Observationally driven 3D magnetohydrodynamics model of the solar corona above an active region

Bingert

et al. 2014

A&A

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Aims. We present the first model that couples the formation of the corona of a solar active region to a model of the emergence of a sunspot pair. This allows us to study when, where, and why active region loops form, and how they evolve. Methods. We use a 3D radiation magnetohydrodynamics (MHD) simulation of the emergence of an active region through the upper convection zone and the photosphere as a lower boundary for a 3D MHD coronal model. The coronal model accounts for the braiding of the magnetic fieldlines, which induces currents in the corona to heat up the plasma. We synthesize the coronal emission for a direct comparison to observations. Starting with a basically field-free atmosphere we follow the filling of the corona with magnetic field and plasma. Results. Numerous individually identifiable hot coronal loops form, and reach temperatures well above 1 MK with densities comparable to observations. The footpoints of these loops are found where small patches of magnetic flux concentrations move into the sunspots. The loop formation is triggered by an increase in upward-directed Poynting flux at their footpoints in the photosphere. In the synthesized extreme ultraviolet (EUV) emission these loops develop within a few minutes. The first EUV loop appears as a thin tube, then rises and expands significantly in the horizontal direction. Later, the spatially inhomogeneous heat input leads to a fragmented system of multiple loops or strands in a growing envelope.

Section: Introductionsupporting

confidence: 80%

“…2.3). Our previous models (e.g., Bingert & Peter 2011Bourdin et al 2013) used an observed magnetic field and an observed or generated velocity field to prescribe the bottom boundary conditions, which was then driving the model. Here we impose the properties from the flux-emergence simulation in a similar fashion.…”

Section: Coronal Modelmentioning

confidence: 99%

A model for the formation of the active region corona driven by magnetic flux emergence

Chen

Bingert

et al. 2014

A&A

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“…On the other hand, the field-line braiding mechanism proposed by Parker (1972) induces currents in the corona that are dissipated by Ohmic (direct current, DC) heating. This is described well in magneto-hydrodynamics (MHD), and observationally driven computational models show an energy input together with a dissipation mechanism that creates loop structures within the corona that compare well to observations (Bourdin et al 2013(Bourdin et al , 2015.…”

Section: Introductionsupporting

confidence: 61%

“…We selected field lines that (a) are closed and do not connect to the upper box boundary; (b) have a length of 18 Mm to 150 Mm; and (c) reach a minimum temperature of at least 75 000 K before reaching a height of 18 Mm. We also made sure that the field lines crossing the EUV-intensity maximum of the most prominent loops (SL 1-3 and CL 1+2; see Bourdin et al 2013) were included in this ensemble of about 67 000 field lines, as well as approximately 200 field lines neighboring these loops.…”

Section: Coronal Model and Field-line Ensemblementioning

confidence: 99%

Scaling laws of coronal loops compared to a 3D MHD model of an active region

Bourdin

Bingert

2016

A&A

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Context. The structure and heating of coronal loops have been investigated for decades. Established scaling laws relate fundamental quantities like the loop apex temperature, pressure, length, and coronal heating. Aims. We test these scaling laws against a large-scale 3D magneto-hydrodynamics (MHD) model of the solar corona, which became feasible with current high-performance computing. Methods. We drove an active region simulation with photospheric observations and find strong similarities to the observed coronal loops in X-rays and extreme-ultraviolet (EUV) wavelength. A 3D reconstruction of stereoscopic observations shows that our model loops have a realistic spatial structure. We compared scaling laws to our model data extracted along an ensemble of field lines. Finally, we fit a new scaling law that represents hot loops and also cooler structures, which was not possible before based only on observations. Results. Our model data gives some support for scaling laws that were established for hot and EUV-emissive coronal loops. For the Rosner-Tucker-Vaiana (RTV) scaling law we find an offset to our model data, which can be explained by 1D considerations of a static loop with a constant heat input and conduction. With a fit to our model data we set up a new scaling law for the coronal heat input along magnetic field lines. Conclusions. RTV-like scaling laws were fitted to hot loops and therefore do not predict well the coronal heat input for cooler structures that are barely observable. The basic differences between 1D and self-consistent 3D modeling account for deviations between earlier scaling laws and ours. We also conclude that a heating mechanism by MHD-turbulent dissipation within a braided flux tube would heat the corona stronger than is consistent with our model corona.

“…In general, loop structures found in observations, such as thin or thick loops at constant cross section, or expanding envelopes of several thin loops, are also seen in the three-dimensional MHD models [103]. A three-dimensional MHD model that was run to match an actual observation did even reproduce the location of a set of loops in three-dimensional space, as revealed by a comparison to stereoscopic observations [48].…”

Section: (A) Synthetic Observationsmentioning

confidence: 82%

What can large-scale magnetohydrodynamic numerical experiments tell us about coronal heating?

Phil. Trans. R. Soc. A.

2015

Self Cite

The upper atmosphere of the Sun is governed by the complex structure of the magnetic field. This controls the heating of the coronal plasma to over a million kelvin. Numerical experiments in the form of threedimensional magnetohydrodynamic simulations are used to investigate the intimate interaction between magnetic field and plasma. These models allow one to synthesize the coronal emission just as it would be observed by real solar instrumentation. Largescale models encompassing a whole active region form evolving coronal loops with properties similar to those seen in extreme ultraviolet light from the Sun, and reproduce a number of average observed quantities. This suggests that the spatial and temporal distributions of the heating as well as the energy distribution of individual heat deposition events in the model are a good representation of the real Sun. This provides evidence that the braiding of fieldlines through magneto-convective motions in the photosphere is a good concept to heat the upper atmosphere of the Sun.