Magnetic Helicity Estimations in Models and Observations of the Solar Magnetic Field. III. Twist Number Method

Guo, Yang; Pariat, É.; Valori, G.; Anfinogentov, Sergey; Chen, F.; Georgoulis, Manolis K.; Liu, Y.; Moraitis, K.; Thalmann, Julia K.; Yang, Shangbin

doi:10.3847/1538-4357/aa6aa8

Cited by 53 publications

(68 citation statements)

References 57 publications

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“…In the movie associated with Figure 2, it is possible to see that the flows trace the curvilinear paths of the fanning field lines from ≈ 01:30 UT until eruption. As we have assumed a fluxrope configuration of the magnetic environment surrounding and suspending the filamentary material, by studying the magnetic topology of simulations by authors such as Roussev et al (2003), Mei et al (2017), and Guo et al (2017), it is difficult to reconcile how the highly twisted field lines of these studied flux ropes could produce the observed linear-like motions of plasma; plasma-β (plasma pressure/magnetic pressure) is low in the corona and therefore charged material is line-tied. Interestingly, extrapolations of less strongly twisted flux ropes, such as those by Su et al (2011) andJiang et al (2014), are easier to compare to the observations as they contain very weakly twisted field lines that pass through the axes of the extrapolated flux ropes.…”

Section: Discussionmentioning

confidence: 99%

Understanding the Role of Mass-Unloading in a Filament Eruption

et al. 2018

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We describe a partial filament eruption on 11 December 2011 that demonstrates that the inclusion of mass is an important next step for understanding solar eruptions. Observations from the Solar Terrestrial Relations Observatory-Behind (STEREO-B) and the Solar Dynamics Observatory (SDO) spacecraft were used to remove line-of-sight projection effects in filament motion and correlate the effect of plasma dynamics with the evolution of the filament height. Flux cancellation and nearby flux emergence are shown to have played a role in increasing the height of the filament prior to eruption. The two viewpoints allow the quantitative estimation of a large mass-unloading, the subsequent radial expansion, and the eruption of the filament to be investigated. A 1.8 to 4.1 lower-limit ratio between gravitational and magnetic-tension forces was found. We therefore conclude that following the loss-of-equilibrium of the flux-rope, the radial expansion of the flux-rope was restrained by the filamentary material until 70% of the mass had evacuated the structure through massunloading.

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Section: Discussionmentioning

confidence: 99%

Understanding the Role of Mass-Unloading in a Filament Eruption

et al. 2018

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“…Therefore, the sum of the twist and writhe is closer to being independent from the choice of the plane used to determine the axis (as expected; Török et al 2010). Guo et al (2017) concluded that a good proxy of the magnetic helicity in the current-carrying field inside a finite volume is…”

Section: Twist and Writhementioning

confidence: 99%

“…To quantify the twist in our very asymmetric case, the twist of individual field lines in the flux rope was calculated around an axis, and the average was taken (Guo et al 2010(Guo et al , 2013. Following Guo et al (2017), the axial field line is defined as the field line with the smallest ratio of tangential-to-normal magnetic field components with respect to a plane roughly perpendicular to the body of the flux rope.…”

Section: Twist and Writhementioning

confidence: 99%

An Observationally Constrained Model of a Flux Rope that Formed in the Solar Corona

James

Valori

Green

et al. 2018

ApJ

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Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the coronae of stars. Understanding the plasma processes involved in CME initiation has applications for space weather forecasting and laboratory plasma experiments. James et al. used extreme-ultraviolet (EUV) observations to conclude that a magnetic flux rope formed in the solar corona above NOAA Active Region 11504 before it erupted on 2012 June 14 (SOL2012-06-14). In this work, we use data from the Solar Dynamics Observatory (SDO) to model the coronal magnetic field of the active region one hour prior to eruption using a nonlinear force-free field extrapolation, and find a flux rope reaching a maximum height of 150 Mm above the photosphere. Estimations of the average twist of the strongly asymmetric extrapolated flux rope are between 1.35 and 1.88 turns, depending on the choice of axis, although the erupting structure was not observed to kink. The decay index near the apex of the axis of the extrapolated flux rope is comparable to typical critical values required for the onset of the torus instability, so we suggest that the torus instability drove the eruption.

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“…Apart from field line helicity there are other methods that employ the magnetic connectivity to compute quantities related to magnetic helicity. An example is the twist number method (Guo et al 2017), where the twist component of helicity is computed in configurations consisting of twisted magnetic flux ropes. In an alternative method that approximates helicity through its twist, Malanushenko et al (2011) compute RMH by matching the shapes of coronal loops with field lines of a linear force-free field.…”

Section: Introductionmentioning

confidence: 99%

Relative magnetic field line helicity

Moraitis

Pariat

Valori

et al. 2019

A&A

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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 that 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 derive the corresponding field line helicity, and we note the assumptions it is based on. Results. We check 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 compare 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.

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Magnetic Helicity Estimations in Models and Observations of the Solar Magnetic Field. III. Twist Number Method

Cited by 53 publications

References 57 publications

Understanding the Role of Mass-Unloading in a Filament Eruption

Understanding the Role of Mass-Unloading in a Filament Eruption

An Observationally Constrained Model of a Flux Rope that Formed in the Solar Corona

Relative magnetic field line helicity

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