Solar Coronal Heating

[bystander]

Plasma jets are prime suspect in solar mystery
University Corporation and National Center for Atmospheric Research | 2010 Dec 30

Longstanding Mystery of Sun's Hot Outer Atmosphere Solved
National Science Foundation | 2011 Jan 06

Narrow jets of material, called spicules, streak upward from the Sun's surface at high speeds. [i](Credit: NASA/SDO/AIA)[/i]

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One of the most enduring mysteries in solar physics is why the Sun’s outer atmosphere, or corona, is millions of degrees hotter than its surface. Now scientists believe they have discovered a major source of hot gas that replenishes the corona: narrow jets of plasma, known as spicules, shooting up from just above the Sun’s surface. The finding addresses a fundamental question in astrophysics: how energy moves from the Sun’s interior to create its hot outer atmosphere.

“It’s always been quite a puzzle to figure out why the Sun’s atmosphere is hotter than its surface,” says Scott McIntosh, a scientist at the National Center for Atmospheric Research (NCAR), a coauthor of the study. “By identifying that these jets insert heated plasma into the Sun’s outer atmosphere, we gain a greater knowledge of the corona and possibly improve our understanding of the Sun’s subtle influence on Earth’s upper atmosphere.”

The new study, published this week in the journal Science, was conducted by scientists from Lockheed Martin’s Solar and Astrophysics Laboratory (LMSAL), NCAR, and the University of Oslo. It was supported by NASA and the National Science Foundation, NCAR’s sponsor.
...
The research team focused on spicules, which are fountains of plasma propelled upward from near the surface of the Sun into its outer atmosphere. For decades scientists thought that spicules might be sending heat into the corona. However, following observational research in the 1980s, it was found that spicule plasma did not reach coronal temperatures, and so this line of study largely fell out of vogue.

“Heating of spicules to millions of degrees has never been directly observed, so their role in coronal heating had been dismissed as unlikely,” says Bart De Pontieu, the lead author and a solar physicist at LMSAL.

In 2007, De Pontieu, McIntosh, and their colleagues identified a new class of spicules that moved much faster and were shorter lived than the traditional spicules. These “Type II” spicules shoot upward at high speeds, often in excess of 60 miles per second (100 kilometers per second), before disappearing. The rapid disappearance of these jets suggested that the plasma they carried might get very hot, but direct observational evidence of this process was missing.

The researchers used new observations from the Atmospheric Imaging Assembly on NASA's recently launched Solar Dynamics Observatory and NASA's Focal Plane Package for the Solar Optical Telescope (SOT) on the Japanese Hinode satellite to test their hypothesis.

“The high spatial and temporal resolution of the newer instruments was crucial in revealing this previously hidden coronal mass supply,” says McIntosh, a solar physicist at NCAR’s High Altitude Observatory. “Our observations reveal, for the first time, the one-to-one connection between plasma that is heated to millions of degrees kelvin and the spicules that insert this plasma into the corona.”

The findings provide an observational challenge to existing theories of coronal heating. During the past few decades, scientists have proposed a wide variety of theoretical models, but the lack of detailed observation has significantly hampered progress. “One of our biggest challenges is to understand what drives and heats the material in the spicules,” says De Pontieu.

A key step, according to De Pontieu, will be to better understand the interface region between the Sun’s visible surface, or photosphere, and its corona. Another NASA mission, the Interface Region Imaging Spectrograph (IRIS), is scheduled for launch in 2012. IRIS will provide high-fidelity data on the complex processes and enormous contrasts of density, temperature, and magnetic field between the photosphere and corona. Researchers hope this will reveal more about the spicule heating and launch mechanisms.

