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Topic: Magnetic helicity


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  Dynamo Conference, NATO 2000, Cargese, participants and abstracts
The magnetic helicity is a fundamental quantity in magnetohydrodynamics because it is conserved in the limit of infinite electrical conductivity.
The reason for the anomalous scalings of the magnetic helicity correlation function is that the magnetic field in the equation for the two-point correlation function of the magnetic helicity plays a role of a pumping with anomalous scalings.
Magnetic fields exhibit dipolar, quadrupolar or hemispherical structures as long as convection in the polar regions of the spherical fluid shells is inhibited owing to a sufficiently high rate of rotation.
www.math.colostate.edu /~juliana/CARGESE2000/listpt.html   (9552 words)

  
 Recent Reprints
This paper reviews several aspects of magnetic helicity, including its history from Gauss to the present, its relation to field structure, its role in Taylor relaxation, and how it is defined for sub-volumes of space.
Magnetic helicity is conserved in ideal MHD and approximately conserved during reconnection.
In recent years, solar observers have discovered a striking pattern in the distribution of coronal magnetic structures: northern hemisphere structures tend to have negative magnetic helicity, while structures in the south tend to have positive magnetic helicity.
www.ucl.ac.uk /~ucahmab/HelicityReprints/HelicityReprints.htm   (1307 words)

  
 SOI Coronal Synoptic Fields Science Page
Comparison of the predicted coronal magnetic field configuration with observed images by other SOHO instruments may be used to test and determine the explosive release model(s) of the free magnetic energy that works in explosive events in the solar atmosphere.
In addition to the photospheric magnetic field as an input to the model, the plasma properties is also necessary to be specified at the photosphere, which is difficult to be realistic at the present.
With the assumption of radial photospheric magnetic field, the line-of-sight component in the magnetogram (Level 1) is divided by \cos(\theta)cos(\phi) to obtain the radial field.
soi.stanford.edu /science/CS.html   (2620 words)

  
 Statement of Research Interests   (Site not responding. Last check: 2007-11-05)
Helicity is a measure of the linkage between curves or lines in space.
However, magnetic helicity, which measures the linkage of magnetic lines of force, is conserved, and that seems to be intimately related to the origin of galactic magnetic fields.
The orientation of the large-scale magnetic fields in M51 from Neiminger (1992) using the polarization of synchrotron radiation.
cfa-www.harvard.edu /hco/astro/people/homepages/field/research.html   (1026 words)

  
 Corona and Heliosphere
The prominence observations of Lin, Penn and Kuhn (1998) showing a reversal of sign of the axial magnetic field of the prominence suggests that the magnetic configurations in prominences may be richer than previously thought.
Magnetic flux rope models provide a physical explanation for the cavity, forming a region of depleted density surrounding the prominence core (Low and Hundhausen, 1995).
The quiescent cavities observed in white light are thus consistent with a precursor magnetic flux rope in the corona.
www.hao.ucar.edu /Public/research/ch.html   (3818 words)

  
 HelicityReprints
This association is physically consistent with the conclusion, based independently on measurements of prominence magnetic fields, that magnetic flux ropes occur preferentially with negative and positive helicities in the northern and southern solar hemispheres, respectively.
In both cases, magnetic helicity is computed as the sum of mutual helicity over all possible pairs of magnetic flux tubes weighted by their magnetic fluxes.
For open configurations, the magnetic helicity is derived directly from the geometry of the interlaced flux tubes so it can be computed without reference to a ground state (such as a potential field).
www.ucl.ac.uk /~ucahmab/HelicityReprints/HelicitySummaries.htm   (1833 words)

  
 The Spheromak
The coupling of the helicity to the dynamo in the plasma is understood qualitatively, but it is dependent on current driven modes and magnetic turbulence.
Magnetic turbulence and its consequences for the dynamo and for energy confinement.
There is a particularly strong synergism with the RFP; the OH transformer generates helicity primarily on the magnetic axis, whereas the coaxial gun used in a spheromak generates it near the seperatrix.
www-ferp.ucsd.edu /PUBLIC/AC-PANEL/REC-DOCS/W-PAPERS/spher.html   (4562 words)

