Global Dynamics of Subsurface Solar Active Regions

Jouve, L.; Brun, A. S.; Aulanier, G.

doi:10.1088/0004-637x/762/1/4

Cited by 45 publications

(50 citation statements)

References 73 publications

(106 reference statements)

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“…This means that a given sunspot group is never energized as a whole. These three observational properties are actually consistent with the solar convection-driven breaking of large sub-photospheric flux tubes into a series of smaller deformed structures, as found in numerical simulations (Fan et al 2003;Jouve et al 2013). They show that these deformed structures should eventually emerge through the photosphere as grouped but distinct magnetic bipoles.…”

Section: Taking Into Account the Fragmentation Of Fluxsupporting

confidence: 83%

The standard flare model in three dimensions

Aulanier

Démoulin

Schrijver

et al. 2012

A&A

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Context. Solar flares strongly affect the Sun's atmosphere as well as the Earth's environment. Quantifying the maximum possible energy of solar flares of the present-day Sun, if any, is thus a key question in heliophysics. Aims. The largest solar flares observed over the past few decades have reached energies of a few times 10 32 erg, possibly up to 10 33 erg. Flares in active Sun-like stars reach up to about 10 36 erg. In the absence of direct observations of solar flares within this range, complementary methods of investigation are needed to assess the probability of solar flares beyond those in the observational record. Methods. Using historical reports for sunspot and solar active region properties in the photosphere, we scaled to observed solar values a realistic dimensionless 3D MHD simulation for eruptive flares, which originate from a highly sheared bipole. This enabled us to calculate the magnetic fluxes and flare energies in the model in a wide paramater space. Results. Firstly, commonly observed solar conditions lead to modeled magnetic fluxes and flare energies that are comparable to those estimated from observations. Secondly, we evaluate from observations that 30% of the area of sunspot groups are typically involved in flares. This is related to the strong fragmentation of these groups, which naturally results from sub-photospheric convection. When the model is scaled to 30% of the area of the largest sunspot group ever reported, with its peak magnetic field being set to the strongest value ever measured in a sunspot, it produces a flare with a maximum energy of ∼6 × 10 33 erg. Conclusions. The results of the model suggest that the Sun is able to produce flares up to about six times as energetic in total solar irradiance fluence as the strongest directly observed flare of Nov. 4, 2003. Sunspot groups larger than historically reported would yield superflares for spot pairs that would exceed tens of degrees in extent. We thus conjecture that superflare-productive Sun-like stars should have a much stronger dynamo than in the Sun.

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Section: Taking Into Account the Fragmentation Of Fluxsupporting

confidence: 83%

The standard flare model in three dimensions

Aulanier

Démoulin

Schrijver

et al. 2012

A&A

Self Cite

202

144

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“…They were originally reported by López Fuentes et al (2000) and observed later in other works (see, e.g., Li et al, 2007;Chandra et al, 2009;Luoni et al, 2011). Their presence in emerging flux-ropes has been found by numerical simulations (Archontis and Hood, 2010;MacTaggart, 2011;Jouve, Brun, and Aulanier, 2013). Luoni et al (2011) compared the helicity-sign inferred from the orientation of magnetic tongues, observed in line-of-sight magnetograms, with other known helicity sign proxies for a sample of 40 ARs.…”