Really Hot Doin's Discovered on the Sun
Science NOW | Richard A Kerr | 2011 Jan 06

[b]In the eye of the beholder.[/b] Sharper views of the sun at a variety of wavelengths reveal small jets from the solar surface that help heat the overlying corona to 1 million˚C. [i](Credit: Bart De Pontieu)[/i]

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The mystery of the solar corona is obvious enough. The vanishingly thin atmosphere of the sun—the wispy stuff that can be glimpsed faintly during total solar eclipses—simmers at 1 million˚C, 200 times hotter than the "fire" beneath it. What gives? Researchers now believe they have caught the sun in the act of heating bits of itself to coronal temperatures and jetting those bits up into the corona.

Researchers have had plenty of ideas about why the corona is so hot but no convincing way to test them. Whatever the process, it was happening on too fine a spatial scale for instruments to discern clearly. But that changed with the data dumps following the launches of the Japanese-led Hinode mission in 2006 and NASA's Solar Dynamics Observatory (SDO) in February of last year. At visible wavelengths, Hinode's imager can resolve features on the solar surface as small as 150 kilometers. At the same time, an extreme ultraviolet imager on SDO can follow small features at eight different ultraviolet wavelengths that gauge temperatures from 20,000˚C to 1 million˚C. SDO images the whole face of the sun every 12 seconds, sending back 1.5 terabytes of data each day.

The combination of Hinode and SDO observations has now shown at least one way the corona gets heated. Solar physicist Bart De Pontieu of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California, and his colleagues report in the 7 January issue of Science that they can trace jets of plasma, or ionized gas, rising into the corona. The jets ascend at 180,000 to 360,000 kilometers per hour from 300-kilometer-wide bright spots on the surface called spicules. Within its less-than-2-minute lifetime, a jet's temperature soars, some parts reaching corona-like temperatures of about a million degrees. And, most telling, "we found them everywhere," says De Pontieu, and "they go up, but they don't come down."

A rough estimate suggests that the energy that these roaring jets deliver to the corona might well account for its heating, says De Pontieu. "We're not saying it's the dominant mode," he adds. "We are saying it's time to reconsider what kinds of processes are involved." Many theorists had placed their heating mechanisms up in the corona itself, but De Pontieu sees the focus moving down to near the sun's visible surface where these jets originate.

"They've discovered a new phenomenon," says solar astrophysicist Spiro Antiochos of NASA's Goddard Space Flight Center in Greenbelt, Maryland. The pervasiveness of the hot, corona-penetrating jets "doesn't prove their phenomenon is the only one heating the corona. It does show it's a ubiquitous phenomenon. That's one test." The next test should be more detailed modeling, Antiochos says, followed by even better observations using NASA's Interface Region Imaging Spectrograph, to be launched in December 2012. Modeling and new observations might even reveal what drives these jets.

Plasma jets key to enduring solar mystery
Nature News | Jon Cartwright | 2011 Jan 06

Why the Sun's corona is hotter than its surface.

It's been a mystery for more than half a century: why, in the short distance from the Sun's surface to its corona, or outer atmosphere, does the temperature leap from a few thousand to a few million degrees? The answer, researchers say, might lie in hot jets of plasma erupting from the Sun's surface¹.

"It's truly a breakthrough in the longstanding puzzle of how the corona gets so hot," says Rob Rutten, a solar physics expert at Utrecht University in the Netherlands who was not involved with the work. "The jets behave like bullets shot upwards, causing hot coronal temperature fronts in front of them."

Over the years, theorists have offered various explanations for the hot corona. One idea is that the Sun's violent inner motion shakes its magnetic field lines, sending waves through the atmosphere and into the corona that deposit their energy as heat². Another posits that the magnetic field lines become so twisted that they snap, accelerating and heating the coronal gas³. However, there has been little observational evidence to support either of these theories.

Plasma jets have also been considered as a possible heating mechanism. These jets are known to travel several hundred kilometres from the 'chromosphere' layer just above the Sun's surface to the corona. Yet in the past, rough observations of plasma jets suggested them to be too cool for coronal heating, with temperatures similar to that of the chromosphere itself — just a few thousand degrees.