  
 Magnetic Helicity Abstract   (Site not responding. Last check: 2007-11-05)
Magnetic helicity refers to the twistedness, knottedness, and connectedness of a topological magnetic configuration.
In 1958 Woltjer introduced the idea of helicity conservation in force free fields, and in 1974 Taylor used that idea in explaining the stability of a plasma in a reverse field pinch by showing that it represented the state of lowest magnetic energy.
It is also thought that helicity is nearly conserved during magnetic reconnection, a process in which oppositely directed field lines merge together and annihilate each other.
wwwrel.ph.utexas.edu /Public/Students/rcrane/abstract/abstract.html   (235 words)

  
 Chapman Conference on Magnetic Helicity in Space and Laboratory Plasmas GENERAL INFORMATION
On the other hand, magnetic helicity has been intensively studied in the laboratory and in interplanetary plasma physics.
Non-solar participants will bring an understanding of helicity that is relatively well-developed within the framework of their own discipline, as well as an enthusiasm for applying it to a new discipline.
Helicity in the Solar Chromosphere, Corona, Wind, and IMF
www.agu.org /meetings/chapman/chapman_archive/cc98bcall.html   (507 words)

  
 [83.06] Calculation of Magnetic Helicity in a Force-Free Field   (Site not responding. Last check: 2007-11-05)
By studying the nature of magnetic helicity, we are studying the solar magnetic field driven by the dynamo.
Using this degree of twist and the known magnetic energy for a given active region, we were able to measure the magnetic helicity in this force-free region.
We found a 73% hemispheric preference in the sign of the helicity - in the northern hemisphere, 70% of the active regions exhibited negative helicity; in the southern hemisphere, 76.5% of the active regions exhibited positive helicity.
www.aas.org /publications/baas/v34n4/aas201/793.htm   (209 words)

  
 3 Results   (Site not responding. Last check: 2007-11-05)
Without magnetic fields of sufficient strength we find in all cases clear signs of the AKA effect, which manifests itself through enhanced power in the lowest Fourier modes of the kinetic energy spectrum; see Fig.
The opposite signs of kinetic and magnetic helicities are in agreement with what is found for helically forced turbulence; see Brandenburg (2001), where the forcing was however chosen to have positive helicity.
However, because of (approximate) helicity conservation the integrated helicity spectrum has to vanish, which is why at large scales the sign of magnetic helicity is reversed.
aanda.u-strasbg.fr:2002 /papers/aa/full/2001/45/aa1588/node3.html   (1303 words)

  
 Engaging with Questions of Higher Order: cognitive vigilance required for higher degrees of twistedness
It is, however, a typical property of astrophysical plasmas, that the dynamics of magnetic fields is alternating between an ideal motion, where all forms of knottedness and linkage of the field are conserved (topology conservation), and a kind of disruption of the magnetic structure, the so called magnetic reconnection.
Magnetic helicity is a function of the vector potential and the magnetic field.
The helicity of a smooth vector field defined on a domain in 3-space is the standard measure of the extent to which the field lines wrap and coil around one another; it plays important roles in fluid mechanics, magnetohydrodynamics and plasma physics.
www.laetusinpraesens.org /docs00s/coils.php   (12012 words)

  
 Physics of HIT II
It is a measure of how helical or spiraled the magnetic field lines are, in other words, a measure of flux linkage.
Helicity is no longer conserved; it is dissipated on a resistive time scale.
This energy is within the magnetic field lines and is determined by their shape.
www.aa.washington.edu /AERP/HIT/physics.html   (433 words)

  
 Hydromagnetic Dynamo Theory - Scholarpedia
The evolution equation for the magnetic helicity in the small scale field, , must then be amended by the same term, but with the opposite sign so that the sum of both terms adds up to the original helicity equation.
In the case of the sun the shedding of magnetic helicity may occur mostly via coronal mass ejections, but this is still an open research area.
The longitudinally averaged component of the radial magnetic field of the sun shows a markedly regular spatio-temporal pattern where the radial magnetic field alternates in time over the 11 year cycle and also changes sign across the equator.
www.scholarpedia.org /article/Hydromagnetic_Dynamo_Theory   (1932 words)

