscillations in the magnetic fields near Earth, known as the magnetosphere.

    3
    The WIND Satellite launched on November 1, 1994. The first of NASA’s Global Geospace Science (GGS) program.

    Magnetosphere of Earth, original bitmap from NASA. SVG rendering by Aaron Kaase
    Magnetosphere of Earth, original bitmap from NASA. SVG rendering by Aaron Kaase

    “It has been thought that the magnetosphere rang like a bell when the solar wind hit it with a sudden increase in pressure,” said Larry Kepko, a magnetospheric scientist at Goddard. “We went in for a closer look and found these periodicities in the solar wind. The magnetosphere was acting more like a drum than a bell.”

    But Wind only gave the researchers measurements of the slow solar wind’s density and velocity, and could not identify its source. For that, they needed composition data.

    Furthermore, in order to solve this problem, scientists from different disciplines needed to work together to come up with an explanation of the entire system. Kepko studies the magnetosphere, while Viall studies the sun. By observing what’s close to Earth and what’s happening at the sun, the team could determine the source of the slow solar wind.

    The scientists turned to NASA’s Advanced Composition Explorer. ACE launched in 1997 to study and measure the composition of several types of space material including the solar wind and cosmic rays. It can observe solar particles and helps researchers determine the elemental composition and speeds of solar wind.

    “Without the ACE data, we wouldn’t have been able to do this research,” Kepko said. “There’s no other instrument that gives us the information at the time resolution we needed.”

    The team is continuing to look at composition data to find other instances of the periodic density structures to determine if the source for all slow solar wind is magnetic reconnection. Their case study clearly shows that this particular event was the result of magnetic reconnection, but they wish to find other examples to show this is the most common mechanism for powering the slow solar wind.

    As the team gathers more information about magnetic reconnection and its effects near the sun, it will add to a growing body of knowledge about the phenomenon in general — because magnetic reconnection events take place throughout the universe.

    “If we can understand this phenomenon here, where we can actually measure the magnetic field, we can get a better handle on how these fundamental physics processes take place in other places in the universe,” Viall said.

    *Science paper: Geophysical Research Letters
    Implications of L1 observations for slow solar windformation by solar reconnection

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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.

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  • richardmitnick 3:19 pm on May 12, 2016 Permalink | Reply
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    From GIZMODO: “A Major Mystery About Earth’s Magnetic Field Has Just Been Solved” 

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    GIZMODO

    5.12.16
    Maddie Stone

    NASA/MMS
    NASA/MMS

    1
    NASA MMS in flight. University of Maryland

    For the first time, physicists have observed a mysterious process called magnetic reconnection—wherein opposing magnetic field lines join up, releasing a tremendous burst of energy. The discovery, published* today in Science, may help us unlock the secrets of space weather and learn about some of the weirdest, most magnetic objects in the universe.

    The magnetosphere, an invisible magnetic field surrounding our planet, is a critical shield for life on Earth.

    Magnetosphere of Earth
    Magnetosphere of Earth, original bitmap from NASA. SVG rendering by Aaron Kaase

    It protects us from all sorts of high energy particles emitted by the sun on a daily basis. When a particularly large burst of solar energy hits the edge of the magnetosphere (called the magnetopause), it can trigger space weather. This includes geomagnetic storms that light up the northern and southern skies with auroras, occasionally knocking out our satellites and power grids.

    A better understanding of space weather is key to helping us prepare for the next massive geomagnetic storm—a once-in-a-century event that could quite literally cause a global power surge. Magnetic reconnection is at the heart of the mystery, underlying both the formation of solar eruptions and how they interact with our planet.


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

    “The process of space weather starts on the sun—reconnection there produces coronal mass ejections and solar flares, both of which lead to space weather at the Earth,” James Burch, a space weather scientist at the Southwest Research Institute told Gizmodo. “When the solar wind and its embedded magnetic field lines collide with Earth’s magnetosphere at a high angle, then you have a direct connection between the sun and the Earth.”

    Now, for the first time, Burch and his colleagues have observed that sun-Earth connection at the subatomic scale, using data collected by NASA’s Magnetospheric Multiscale (MMS) mission. This high-resolution physics laboratory consists of four identical spacecraft that fly in pyramid formation around Earth’s magnetopause, collecting precise information on tiny charged particles every 30 milliseconds.

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    Artist’s concept of the four MMS satellites flying in formation. Image: University of Maryland

    Almost as soon as the mission launched in March of 2015, researchers started observing magnetic reconnection at unprecedented resolution. The most detailed of those is the subject of the new paper. “We hit the jackpot,” Roy Torbert, MMS deputy principal investigator said in a statement. “The spacecraft passed directly through the electron dissipation region, and we were able to perform the first-ever physics experiment in this environment.”

    The features of reconnection recorded in the data include a drop in the magnetic field to near zero, and a power spike generated by accelerating electrons. “We realized that the process of reconnection is really driven by electrons,” Burch said. “Before, all measurements had been made at much larger scales. People could see dramatic effects, but these are the result of reconnection, not the cause.”

    Burch and his colleagues are continuing to study five other instances of magnetic reconnection recently observed by the MMS, and they’re hopeful the mission will yield more events for years to come. In addition to shedding light on space weather, magnetic reconnection can help us understand exotic astronomical objects like magnetars, as well as the strong magnetic environments created by fusion reactors.