In a paper published today in Science¹, however, Bart De Pontieu of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California, and his colleagues show that most of the plasma making up the jets is heated to hundreds of thousands of degrees on its way to the corona, with a small fraction reaching millions of degrees. On the basis of the jets' frequency and intensity, the researchers estimate that they deliver energy "of the order that is required" for the corona to sustain its high temperature. "We are not saying that this is the only mechanism to heat the corona," says De Pontieu. "Clearly, however, these events deserve more attention."

1. The Origins of Hot Plasma in the Solar Corona - B De Pontieu et al
   • Science 331(6013) 55 (2011 Jan 07) DOI: 10.1126/science.1197738
2. Heating of the solar corona by the resonant absorption of Alfven waves - JM Davila
   • Astrophysical Journal 317 514 (1987 Jun 01) DOI: 10.1086/165295
3. Tangential discontinuities and the optical analogy for stationary fields IV. High speed fluid sheets - EN Parker
   • Geophysical & Astrophysical Fluid Dynamics 50(4) 229 (Feb 1990) DOI: 10.1080/03091929008204106

Hotspots in Fountains on the Sun's Surface Help Explain Coronal Heating Mystery
NASA Goddard Space Flight Center | 2011 Jan 06

[bystander]

Feeling the Heat
Scientific American (preview)
John Matson | 2011 Feb 19

Courtesy of SOHO/EIT consortium/ESA/NASA

It is a question that has hounded solar phys­icists since the 1940s: Why is the outer layer of the sun’s atmosphere, the region farthest from the heat-producing core, hotter than both the lower atmosphere and the sun’s surface?

Experts have put forth various explanations, from sound waves or magnetic waves dissipating in the upper solar atmosphere, or corona, to short bursts of energy known as nanoflares that erupt as tangled magnetic field lines in the corona reconnect. Now observations from a new generation of sun-observing spacecraft are implicating a different mechanism, one that could provide the corona with a significant portion of its heat by continually delivering hot ionized gas, or plasma, to the upper atmosphere.

Spicules, which are short-lived fount­ains of plasma shooting up from the sun’s chromosphere, or lower atmos­phere, seem to play a role in heating the corona to searing temperatures at millions of degrees kelvins, investigators have found. Spicules, whose origins are somewhat mysterious, last just 100 seconds, rising from the chromosphere at speeds of 50 to 100 kilometers per second. As lead study author Bart De Pontieu of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, Calif., points out, that is fast enough to travel from San Francisco to London in minutes. De Pontieu and his colleagues reported their findings in the journal Science.

The group based its study on observations from NASA’s new Solar Dynamics Observatory, launched in 2010, and the Japanese Hinode spacecraft, which began service in 2006. Both solar observatories can take detailed images of the sun every several seconds, the kind of quick-time observation needed to identify transient or rapidly changing phenomena.

As spicules measuring in the tens of thousands of degrees kelvins rise from the chromosphere, the researchers noticed, patches of the corona above flare up at one million to two million degrees.

The researchers do not yet know what launches the chromospheric plasma at such high speeds nor what heats it to the extreme temperatures it reaches in the corona. But the link between spicules and coronal heating holds promise for closing the books on a 70-year-old mystery, says Kenneth Phillips of University College London.

Although spicules seem to be important phenomena in certain regions of the sun, time will tell whether they deliver enough hot plasma on a global scale to explain the corona’s tremendous heat, says James Klimchuk of the NASA Goddard Space Flight Center in Greenbelt, Md. Klimchuk calls the new observations “very exciting” but notes that his own preliminary calculations indicate that spicules provide only a small share of the hot plasma in the corona, leaving plenty of room for other, more conventional modes of coronal heating.

For his part, De Pontieu sounds a similar note of caution that the long-standing problem of the corona’s temperature has yet to be conclusively resolved. “I think it’s important to point out that we have not solved coronal heating, but we have provided a piece of the puzzle,” he says. “We’ll see down the line whether this proves to be a dominant process or simply a contributor.”

