scholarly journals Relative field-line helicity in bounded domains

2018 ◽  
Vol 84 (6) ◽  
Author(s):  
Anthony R. Yeates ◽  
Marcus H. Page
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Active Regions ◽  
Magnetic Helicity ◽  
Numerical Code ◽  
Computational Domain ◽  
Reference Field ◽  
Astrophysical Plasmas ◽  
Vector Potentials ◽  
Relative Field

Models for astrophysical plasmas often have magnetic field lines that leave the boundary rather than closing within the computational domain. Thus, the relative magnetic helicity is frequently used in place of the usual magnetic helicity, so as to restore gauge invariance. We show how to decompose the relative helicity into a relative field-line helicity that is an ideal-magnetohydrodynamic invariant for each individual magnetic field line, and vanishes along any field line where the original field matches the reference field. Physically, this relative field-line helicity is a magnetic flux, whose specific definition depends on the gauge of the reference vector potential on the boundary. We propose a particular ‘minimal’ gauge that depends only on the reference field and minimises this boundary contribution, so as to reveal topological information about the original magnetic field. We illustrate the effect of different gauge choices using the Low–Lou and Titov–Démoulin models of solar active regions. Our numerical code to compute appropriate vector potentials and relative field-line helicity in Cartesian domains is open source and freely available.

scholarly journals The development of magnetic field line wander by plasma turbulence

2017 ◽  
Vol 83 (4) ◽  
Author(s):  
Gregory G. Howes ◽  
Sofiane Bourouaine
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Magnetic Field Line ◽  
Large Scale ◽  
Magnetic Energy ◽  
Plasma Turbulence ◽  
Alfven Wave ◽  
Astrophysical Plasmas ◽  
Field Lines ◽  
The Magnetic Field

Plasma turbulence occurs ubiquitously in space and astrophysical plasmas, mediating the nonlinear transfer of energy from large-scale electromagnetic fields and plasma flows to small scales at which the energy may be ultimately converted to plasma heat. But plasma turbulence also generically leads to a tangling of the magnetic field that threads through the plasma. The resulting wander of the magnetic field lines may significantly impact a number of important physical processes, including the propagation of cosmic rays and energetic particles, confinement in magnetic fusion devices and the fundamental processes of turbulence, magnetic reconnection and particle acceleration. The various potential impacts of magnetic field line wander are reviewed in detail, and a number of important theoretical considerations are identified that may influence the development and saturation of magnetic field line wander in astrophysical plasma turbulence. The results of nonlinear gyrokinetic simulations of kinetic Alfvén wave turbulence of sub-ion length scales are evaluated to understand the development and saturation of the turbulent magnetic energy spectrum and of the magnetic field line wander. It is found that turbulent space and astrophysical plasmas are generally expected to contain a stochastic magnetic field due to the tangling of the field by strong plasma turbulence. Future work will explore how the saturated magnetic field line wander varies as a function of the amplitude of the plasma turbulence and the ratio of the thermal to magnetic pressure, known as the plasma beta.

scholarly journals Observations of magnetic and kinetic helicity proxies

2012 ◽  
Vol 8 (S294) ◽  
pp. 13-24
Author(s):  
Hongqi Zhang
Keyword(s):  
Magnetic Field ◽  
Velocity Field ◽  
Magnetic Fields ◽  
Solar Atmosphere ◽  
Active Regions ◽  
Solar Dynamo ◽  
Magnetic Helicity ◽  
Vector Magnetograms ◽  
Topological Configuration ◽  
The Relationship

AbstractThe helicity is important to present the basic topological configuration of magnetic field in solar atmosphere. The distribution of magnetic helicity in solar atmosphere is presented by means of the observational (vector) magnetograms. As the kinetic helicity in the solar subatmosphere can be inferred from the velocity field based on the technique of the helioseismology and used to compare with the magnetic helicity in the solar atmosphere, the observational helicities provide the important chance for the confirmation on the generation of magnetic fields in the subatmosphere and solar dynamo models also. In this paper, we present the observational magnetic and kinetic helicity in solar active regions and corresponding questions, except the relationship with solar eruptive phenomena.