    “The quality of the MMS data is absolutely inspiring,” said James Drake, a physicist at the University of Maryland and a co-author on the study. “It’s not clear that there will ever be another mission quite like this one.”

    *Science paper:
    Electron-scale measurements of magnetic reconnection in space

    See the full article here .

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  • richardmitnick 11:23 am on April 6, 2016 Permalink | Reply
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    From Physics- “Focus: Space Wave Gives Electrons a Shove” 

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    Access mp4 video here .
    NASA’s MMS Formation Will Give Unique Look at Magnetic Reconnection

    Physics

    April 5, 2016
    Michael Schirber

    A new satellite mission has observed electron acceleration by electric field waves moving along the magnetic boundary between the Earth and the solar wind.

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    This artist’s drawing shows the four MMS spacecraft flying through the magnetopause, where the magnetic field of the solar wind (yellow-orange) confronts the Earth’s magnetic field (blue). At this boundary, magnetic field reconnection converts the field energy into particle energy. MMS has observed electric-field waves that likely play a role in this conversion.

    NASA/MMS
    NASA/MMS

    Charged particles around the Earth, Sun, and other astrophysical bodies appear to be accelerated to high energies in regions where magnetic fields break up and reconnect, but the exact mechanism is unclear. A recently launched multi-satellite mission has now flown through such a region and directly observed electron acceleration by fast-moving electric-field waves, suggesting a possible role for these waves in the production of high-energy particles. The new data may be an important step in unraveling the mysteries behind solar flares and other energetic cosmic events.

    To learn about the strong interactions between particles and magnetic fields that occur near many planets and stars, researchers study the magnetopause, where the solar wind meets the Earth’s magnetic field. The solar wind is a collection of mostly protons and electrons streaming out from the Sun, carrying with it the interplanetary magnetic field (IMF), which spirals outward from the Sun. Within the magnetopause, the IMF and the geomagnetic field often point in nearly opposite directions in a region called the X-line. The field misalignment forces the field lines to break and reconnect. This reconnection, which also occurs around the Sun and in other plasma regions, converts magnetic energy into kinetic energy for charged particles. Studying reconnection is important for understanding the generation of high-energy particles around the Earth (which endanger satellites and high-altitude airplane passengers) and also for explaining high-energy events like solar flares.

    Acceleration of electrons in a reconnection region has been much harder to measure than the acceleration of ions. Forrest Mozer of the University of California, Berkeley, and his colleagues now report on a direct observation of electron acceleration occurring in the magnetopause, using the Magnetospheric Multiscale (MMS) mission. Launched in the spring of 2015, MMS consists of four satellites flying in a tetrahedral formation. Each probe records electric and magnetic fields as well as the numbers of electrons and ions in various energy ranges. Compared with other multi-satellite missions, the MMS probes have a smaller separation (as little as 10 km), which affords them much higher spatial resolution for measuring localized acceleration mechanisms in reconnection regions.

    On October 5, 2015, the MMS flotilla was passing the X-line in the dayside magnetopause. Two of the probes recorded a set of sharp spikes in the electric field pointing parallel to the local magnetic field. The spikes were part of a traveling wave called a time domain structure (TDS). TDSs have been detected many times by satellite missions in other regions [1]. They have not generally been considered as acceleration mechanisms because the electric field in a spike has both positive (push) and negative (pull) peaks, giving a net electric potential of only 10 volts or less. “They were thought to be the result of some other processes rather than significant mechanisms on their own,” Mozer says.

    However, a TDS can accelerate particles because it acts like a fast-moving barrier that “bumps into slow-moving electrons,” Mozer explains. By comparing the spike arrival times at the two MMS spacecraft, he and his colleagues were able to directly measure the velocity of a TDS, whereas previous TDS observations could only infer the velocity. They found that the wave was moving away from the X-line at 4000 km/s. The MMS instruments confirmed that the wave led to particle acceleration by observing a 50% jump in the number of modestly-high-energy electrons after the wave’s passing. The TDS boosted electrons to around 200 eV, 40 times their initial energy.

    This is not the first detection of TDS-induced acceleration [2], but it is the first direct observation of electron acceleration by TDS within a reconnection region. Mozer admits that the energy gain is not enough to explain 100-keV electrons that have been observed in other reconnection zones. But he believes that faster moving TDSs may be observed in the future. “I think we are seeing just the tip of the iceberg,” Mozer says.

    The MMS observations are new and significant because they are “able to characterize TDSs and directly investigate the associated particle acceleration simultaneously,” says Daniel Graham of the Swedish Institute of Space Physics. James Drake of the University of Maryland says the acceleration “was more than you might expect for such a small electric field.” Still, the total energy gain is not very large, so other mechanisms may play a dominant role in accelerating electrons. Even so, Drake believes these new observations are important for providing benchmarks for computer simulations of reconnection regions.

    This research is published in Physical Review Letters.

    The science team:
    F. S. Mozer, O. A. Agapitov, A. Artemyev, J. L. Burch, R. E. Ergun, B. L. Giles, D. Mourenas, R. B. Torbert, T. D. Phan, and I. Vasko
    Affiliations:
    No affiliations given

    See the full article here .

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    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

     
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