[bystander]

GSFC: SDO Spots Extra Energy in the Sun's Corona
NASA GSFC SDO | Karen C Fox | 2011 July 27

These jets, known as spicules, were captured in an SDO image on April 25, 2010.
Combined with the energy from ripples in the magnetic field, they may contain
enough energy to power the solar wind that streams from the sun toward Earth
at 1.5 million miles per hour. Credit: NASA/GSFC/SDO/AIA

Like giant strands of seaweed some 32,000 miles high, material shooting up from the sun sways back and forth with the atmosphere. In the ocean, it's moving water that pulls the seaweed along for a ride; in the sun's corona, magnetic field ripples called Alfvén waves cause the swaying.

For years these waves were too difficult to detect directly, but NASA's Solar Dynamics Observatory (SDO) is now able to track the movements of this solar "seaweed" and measure how much energy is carried by the Alfvén waves. The research shows that the waves carry more energy than previously thought, and possibly enough to drive two solar phenomena whose causes remain points of debate: the intense heating of the corona to some 20 times hotter than the sun's surface and solar winds that blast up to 1.5 million miles per hour.

"SDO has amazing resolution so you can actually see individual waves," says Scott McIntosh at the National Center for Atmospheric Research in Boulder, Colo. "Now we can see that instead of these waves having about 1000th the energy needed as we previously thought, it has the equivalent of about 1100W light bulb for every 11 square feet of the sun's surface, which is enough to heat the sun's atmosphere and drive the solar wind."

McIntosh published his research in a Nature article appearing on July 28. Alfvén waves, he says, are actually fairly simple. They are waves that travel up and down a magnetic field line much the way a wave travels up and down a plucked string. The material surrounding the sun -- electrified gas called plasma – moves in concert with magnetic fields. SDO can see this material in motion and so can track the Alfvén waves.

Alfvén waves are part of a much more complex system of magnetic fields and plasma surrounding the sun. Understanding that system could help answer general questions such as what initiates geomagnetic storms near Earth and more focused questions such as what causes coronal heating and speeds of the solar wind – a field of inquiry in which there are few agreed-upon answers.

"We know there are mechanisms that supply a huge reservoir of energy at the sun's surface," says space scientist Vladimir Airapetian at NASA's Goddard Space Flight Center in Greenbelt, Md. "This energy is pumped into magnetic field energy, carried up into the sun's atmosphere and then released as heat." But determining the details of this mechanism has long been debated. Airapetian points out that a study like this confirms Alfvén waves may be part of that process, but that even with SDO we do not yet have the imaging resolution to prove it definitively.

When the waves were first observed in 2007 (more than six decades after being hypothesized by Hannes Alfvén in 1942), it was clear that they could in theory carry energy up from the sun's surface to its atmosphere. However, the 2007 observations showed them to be too weak to contain the great amounts of energy needed to heat the corona so dramatically.

This study says that those original numbers may have been underestimated. McIntosh, in collaboration with a team from Lockheed Martin, Norway's University of Oslo, and Belgium's Catholic University of Leuven, analyzed the great oscillations in movies from SDO's Atmospheric Imagine Assembly (AIA) instrument captured on April 25, 2010.

"Our code name for this research was 'The Wiggles,'" says McIntosh. "Because the movies really look like the sun was made of Jell-O wiggling back and forth everywhere. Clearly, these wiggles carry energy."

The team tracked the motions of this wiggly material spewing up -- in great jets known as spicules – as well as how much the spicules sway back and forth. They compared these observations to models of how such material would behave if undergoing motion from the Alfvén waves and found them to be a good match.

Going forward, they could analyze the shape, speed, and energy of the waves. The sinusoidal curves deviated outward at speeds of over 30 miles per second and repeated themselves every 150 to 550 seconds. These speeds mean the waves would be energetic enough to accelerate the fast solar wind and heat the quiet corona. The shortness of the repetition – known as the period of the wave – is also important. The shorter the period, the easier it is for the wave to release its energy into the coronal atmosphere, a crucial step in the process.

