On the influence of magnetic helicity on X-rays emission of solar and stellar coronae

Warnecke, Jörn; Peter, Hardi

doi:10.48550/arxiv.1910.06896

Cited by 4 publications

(4 citation statements)

References 47 publications

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“…The twisted magnetic field generated from the MF simulations may also serve as the initial conditions for the full MHD runs including plasma, where one intends to study the thermodynamic evolution of flaring plasma, as well as other aspects. Compared to Warnecke & Peter (2019b), where the helicity is injected directly at the boundary, this approach is much closer to the observed loop structure and hence is more self-consistent with photospheric input data.…”

Section: Discussionsupporting

confidence: 57%

Data-driven Simulations of Magnetic Field Evolution in Active Region 11429: Magneto-frictional Method Using PENCIL CODE

Vemareddy,

Warnecke,

Bourdin

2024

Res. Astron. Astrophys.

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Coronal magnetic fields evolve quasi statically over long time scales and dynamically over short time scales. As of now there exists no regular measurements of coronal magnetic fields, and therefore generating the coronal magnetic field evolution using the observations of the magnetic field at the photosphere is of fundamental requirement to understand the origin of the transient phenomena from the solar active regions. Using the magnetofriction (MF) approach, we aim to simulate the coronal field evolution in the solar active region 11429. The MF method is implemented in open source \PC along with a driver module to drive the initial field with different boundary conditions prescribed from observed vector magnetic fields at the photosphere. In order to work with vector potential and the observations, we prescribe three types of bottom boundary drivers with varying free-magnetic energy. The MF simulation reproduces the magnetic structure which better matches to the sigmoidal morphology exhibited by AIA images at the pre-eruptive time. We found that the already sheared field further driven by the sheared magnetic field will maintain and further build the highly sheared coronal magnetic configuration such as seen in AR 11429. Data-driven MF simulation is a viable tool to generate the coronal magnetic field evolution capturing the formation of the twisted flux rope and its eruption.

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Section: Discussionsupporting

confidence: 57%

Data-driven Simulations of Magnetic Field Evolution in Active Region 11429: Magneto-frictional Method Using PENCIL CODE

Vemareddy,

Warnecke,

Bourdin

2024

Res. Astron. Astrophys.

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“…Magnetic helicity in stellar research is not as well studied. Most recently, Warnecke & Peter (2019) have modelled the impact of helicity on stellar X-ray emission, suggesting that the rise in X-ray emission with increasing rotation rate may be related to the underlying variation in the helicity.…”

Section: Introductionmentioning

confidence: 99%

Measuring stellar magnetic helicity density

Lund

Jardine

Lehmann

et al. 2020

Monthly Notices of the Royal Astronomical Society

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Helicity is a fundamental property of a magnetic field but to date it has only been possible to observe its evolution in one star -the Sun. In this paper we provide a simple technique for mapping the large-scale helicity density across the surface of any star using only observable quantities: the poloidal and toroidal magnetic field components (which can be determined from Zeeman-Doppler imaging) and the stellar radius. We use a sample of 51 stars across a mass range of 0.1-1.34 M to show how the helicity density relates to stellar mass, Rossby number, magnetic energy and age. We find that the large-scale helicity density increases with decreasing Rossby number R o , peaking at R o 0.1, with a saturation or decrease below that. For both fully-and partially-convective stars we find that the mean absolute helicity density scales with the mean squared toroidal magnetic flux density according to the power law: | h | ∝ B tor 2 0.86 ± 0.04 . The scatter in this relation is consistent with the variation across a solar cycle, which we compute using simulations and observations across solar cycles 23 and 24 respectively. We find a significant decrease in helicity density with age.

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“…To name a few examples, it is suggested as a possible proxy to quantify the eruptivity of solar active regions (ARs) (Pariat et al 2017;Thalmann et al 2019), or as a plausible predictor of the solar cycle (Hawkes & Berger 2018). The influence of magnetic helicity on coronal emission, to explain the observed enhancement in X-ray luminosity with increasing stellar rotation, has also been explored (Warnecke & Peter 2019). Arguably, magnetic helicity plays the most crucial part in dynamo theory, which is often called upon to explain the generation and maintenance of the so-lar magnetic field (Moffatt 1978;Brandenburg & Subramanian 2005;Brandenburg 2018;Rincon 2019).…”

Section: Introductionmentioning

confidence: 99%

Inferring magnetic helicity spectrum in spherical domains: the method and example applications

Prabhu,

Singh,

Käpylä

et al. 2021

Preprint

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Context. Obtaining observational constraints on the role of turbulent effects for the solar dynamo is a difficult, yet crucial, task. Without such knowledge, the full picture of the operation mechanism of the solar dynamo cannot be formed. Aims. The magnetic helicity spectrum provides important information about the α effect. Here we demonstrate a formalism in spherical geometry to infer magnetic helicity spectra directly from observations of the magnetic field, taking into account the sign change of magnetic helicity across the Sun's equator. Methods. Using an angular correlation function of the magnetic field, we develop a method to infer spectra for magnetic energy and helicity. The retrieval of the latter relies on a fundamental definition of helicity in terms of linkage of magnetic flux. We apply the two-scale approach, previously used in Cartesian geometry, to spherical geometry for systems where a sign reversal of helicity is expected across the equator at both small and large scales. Results. We test the method by applying it to an analytical model of a fully helical field, and to magneto-hydrodynamic simulations of a turbulent dynamo. The helicity spectra computed from the vector potential available in the models are in excellent agreement to the spectra computed solely from the magnetic field using our method. In a next test, we use our method to obtain the helicity spectrum from a synoptic magnetic field map corresponding to a Carrington rotation. We observe clear signs of a bihelical spectrum of magnetic helicity, which is in complete accordance to the previously reported spectra in literature from the same map. Conclusions. Our formalism makes it possible to infer magnetic helicity in spherical geometry, without the necessity of computing the magnetic vector potential. This has the advantage of being gauge invariant. It has many applications in solar and stellar observations, but can also be used to analyze global magnetoconvection models of stars and compare them with observations.

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On the influence of magnetic helicity on X-rays emission of solar and stellar coronae

Cited by 4 publications

References 47 publications

Data-driven Simulations of Magnetic Field Evolution in Active Region 11429: Magneto-frictional Method Using PENCIL CODE

Data-driven Simulations of Magnetic Field Evolution in Active Region 11429: Magneto-frictional Method Using PENCIL CODE

Measuring stellar magnetic helicity density

Inferring magnetic helicity spectrum in spherical domains: the method and example applications

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