Magnetohydrodynamics of Plasma Relaxation

Ortolani, S.; Schnack, D. D.

doi:10.1142/1564

Cited by 313 publications

(421 citation statements)

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“…A better method is therefore to turn the problem into a hyperbolic one and to solve an additional evolution equation for the velocity correction where the driving force is the Lorentz force. This approach is sometimes called the "force-free model" (FFM), even though the field in this model is never exactly force-free anywhere (Mikić et al 1988;Ortolani & Schnack 1993). In the context of force-free magnetic field extrapolations this method is also known as the stress-and-relax method (Valori et al 2005).…”

Section: Introductionmentioning

confidence: 99%

Surface appearance of dynamo-generated large-scale fields

Warnecke

Brandenburg

2010

A&A

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Aims. Twisted magnetic fields are frequently seen to emerge above the visible surface of the Sun. This emergence is usually associated with the rise of buoyant magnetic flux structures. Here we ask how magnetic fields from a turbulent large-scale dynamo appear above the surface if there is no magnetic buoyancy. Methods. The computational domain is split into two parts. In the lower part, which we refer to as the turbulence zone, the flow is driven by an assumed helical forcing function leading to dynamo action. Above this region, which we refer to as the exterior, a nearly force-free magnetic field is computed at each time step using the stress-and-relax method. Results. Twisted arcade-like field structures are found to emerge in the exterior above the turbulence zone. Strong current sheets tend to form above the neutral line, where the vertical field component vanishes. Time series of the magnetic field structure show recurrent plasmoid ejections. The degree to which the exterior field is force free is estimated as the ratio of the dot product of current density and magnetic field strength to their respective rms values. This ratio reaches values of up to 95% in the exterior. A weak outward flow is driven by the residual Lorentz force.

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Section: Introductionmentioning

confidence: 99%

Surface appearance of dynamo-generated large-scale fields

Warnecke

Brandenburg

2010

A&A

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“…The energy release due to many localised reconnections can be predicted using relaxation theory (Ortolani & Schnack 1993;Taylor 1974Taylor , 1986Taylor , 2000 which was first developed to explain the generation of reversed field in laboratory Reverse Field Pinch devices (Taylor 1974(Taylor , 1986) and later applied to the solar corona (Browning 1988;Dixon et al 1988;Heyvaerts & Priest 1984;Lothian & Browning2000;Vekstein et al 1993;Wolfson et al 1994). …”

Section: Introductionmentioning

confidence: 99%

Solar coronal heating by relaxation events

Browning¹,

Linden²

2003

A&A

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Abstract.A coronal heating model is proposed which predicts heating by a series of discrete events of various energies, analogous to the observed range of events from large scale flares through various transient brightening phenomena down to the often discussed "nanoflares". We suggest that an energy release event occurs when a field becomes linearly unstable to ideal MHD modes, with dissipation during the nonlinear phase of such an instability due to reconnection in fine-scale structures such as current sheets. The energy release during this complex dynamic period can be evaluated by assuming the field relaxes to a minimum energy state subject to the constraint of helicity conservation. A model problem is studied: a cylindrical coronal loop, with a current profile generated by slow twisting of the photospheric footpoints parameterised by two values of α (the ratio of current density to field strength). Different initial α profiles, corresponding to different footpoint twisting profiles, lead to energy release events of a wide range of magnitudes, but our model predicts an observationally realistic minimum size for these events.

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“…In plasma fusion devices where such MRD occur, helicity fluxes are the key sustainer of the EMF (e.g. Strauss 1985Strauss , 1986Bhattacharjee & Hameri 1986;Ortolani & Schnack 1993;Ji et al 1995;Bellan 2000;Ji & Prager 2002) In astrophysics such MR-LSDs likely occur in coronae, where the field is injected from below on small scales relative to the corona and further relaxation occurs. Astrophysical rotators with coronae likely have flow driven dynamos in the interior, coupled to MRDs in coronae.…”

Section: Types Of Dynamos and Approaches To Study Themmentioning

confidence: 99%

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

Blackman¹

2016

Space Sciences Series of ISSI

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Magnetic fields of laboratory, planetary, stellar, and galactic plasmas commonly exhibit significant order on large temporal or spatial scales compared to the otherwise random motions within the hosting system. Such ordered fields can be measured in the case of planets, stars, and galaxies, or inferred indirectly by the action of their dynamical influence, such as jets. Whether large scale fields are amplified in situ or a remnant from previous stages of an object's history is often debated for objects without a definitive magnetic activity cycle. Magnetic helicity, a measure of twist and linkage of magnetic field lines, is a unifying tool for understanding large scale field evolution for both mechanisms of origin. Its importance stems from its two basic properties: (1) magnetic helicity is typically better conserved than magnetic energy; and (2) the magnetic energy associated with a fixed amount of magnetic helicity is minimized when the system relaxes this helical structure to the largest scale available. Here I discuss how magnetic helicity has come to help us understand the saturation of and sustenance of large scale dynamos, the need for either local or global helicity fluxes to avoid dynamo quenching, and the associated observational consequences. I also discuss how magnetic helicity acts as a hindrance to turbulent diffusion of large scale fields, and thus a helper for fossil remnant large scale field origin models in some contexts. I briefly discuss the connection between large scale fields and accretion disk theory as well. The goal here is to provide a conceptual primer to help the reader efficiently penetrate the literature.

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Magnetohydrodynamics of Plasma Relaxation

Cited by 313 publications

References 0 publications

Surface appearance of dynamo-generated large-scale fields

Surface appearance of dynamo-generated large-scale fields

Solar coronal heating by relaxation events

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

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