  
 Solar Active Regions and 3D Magnetic Structure - Scientific Rationale
The magnetic field of the sun is responsible for most of its visible dynamic features, including the most energetic events that can effect the near earth space environment, producing space weather.
The solar magnetic field is generated below the visible layer (the photosphere) and erupts into the solar atmosphere.
In the recent past there have been spectacular advances in various topics related to this field such as: magnetic helicity, temporal evolution of magnetic field creating large-scale structure; thermal and magnetic instabilities leading to fine-scale structure; wave dissipation and reconnection providing coronal heating; instability and nonequilibrium states leading to eruptions.
www.csun.edu /phys/IAUJD3/scientific-rationale.html   (835 words)

  
 Center for Magnetic Self Organization in Laboratory and Astrophysical Plasmas
This first goal is to establish a realistic formalism of gauge-invariant definitions of magnetic helicity in such cases and to examine its conservation properties in laboratory plasmas.
Although conservation of magnetic helicity has been one of fundamental assumptions in the theories of magnetic self-organization, its physical significance has never been clear.
An example case is that a finite helicity flux either in radial outward direction or through the equator plane may be required to explain the fast dynamo phenomena on the sun (the solar cycles).
www.cmso.info /html/helicity.htm   (314 words)

  
 The Field that Ate the Milky Way
Astronomers detect magnetic fields by their effects on cosmic rays and on the polarization of light from distant bodies.
The fields of about 1 microgauss (one millionth of the earth's magnetic field strength) seem to exist in all galaxies observed so far, but their origins remain one of the great mysteries of the Universe's evolution.
By adding helicity to the mix, the researchers were able to make realistic predictions for the evolution of magnetic fields from the big bang up to the era when galaxies began to form, 10 billion years ago.
focus.aps.org /story/v12/st24   (656 words)

  
 [No title]   (Site not responding. Last check: 2007-11-05)
Observations of a number of solar magnetic features and of interplanetary magnetic clouds indicate that the Sun preferentially hosts left-handed, negative-helicity structures in its northern hemisphere, and their opposite counterparts in the south, independent of sunspot cycle.
Furthermore, the total helicity generated in a nominal bipolar region is about 10% of its squared magnetic flux, or about 1x10^43 Mx^2 for a bipole containing 1x10^22 Mx of flux.
Thus, the Sun's magnetic helicity balance can be accounted for quantitatively, and its patterns of helicity can be understood qualititatively, based on the action of differential rotation on its emerged bipolar sources of flux.
www-istp.gsfc.nasa.gov /istp/solar_events/ws_html/CDevore.html   (219 words)

  
 Helical Gallery
Magnetic Helicity at Sun, in the Solar Wind and Magnetospheres.
Magnetic Helicity in Space and Laboratory Plasmas, M.
Magnetic Helicity at the Sun, in Solar Wind and Magnetospheres; Vistas from X-Ray Observatories,
www.nso.edu /staff/apevtsov/www/gallery.html   (205 words)

  
 Processes at Active Region Size Scales   (Site not responding. Last check: 2007-11-05)
In large measure the helicity should be conserved during the passage of the rope through the atmosphere, but the extent to which it is modified by flows locally near the surface must also be determined.
The traditional view is that the twist and the relative motions of magnetic elements at the surface results from the action of flows near the surface.
A contrasting view is that the motion of magnetic elements and the change in magnetic helicity with time results from the buoyant rise of pre-existing structures from below.
web.hao.ucar.edu /public/workshop/smi/proc_active_region.html   (692 words)

  
 Spheromaks
Spheromaks are plasmas with very large internal currents and internal magnetic fields that are aligned so as to be in a nearly force-free equilibrium, i.e., the currents are very nearly parallel to the magnetic fields.
Spheromaks are inherently three-dimensional and involve the concept of magnetic helicity which is a measure of the twistedness of a magnetic flux tube.
Making spheromaks is analogous to blowing bubbles: the component of the magnetic stress tensor parallel to the magnetic field acts like the surface tension in the soap film while the perpendicular component acts like the air pressure inflating the bubble.
ve4xm.caltech.edu /Bellan_plasma_page/spheroma.htm   (306 words)