Earlier work with this same data also showed that the spicules achieved coronal temperatures of at least 1.8 million degrees Fahrenheit. Together the heat and Alfvén waves do seem to have enough energy to keep the roiling corona so hot. The energy is not quite enough to account for the largest bursts of radiation in the corona, however.

"Knowing there may be enough energy in the waves is only one half of the problem," says Goddard's Airapetian. "The next question is to find out what fraction of that energy is converted into heat. It could be all of it, or it could be 20 percent of it – so we need to know the details of that conversion."

In practice, that means studying more about the waves to understand just how they impart their energy into the surrounding atmosphere.

"We still don't perfectly understand the process going on, but we're getting better and better observations," says McIntosh. "The next step is for people to improve the theories and models to really capture the essence of the physics that's happening."

Wave power can drive Sun’s intense heat
University Corporation for Atmospheric Research (UCAR) | 2011 July 27

Click to play embedded YouTube video.

Click to play embedded YouTube video.

Click to play embedded YouTube video.

A new study sheds light on why the Sun’s outer atmosphere, or corona, is more than 20 times hotter than its surface. The research, led by the National Center for Atmospheric Research (NCAR), may bring scientists a step closer to understanding the solar cycle and the Sun’s impacts on Earth.

The study uses satellite observations to reveal that magnetic oscillations carrying energy from the Sun’s surface into its corona are far more vigorous than previously thought. These waves are energetic enough to heat the corona and drive the solar wind, a stream of charged particles ejected from the Sun that affects the entire solar system.

“We now understand how hot mass can shoot upward from the solar interior, providing enough energy to maintain the corona at a million degrees and fire off particles into the high-speed solar wind,” says Scott McIntosh, the study’s lead author and a scientist in NCAR’s High Altitude Observatory. “This new research will help us solve essential mysteries about how energy gets out of the Sun and into the solar system.”

The study, published this week in the journal Nature, was conducted by a team of scientists from NCAR, Lockheed Martin Solar and Astrophysics Lab, Norway’s University of Oslo, and Belgium’s Catholic University of Leuven. It was funded by NASA. NCAR is sponsored by the National Science Foundation.

Jets and waves

The flow of mass and energy from the corona influences how much ultraviolet radiation reaches Earth. It also drives upper-atmospheric disturbances known as geomagnetic storms, which can disrupt technologies ranging from telecommunications to electrical transmission.

The new study focuses on the role of oscillations in the corona, known as Alfvén waves, in moving energy through the corona.

Alfvén waves were directly observed for the first time in 2007. Scientists recognized them as a mechanism for transporting energy upward along the Sun’s magnetic field into the corona. But the 2007 observations showed amplitudes on the order of about 1,600 feet (0.5 kilometers) per second, far too small to heat the corona to its high levels or to drive the solar wind.

The new satellite observations used in the current study reveal Alfvén waves that are over a hundred times stronger than previously measured, with amplitudes on the order of 12 miles (20 km) per second—enough to heat the Sun’s outer atmosphere to millions of degrees and drive the solar wind. The waves are easily seen in high-resolution images of the outer atmosphere as they cause high-speed jets of hot material, called spicules, to sway.

“The new satellite observations are giving us a close look for the first time at how energy and mass move through the Sun’s outer atmosphere,” McIntosh says.

The research builds on ongoing efforts to study the connection between spicules and Alfvén waves. Scientists have known about spicules for decades but were unable to determine if their mass got hot enough to provide heat for the corona until earlier this year, when McIntosh and colleagues published research in the journal Science that used satellite observations to reveal that a new class of the phenomenon, dubbed “Type II” spicules, moves much faster and reaches coronal temperatures. (Related news release, January 2011.)

The new study reveals the role of Alfvén waves. These oscillations play a critical role in transporting heat from the Sun by riding on the spicules and carrying energy into the corona.

