Simulations Of Solar Magneto-convection Research Articles

Influence of ambipolar and Hall effects on vorticity in three-dimensional simulations of magneto-convection: Vorticity with ambipolar and Hall effect

This paper presents the results of the analysis of three-dimensional simulations of solar magneto-convection that include the joint action of the ambipolar diffusion and the Hall effect. Three simulation runs are compared: One including both ambipolar diffusion and the Hall effect; one including only ambipolar diffusion and one without any of these two effects. The magnetic field is amplified from initial field to saturation level by the action of turbulent local dynamo. In each of these cases, we study 2 h of simulated solar time after the local dynamo reaches the saturation regime. We analyse the power spectra of vorticity, of magnetic field fluctuations and of the different components of the magnetic Poynting flux responsible for the transport of vertical or horizontal perturbations. Our preliminary results show that the ambipolar diffusion produces a strong reduction of vorticity in the upper chromospheric layers and that it dissipates the vortical perturbations converting them into thermal energy. The Hall effect acts in the opposite way, strongly enhancing the vorticity. When the Hall effect is included, the magnetic field in the simulations becomes, on average, more vertical and long-lived flux tube-like structures are produced. We trace a single magnetic structure to study its evolution pattern and the magnetic field intensification, and their possible relation to the Hall effect. This article is part of the Theo Murphy meeting issue 'High-resolution wave dynamics in the lower solar atmosphere'.

Reflections on magnetoconvection

The interaction between magnetic fields and convection can be observed directly in stars like the Sun. This selective review uses idealized numerical experiments to illustrate and extend results obtained from bifurcation theory. It covers a wide variety of steady and time-dependent behaviour, in incompressible and compressible fluids, in two and three dimensions. These results illuminate behaviour that appears in recent “realistic” simulations of solar magnetoconvection.

A comparison of measured and simulated solar network contrast

AbstractLong-term trends in the solar spectral irradiance are important to determine the impact on Earth's climate. These long-term changes are thought to be caused mainly by changes in the surface area covered by small-scale magnetic elements. The direct measurement of the contrast to determine the impact of these small-scale magnetic elements is, however, limited to a few wavelengths, and is, even for space instruments, affected by scattered light and instrument defocus. In this work we calculate emergent intensities from 3-D simulations of solar magneto-convection and validate the outcome by comparing with observations from Hinode/SOT. In this manner we aim to construct the contrast at wavelengths ranging from the NUV to the FIR.

The Zeeman Effect in the G Band

We investigate the possibility of measuring magnetic field strength in G-band bright points through the analysis of Zeeman polarization in molecular CH lines. To this end we solve the equations of polarized radiative transfer in the G band through a standard plane-parallel model of the solar atmosphere with an imposed magnetic field and through a more realistic snapshot from a simulation of solar magnetoconvection. This region of the spectrum is crowded with many atomic and molecular lines. Nevertheless, we find several instances of isolated groups of CH lines that are predicted to produce a measurable Stokes V signal in the presence of magnetic fields. In part this is possible because the effective Lande factors of lines in the stronger main branch of the CH A2Δ-X2Π transition tend to zero rather quickly for increasing total angular momentum J, resulting in a Stokes V spectrum of the G band that is less crowded than the corresponding Stokes I spectrum. We indicate that, by contrast, the effective Lande factors of the R and P satellite subbranches of this transition tend to ±1 for increasing J. However, these lines are in general considerably weaker and do not contribute significantly to the polarization signal. In one wavelength location near 430.4 nm, the overlap of several magnetically sensitive and nonsensitive CH lines is predicted to result in a single-lobed Stokes V profile, raising the possibility of high spatial resolution narrowband polarimetric imaging. In the magnetoconvection snapshot we find circular polarization signals of the order of 1%, prompting us to conclude that measuring magnetic field strength in small-scale elements through the Zeeman effect in CH lines is a realistic prospect.