Section: Introductionmentioning

confidence: 65%

Evidence of Twisted Flux-Tube Emergence in Active Regions

et al. 2014

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Elongated magnetic polarities are observed during the emergence phase of bipolar active regions (ARs). These extended features, called magnetic "tongues", are interpreted as a consequence of the azimuthal component of the magnetic flux in the toroidal flux-tubes that form ARs. We develop a new systematic and user-independent method to identify AR tongues. Our method is based on the determination and analysis of the evolution of the AR main polarity inversion line (PIL). The effect of the tongues is quantified by measuring the acute angle [τ ] between the orientation of the PIL and the direction orthogonal to the AR main bipolar axis. We apply a simple model to simulate the emergence of a bipolar AR. This model lets us interpret the effect of magnetic tongues on parameters that characterize ARs (e.g. the PIL inclination, and the tilt angles and their evolution). In this, idealized kinematic emergence model, τ is a monotonically increasing function of the twist and has the same sign as the magnetic helicity. We systematically apply our procedure to a set of bipolar ARs (41 ARs) that were observed emerging in line-of-sight magnetograms over eight years. For the majority of the cases studied, the presence of tongues has a small influence on the AR tilt angle since tongues have much lower magnetic flux than the more concentrated main polarities. From the observed evolution of τ , corrected by the temporal evolution of the tilt angle and its final value when the AR is fully emerged, we estimate the average number of turns present in the subphotospheric emerging flux-rope. These values for the 41 observed ARs, except one, are below unity. This indicates that sub-photospheric flux-ropes typically have a low amount of twist, i.e. highly twisted flux-tubes are rare. Our results demonstrate that the evolution of the PIL is a robust indicator of the presence of tongues and constrains the amount of twist present in emerging flux-tubes.

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“…The origin of the twisted magnetic flux-ropes in the corona is most probably related to a combination of flux-emergence, magnetic shearing by surface motions, and magnetic reconnection in the corona, but the exact details are unknown at present (Jouve & Brun 2009;Fan et al 2009;Jouve et al 2013;Pinto & Brun 2013). We only considered here the dynamics of already existing coronal flux-ropes that have reached typical coronal background temperatures.…”

Section: Scope and Caveats Of The Modelmentioning

confidence: 99%

Soft X-ray emission in kink-unstable coronal loops

Pinto

Vilmer

Brun

2015

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Context. Solar flares are associated with intense soft X-ray emission generated by the hot flaring plasma in coronal magnetic loops. Kink-unstable twisted flux-ropes provide a source of magnetic energy that can be released impulsively and may account for the heating of the plasma in flares. Aims. We investigate the temporal, spectral, and spatial evolution of the properties of the thermal continuum X-ray emission produced in such kink-unstable magnetic flux-ropes and discuss the results of the simulations with respect to solar flare observations. Methods. We computed the temporal evolution of the thermal X-ray emission in kink-unstable coronal loops based on a series of magnetohydrodynamical numerical simulations. The numerical setup consisted of a highly twisted loop embedded in a region of uniform and untwisted background coronal magnetic field. We let the kink instability develop, computed the evolution of the plasma properties in the loop (density, temperature) without accounting for mass exchange with the chromosphere. We then deduced the X-ray emission properties of the plasma during the whole flaring episode. Results. During the initial (linear) phase of the instability, plasma heating is mostly adiabatic (as a result of compression). Ohmic diffusion takes over as the instability saturates, leading to strong and impulsive heating (up to more than 20 MK), to a quick enhancement of X-ray emission, and to the hardening of the thermal X-ray spectrum. The temperature distribution of the plasma becomes broad, with the emission measure depending strongly on temperature. Significant emission measures arise for plasma at temperatures higher than 9 MK. The magnetic flux-rope then relaxes progressively towards a lower energy state as it reconnects with the background flux. The loop plasma suffers smaller sporadic heating events, but cools down globally by thermal conduction. The total thermal X-ray emission slowly fades away during this phase, and the high-temperature component of the emission measure distribution converges to the power-law distribution EM ∝ T −4.2 . The twist deduced directly from the X-ray emission patterns is considerably lower than the highest magnetic twist in the simulated flux-ropes.

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Global Dynamics of Subsurface Solar Active Regions

Cited by 45 publications

References 73 publications

The standard flare model in three dimensions

The standard flare model in three dimensions

Evidence of Twisted Flux-Tube Emergence in Active Regions

Soft X-ray emission in kink-unstable coronal loops

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