Photographing our nearest star

The critical satellite observations described in the study come from the Atmospheric Imaging Assembly, a package of instruments aboard NASA’s Solar Dynamics Observatory, which was launched in 2010. The instruments boast high spatial and temporal resolution, enough to detect structures and motions across regions of the Sun as small as 310 miles (500 km) and generate images every 12 seconds at different wavelengths.

“It’s like getting a microscope to study the Sun’s corona, giving us the spatial and temperature coverage to focus in on the way mass and energy circulate.” McIntosh says.

Now that the real power of the waves has been revealed in the corona, the next step in unraveling the mystery of its extreme heat is to study how the waves lose their energy, which is transferred to plasma. To do that, scientists will need to develop computer models that are fine enough in detail to capture how the jets and waves work together to power the atmosphere. By studying the Sun’s underlying physics with these tools, scientists could better understand the Sun’s 11-year sunspot cycle and its impacts on Earth.

Alfvénic waves with sufficient energy to power the quiet solar corona and fast solar wind - SW McIntosh et al

• Nature 475 477 (28 July 2011) DOI: 10.1038/nature10235

Sun's Magnetic Waves Show Surprising Heating Power
Space.com | Charles Q Choi | 2011 July 27

Monster Waves Behind Sun's Coronal Heating Mystery?
Discovery News | Ian O'Neill | 2011 July 27

Magnetic waves bake the sun's corona
Science News | Camille M. Carlisle | 2011 July 27

[bystander]

The Puzzle of the Sun's Coronal Heating
Smithsonian Astrophysical Observatory
Weekly Science Update | 2011 Jul 29

[i]Solar Prominence, 2011 July 12 (Credit: NASA/SDO/AIA)[/i]

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The corona of the sun is the hot (over a million kelvin), gaseous outer region of its atmosphere. The corona is threaded by intense magnetic fields that extend upwards from the surface in loops that are twisted and sheared by the convective stirrings of the underlying dense atmosphere. Understanding the corona and its physical processes is essential to the development of a long-range space weather prediction capability.

The mechanisms that heat the corona are poorly understood. Each magnetic loop is actually a tube containing hot gas, and one theory posits that the enclosed gas is heated by short pulses of energy - so-called nanoflares - generated by the way the field twists and braids around the loop and its connections to the turbulent gas motions in the stellar photosphere. It has never been possible to test this theory, however, in part because (it is thought) each of the nanoflares heats only a small, thin strand of the whole magnetic field loop - and a strand is too small to see clearly.

The Solar Dynamics Observatory (SDO) was launched in February of 2010 with a high spatial resolution ultraviolet camera, the Atmospheric Imaging Assembly (AIA); SAO was a major partner in building AIA. Paola Testa, Ed DeLuca and Leon Golub, together with three colleagues, used AIA to show for the first time that hot coronal loops do indeed contain very fine substructures, consistent with the nanoflaring hypothesis, that can heat the gas in the loops. This initial result still awaits more testing; time variability, for example, will be discussed in future papers. But this result helps to resolve an important outstanding puzzle about the heating of the solar corona.

Solar Dynamics Observatory discovers thin high temperature strands in coronal active regions - F Reale et al

• Astrophysical Journal Letters 736(1) L16 (2011 July 20) DOI: 10.1088/2041-8205/736/1/L16
  arXiv.org > astro-ph > arXiv:1106.1591 > 08 Jun 2011

<< Previous Science Update

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SPICULATION:

Wave power can drive Sun’s intense heat
University Corporation for Atmospheric Research (UCAR) | 2011 July 27