scholarly journals Twist and writhe of δ-island active regions

2010 ◽  
Vol 6 (S273) ◽  
pp. 153-156
Author(s):  
M. C. López Fuentes ◽  
C. H. Mandrini ◽  
P. Démoulin
Keyword(s):  
Magnetic Field ◽  
Flux Tube ◽  
Active Regions ◽  
Magnetic Helicity ◽  
Coronal Structure ◽  
Flux Tubes ◽  
Single Mechanism ◽  
Magnetic Flux Tubes ◽  
Kink Instability ◽  
The Magnetic Field

AbstractWe study the magnetic helicity properties of a set of peculiar active regions (ARs) including δ-islands and other high-tilt bipolar configurations. These ARs are usually identified as the most active in terms of flare and CME production. Due to their observed structure, they have been associated with the emergence of magnetic flux tubes that develop a kink instability. Our main goal is to determine the chirality of the twist and writhe components of the AR magnetic helicity in order to set constrains on the possible mechanisms producing the flux tube deformations. We determine the magnetic twist comparing observations of the AR coronal structure with force-free models of the magnetic field. We infer the flux-tube writhe from the rotation of the main magnetic bipole during the observed evolution. From the relation between the obtained twist and writhe signs we conclude that the development of the kink instability cannot be the single mechanism producing deformed flux-tubes.

scholarly journals Magnetic helicity ejections and coronal activity

2012 ◽  
Vol 8 (S294) ◽  
pp. 519-530 ◽  
Author(s):  
A. Nindos
Keyword(s):  
Magnetic Field ◽  
Physical Quantity ◽  
Coronal Mass Ejections ◽  
Active Regions ◽  
Magnetic Helicity ◽  
The Sun ◽  
Coronal Activity ◽  
Solar Eruptions ◽  
Ideal Tool ◽  
The Magnetic Field

AbstractMagnetic helicity quantifies the degree of linkage and/or twistedness in the magnetic field. It is probably the only physical quantity which is approximately conserved even in resistive MHD. This makes it an ideal tool for the exploration of the physics of solar eruptions. In this article, I discuss the sources of magnetic helicity injected into active regions and I point out that coronal mass ejections (CMEs) are probably necessary to remove at least part of the excess helicity produced in the Sun. I also discuss the importance of magnetic helicity in the overall coronal evolution that may lead to eruptions.

scholarly journals Helicity proxies from linear polarisation of solar active regions

2020 ◽  
Vol 641 ◽  
pp. A46 ◽  
Author(s):  
A. Prabhu ◽  
A. Brandenburg ◽  
M. J. Käpylä ◽  
A. Lagg
Keyword(s):  
Magnetic Field ◽  
Active Regions ◽  
Field Vector ◽  
Magnetic Helicity ◽  
Solar Dynamics Observatory ◽  
Solar Dynamics ◽  
Good Proxy ◽  
The Magnetic Field ◽  
Linear Polarisation ◽  
Polarisation Measurements

Context. The α effect is believed to play a key role in the generation of the solar magnetic field. A fundamental test for its significance in the solar dynamo is to look for magnetic helicity of opposite signs both between the two hemispheres as well as between small and large scales. However, measuring magnetic helicity is compromised by the inability to fully infer the magnetic field vector from observations of solar spectra, caused by what is known as the π ambiguity of spectropolarimetric observations. Aims. We decompose linear polarisation into parity-even and parity-odd E and B polarisations, which are not affected by the π ambiguity. Furthermore, we study whether the correlations of spatial Fourier spectra of B and parity-even quantities such as E or temperature T are a robust proxy for magnetic helicity of solar magnetic fields. Methods. We analysed polarisation measurements of active regions observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics observatory. Theory predicts the magnetic helicity of active regions to have, statistically, opposite signs in the two hemispheres. We then computed the parity-odd EB and TB correlations and tested for a systematic preference of their sign based on the hemisphere of the active regions. Results. We find that: (i) EB and TB correlations are a reliable proxy for magnetic helicity, when computed from linear polarisation measurements away from spectral line cores; and (ii) E polarisation reverses its sign close to the line core. Our analysis reveals that Faraday rotation does not have a significant influence on the computed parity-odd correlations. Conclusions. The EB decomposition of linear polarisation appears to be a good proxy for magnetic helicity independent of the π ambiguity. This allows us to routinely infer magnetic helicity directly from polarisation measurements.