  
 SV 42The Magnetic Helicity of an Interplanetary Hot Flux Rope
Observations of helical magnetic structures in the solar photosphere, corona and solar wind have attracted considerable attention in the last years, with the consequent interest in magnetic helicity (one of the few global quantities that is preserved) studies both in the solar and interplanetary contexts.
We find that the magnetic helicity values determined when using the Gold Hoyle and Lundquist models are the same; while for the constant current model we obtain a value which is lower by a factor 3.
From our point of view, our results mean that whatever be the approach used magnetic helicity is a well-determined quantity and, thus, it is worth using it to understand the link between solar and interplanetary phenomena.
www.arcetri.astro.it /~solwind/abstract/node288.html   (312 words)

  
 ScienceDaily: Evolution Of Galaxy-Spanning Magnetic Fields Explained
Once the large field comes to a certain strength, however, conservation of magnetic helicity will have made the backreaction strong enough to overcome the turbulence and stop further growth of the large field.
Magnetic Fields Crucial To Star Formation, Astronomer Says (September 10, 1999) -- Observations by a University of Illinois astronomer have shown that magnetic fields are a critical component controlling when and how stars form.
Magnetic "Bubbles" May Be Key Force In Galaxy Clusters (January 14, 2002) -- Astronomers have suspected that magnetic fields in space play a key role in the makeup of galaxy clusters the basic building blocks of the universe.
www.sciencedaily.com /releases/2003/01/030113072139.htm   (1983 words)

  
 Magnetic helicity - Wikipedia, the free encyclopedia
In plasma physics, magnetic helicity is the extent to which a magnetic field "wraps around itself".
If magnetic field lines follow the strands of a twisted rope, this configuration would have nonzero magnetic helicity; left handed ropes would have negative values and right handed ropes would have positive values.
It is conserved in electromagnetic fields, even when magnetic reconnection dissipates energy.
en.wikipedia.org /wiki/Magnetic_helicity   (124 words)

  
 Engineering and Design of the Steady Inductive Helicity Injected Torus (HIT-SI)
SIHI directly produces a rotating magnetic field structure, and the current profile is nearly time independent in the frame of the rotating field.
Helicity and power are ejected from the plasma when the voltage is negative, leading to plasma wall interaction.
The profile for helicity ejection is greatly different from that of injection.
www.aa.washington.edu /AERP/HIT/HIT-SI   (1318 words)

  
 [37.15] The Origin of Mass, Magnetic Flux, and Magnetic Helicity in a Solar Prominence   (Site not responding. Last check: 2007-11-05)
Therefore, the supply of mass, magnetic flux, and magnetic helicity is the necessary condition for the formation of solar prominences.
Both the magnetic flux associated with flux cancellation, and magnetic helicity injected by the shearing motion were more than enough for the formation of the prominence.
Our results support that chromospheric magnetic reconnection is in charge of supplying mass and magnetic flux, and photospheric shearing motion is in charge of supplying magnetic helicity.
www.aas.org /publications/baas/v34n2/aas200/75.htm   (252 words)

  
 AGU On Line Book Catalog: Magnetic Helicity in Space and Laboratory Plasmas
This volume presents the first comprehensive review of magnetic helicity in laboratory plasmas, the Sun, interplanetary space, planetary dynamos, and astrophysical objects.
The exceptional chapter bibliographies cover helicity studies in astrophysics, space plasma physics, laboratory plasma, geo-planetary and solar dynamos, solar physics, magnetospheric physics, and mathematics.
Individual chapters are written by leading experts in each sub-field, many of whom pioneered helicity studies in their specialties.
www.agu.org /cgi-bin/agubookstore?book=SPGM1110941   (286 words)

  
 Helicity - Wikipedia, the free encyclopedia
The term helicity has several meanings in physics, all referring (perhaps obliquely) to a phenomenon that resembles a helix.
In biochemistry, helicity may also refer to the amount of alpha helix represented in the secondary structure of a protein.
This disambiguation page lists articles associated with the same title.
en.wikipedia.org /wiki/Helicity   (108 words)

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