<<The flow of mass and energy from the corona influences how much ultraviolet radiation reaches Earth. It also drives upper-atmospheric disturbances known as geomagnetic storms, which can disrupt technologies ranging from telecommunications to electrical transmission. The new study focuses on the role of oscillations in the corona, known as Alfvén waves, in moving energy through the corona. Alfvén waves were directly observed for the first time in 2007. Scientists recognized them as a mechanism for transporting energy upward along the Sun’s magnetic field into the corona. But the 2007 observations showed amplitudes on the order of about 0.5 km/s, far too small to heat the corona to its high levels or to drive the solar wind. The new satellite observations used in the current study reveal Alfvén waves that are over a hundred times stronger than previously measured, with amplitudes on the order of 20 km/s —enough to heat the Sun’s outer atmosphere to millions of degrees and drive the solar wind. The waves are easily seen in high-resolution images of the outer atmosphere as they cause high-speed jets of hot material, called spicules, to sway. “The new satellite observations are giving us a close look for the first time at how energy and mass move through the Sun’s outer atmosphere,” McIntosh says. The research builds on ongoing efforts to study the connection between spicules and Alfvén waves. Scientists have known about spicules for decades but were unable to determine if their mass got hot enough to provide heat for the corona until earlier this year, when McIntosh and colleagues published research in the journal Science that used satellite observations to reveal that a new class of the phenomenon, dubbed “Type II” spicules, moves much faster and reaches coronal temperatures. The new study reveals the role of Alfvén waves. These oscillations play a critical role in transporting heat from the Sun by riding on the spicules and carrying energy into the corona.>>

http://asterisk.apod.com/viewtopic.php? ... 73#p135822
http://asterisk.apod.com/viewtopic.php? ... 73#p135893
http://asterisk.apod.com/viewtopic.php? ... 72#p154072

[size=150][color=#0000FF]Touched by Alfvén's Noodly Appendage[/color][/size]

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Alfvén waves may solve solar-corona mystery
Apr 29, 2009

<<The outer parts of the solar corona are millions of degrees hotter than the surface of the Sun – a fact that has puzzled astrophysicists for quite some time (CERN Courier June 2008 p8). Now David Jess of Queen's University Belfast and colleagues have made major progress in unravelling this mystery. Using the 1 m Swedish Solar Telescope on La Palma in the Canary Islands they observed a tiny conglomeration of highly magnetized bright points on the Sun's surface. Their observations show evidence for Alfvén waves, oscillations of magnetized plasma, moving up from the lower solar atmosphere, which may hold the key to the coronal heating.

Hannes Alfvén predicted such waves in his seminal paper of 1942. Their incompressible nature and ability to penetrate the solar atmosphere have made them likely candidates for heating the solar corona. However, Alfvén waves on the Sun had evaded unambiguous observation until now, owing to difficulties in getting clear-enough images of small parts of the Sun using Earth-based telescopes. The long-wavelength plasma oscillations that Jess and colleagues have observed appear to follow magnetic field lines towards the corona. They seem to be capable of carrying enough heat to the corona to explain its high temperature.>>

<<Hannes Olof Gösta Alfvén (born 30 May 1908 in Norrköping, Sweden; died 2 April 1995 in Djursholm, Sweden) was a Swedish electrical engineer, plasma physicist and winner of the 1970 Nobel Prize in Physics for his work on magnetohydrodynamics (MHD). He described the class of MHD waves now known as Alfvén waves. Alfvén made many contributions to plasma physics, including theories describing the behavior of aurorae, the Van Allen radiation belts, the effect of magnetic storms on the Earth's magnetic field, the terrestrial magnetosphere, and the dynamics of plasmas in the Milky Way galaxy. His theoretical work on field-aligned electric currents in the aurora (based on earlier work by Kristian Birkeland) was confirmed by satellite observations, in 1974, resulting in the discovery of Birkeland currents.

In 1937, Alfvén argued that if plasma pervaded the universe, it could then carry electric currents capable of generating a galactic magnetic field. After winning the Nobel Prize for his works in magnetohydrodynamics, he emphasized that:

• In order to understand the phenomena in a certain plasma region, it is necessary to map not only the magnetic but also the electric field and the electric currents. Space is filled with a network of currents which transfer energy and momentum over large or very large distances. The currents often pinch to filamentary or surface currents. The latter are likely to give space, as also interstellar and intergalactic space, a cellular structure.>>

<<An Alfvén wave in a plasma is a low-frequency (compared to the ion cyclotron frequency) travelling oscillation of the ions and the magnetic field. The ion mass density provides the inertia and the magnetic field line tension provides the restoring force. The wave propagates in the direction of the magnetic field, although waves exist at oblique incidence and smoothly change into the magnetosonic wave when the propagation is perpendicular to the magnetic field. The motion of the ions and the perturbation of the magnetic field are in the same direction and transverse to the direction of propagation. The wave is dispersionless (i.e., phase velocity ω/k = group velocity dω/dk = constant).