scholarly journals Relative magnetic field line helicity

2019 ◽  
Vol 624 ◽  
pp. A51 ◽  
Author(s):  
K. Moraitis ◽  
E. Pariat ◽  
G. Valori ◽  
K. Dalmasse
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Magnetic Field Line ◽  
Solar Physics ◽  
Magnetic Helicity ◽  
Geometrical Complexity ◽  
Wide Range ◽  
Field Lines ◽  
Definition Of ◽  
The Magnetic Field

Context. Magnetic helicity is an important quantity in studies of magnetized plasmas as it provides a measure of the geometrical complexity of the magnetic field in a given volume. A more detailed description of the spatial distribution of magnetic helicity is given by the field line helicity, which expresses the amount of helicity associated to individual field lines rather than in the full analysed volume. Aims. Magnetic helicity is not a gauge-invariant quantity in general, unless it is computed with respect to a reference field, yielding the so-called relative magnetic helicity. The field line helicity corresponding to the relative magnetic helicity has only been examined under specific conditions so far. This work aims to define the field line helicity corresponding to relative magnetic helicity in the most general way. In addition to its general form, we provide the expression for the relative magnetic field line helicity in a few commonly used gauges, and reproduce known results as a limit of our general formulation. Methods. By starting from the definition of relative magnetic helicity, we derived the corresponding field line helicity, and we noted the assumptions on which it is based. Results. We checked that the developed quantity reproduces relative magnetic helicity by using three different numerical simulations. For these cases we also show the morphology of field line helicity in the volume, and on the photospheric plane. As an application to solar situations, we compared the morphology of field line helicity on the photosphere with that of the connectivity-based helicity flux density in two reconstructions of an active region’s magnetic field. We discuss how the derived relative magnetic field line helicity has a wide range of applications, notably in solar physics and magnetic reconnection studies.

scholarly journals Study of helicity properties of peculiar active regions

2009 ◽  
Vol 5 (S264) ◽  
pp. 102-104 ◽  
Author(s):  
M. C. López Fuentes ◽  
C. H. Mandrini ◽  
P. Démoulin
Keyword(s):  
Magnetic Field ◽  
Magnetic Flux ◽  
Active Regions ◽  
Solar Interior ◽  
Magnetic Helicity ◽  
Flux Tubes ◽  
Origin And Evolution ◽  
Magnetic Structures ◽  
Magnetic Flux Tubes ◽  
The Magnetic Field

AbstractPeculiar solar active regions (ARs), such as δ-islands and other high tilt bipoles, are commonly associated with the emergence of severely deformed magnetic flux tubes. Therefore, the study of these ARs provides valuable information on the origin and evolution of magnetic structures in the solar interior. Here, we infer the magnetic helicity properties of the flux tubes associated to a set of peculiar ARs by studying the evolution of photospheric magnetograms (SOHO/MDI) and coronal observations (SOHO/EIT and TRACE) in combination with force-free models of the magnetic field. We discuss how our results relate to different models of the evolution of emerging magnetic flux tubes.

Evolution of Large-Scale Coronal Structures

1994 ◽  
Vol 144 ◽  
pp. 29-33
Author(s):  
P. Ambrož
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Large Scale ◽  
Coronal Magnetic Field ◽  
Source Surface ◽  
Solar Cycles ◽  
Characteristic Relationship ◽  
The Magnetic Field ◽  
Coronal Structures

AbstractThe large-scale coronal structures observed during the sporadically visible solar eclipses were compared with the numerically extrapolated field-line structures of coronal magnetic field. A characteristic relationship between the observed structures of coronal plasma and the magnetic field line configurations was determined. The long-term evolution of large scale coronal structures inferred from photospheric magnetic observations in the course of 11- and 22-year solar cycles is described.Some known parameters, such as the source surface radius, or coronal rotation rate are discussed and actually interpreted. A relation between the large-scale photospheric magnetic field evolution and the coronal structure rearrangement is demonstrated.

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