How this phenomenon became understood:

• 1942: Alfvén suggests the existence of electromagnetic-hydromagnetic waves in a paper published in Nature.
  
  1949: Laboratory experiments by S. Lundquist produce such waves in magnetized mercury, with a velocity that approximated Alfvén's formula.
  
  1949: Enrico Fermi uses Alfvén waves in his theory of cosmic rays. According to Alex Dessler in a 1970 Science journal article, Fermi had heard a lecture at the University of Chicago, Fermi nodded his head exclaiming "of course" and the next day, the physics world said "of course".
  
  1950: Alfvén publishes the first edition of his book, Cosmical Electrodynamics, detailing hydromagnetic waves, and discussing their application to both laboratory and space plasmas.
  
  1952: Additional confirmation appears in experiments by Winston Bostick and Morton Levine with ionized helium
  
  1954: Bo Lehnert produces Alfvén waves in liquid sodium
  
  1958: Eugene Parker suggests hydromagnetic waves in the interstellar medium
  
  1958: Berthold, Harris, and Hope detect Alfvén waves in the ionosphere after the Argus nuclear test, generated by the explosion, and traveling at speeds predicted by Alfvén formula.
  
  1958: Eugene Parker suggests hydromagnetic waves in the Solar corona extending into the Solar wind.
  
  1959: D. F. Jephcott produces Alfvén waves in a gas discharge
  
  1959: C. H. Kelley and J. Yenser produce Alfvén waves in the ambient atmosphere.
  
  1960: Coleman, et al., report the measurement of Alfvén waves by the magnetometer aboard the Pioneer and Explorer satellites
  
  1960: Sugiura suggests evidence of hydromagnetic waves in the Earth's magnetic field
  
  1966: R.O.Motz generates and observes Alfven waves in mercury
  
  1970 Hannes Alfvén wins the 1970 Nobel Prize in physics for "fundamental work and discoveries in magneto-hydrodynamics with fruitful applications in different parts of plasma physics"
  
  1973: Eugene Parker suggests hydromagnetic waves in the intergalactic medium
  
  1974: Hollweg suggests the existence of hydromagnetic waves in interplanetary space
  
  1974: Ip and Mendis suggests the existence of hydromagnetic waves in the coma of Comet Kohoutek.
  
  1999: Aschwanden, et al. and Nakariakov, et al. report the detection of damped transverse oscillations of solar coronal loops observed with the EUV imager on board the Transition Region And Coronal Explorer (TRACE), interpreted as standing kink (or "Alfvénic") oscillations of the loops.
  
  2007: Tomczyk, et al., report the detection of Alfvénic waves in images of the solar corona with the Coronal Multi-Channel Polarimeter (CoMP) instrument at the National Solar Observatory, New Mexico. These waves were interpreted as propagating kink waves by Van Doorsselaere et al. (2008)
  
  2007: Alfvén wave discoveries appear in articles by Jonathan Cirtain and colleagues, Takenori J. Okamoto and colleagues, and Bart De Pontieu and colleagues. De Pontieu’s team also shows that the energy associated with the waves is sufficient to heat the corona and accelerate the solar wind. These results appear in a special collection of 10 articles, by scientists in Japan, Europe and the United States, in the 7 December issue of the journal Science. It was demonstrated that those waves should be interpreted in terms of kink waves of coronal plasma structures by Van Doorsselaere, et al. (2008); Ofman and Wang (2008); and Vasheghani Farahani, et al. (2009).>>