External seminars

Phase Mixing of Propagating Alfvén Waves in a Single-fluid Partially Ionized Solar Plasma 

The phase mixing of Alfvén waves is one of the most promising mechanisms for the heating of the solar atmosphere. The damping of waves in this case requires small transversal scales, relative to the magnetic field direction; this requirement is achieved by considering a transversal inhomogeneity in the equilibrium plasma density profile. Using a single-fluid approximation of a partially ionized chromospheric plasma, we study the effectiveness of the damping of phase-mixed shear Alfvén waves and investigate the effect of varying the ionization degree on the dissipation of waves. Our results show that the dissipation length of shear Alfvén waves strongly depends on the ionization degree of the plasma, but more importantly, in a partially ionized plasma, the damping length of shear Alfvén waves is several orders of magnitude shorter than in the case of a fully ionized plasma, providing evidence that phase mixing could be a large contributor to heating the solar chromosphere. The effectiveness of phase mixing is investigated for various ionization degrees, ranging from very weakly to very strongly ionized plasmas. Our results show that phase-mixed propagating Alfvén waves with a modest amplitude of 2.5 kms^-1 in a partially ionized plasma with ionization degrees in the range μ = 0.518-0.657, corresponding to heights of 1916-2150 km above the solar surface, can provide sufficient heating to balance chromospheric radiative losses in the quiet Sun.  

The Horizontal Poynting Flux in the Solar Photosphere

A crucial step for understanding the origin of the high temperatures in the upper atmosphere is an accurate description of the energy transport. Current studies on the energy flux in the solar atmosphere mainly focus on the vertical electromagnetic flow through the photosphere, ignoring the possible Poynting flux’s contributions to local heating. This study used observational data from Sunrise/IMaX and Bifrost simulations to analyse the electromagnetic energy flux of the lower atmosphere. Based on a simulated quiet Sun atmosphere, we found that only a small fraction of the Poynting energy flows upwards in the photosphere. Most of the electromagnetic energy flows parallel to the surface, and it is mainly concentrated in small regions in the intergranular lanes. We derived an approximation for the horizontal Poynting flux - based only on the line-of-sight magnetic field and the horizontal velocity, variables that can be promptly retrieved from observations. Our proxy for the horizontal portion of the Poynting flux provides a similar distribution to the total electromagnetic flux, especially in regions with high levels of electromagnetic energy. To validate the findings, we also analysed the Sunrise/IMaX data. First, we confirmed that Bifrost data realistically describe photospheric quiet-Sun regions as its data have similar horizontal and line of sight magnetic field distributions compared to the observational data. Based on our proxy, we found very similar horizontal Poynting flux distributions for the observational data and simulated photosphere, with the electromagnetic energy flux reaching 1010 ergs cm−2 s−1. Compared to previous observational estimates of the vertical Poynting flux, the horizontal component of the Poynting flux is considerably more significant than the vertical component. Those findings corroborate that the electromagnetic energy flux in the photosphere is mainly parallel to the solar surface and can be properly described by our proxy for Poynting flux.

Detection and dynamics of the vortex tubes in the solar atmosphere

We present the state-of-art detection method of three-dimensional vortices and apply it to realistic magneto-convections simulations performed by the MURaM code.

The detected vortices extend from the photosphere to the low chromosphere, presenting similar behaviour at all height levels.

The vortices concentrate the magnetic field, and thereby the plasma dynamics inside the vortex is considerably influenced by the Lorentz force.

Rotational motions also perturb the magnetic field lines, but they lead to only slightly bent flux tubes as the magnetic field tension is too high for the vortex flow to significantly twist the magnetic lines.

We find that twisted magnetic flux tubes are created by shear motions in regions where plasma-β>1, regardless of the existence of flow vortices.

MHD wave modes in the solar magnetic flux tubes with elliptical cross-section

Many previous studies of MHD modes in the magnetic flux tubes were focussed on deriving a dispersion relation for cylindrical waveguides.

However, from observations it is well known that, for example, the cross-sectional shape of sunspots and pores are not perfect circles and can often be much better approximated by ellipses. From a theoretical point of view, any imbalance in a waveguide’s diameters, even if very small, will move the study of the problem from cylindrical to elliptical coordinates.

In this talk, I will therefore describe a model that predicts the MHD wave modes that can be trapped and propagate in a compressible magnetic flux tube with an elliptical cross-section embedded in a magnetic environment.

I will discuss the resultant dispersion relations for body and surface modes, then I will show how the ellipticity of a magnetic flux tube effects these solutions (with specific applications to the coronal and photospheric conditions).

From a practical point of view the information from these dispersion diagrams does not show how these MHD modes will manifest themselves in observational data. Therefore, I will also present several visualisations of the eigenfunctions of these MHD wave modes and explain how the eccentricity effects each wave mode.

Surface waves and instabilities in the presence of an inclined magnetic field

While surface waves propagating at tangential discontinuities have been studied in great detail, few studies have been dedicated to the investigation of the nature of waves at contact discontinuities, ie, plasma discontinuity, where the background magnetic field crosses the interface between two media.

In this talk, I will show that by introducing magnetic field inclination, the frequency of waves is rendered complex, where the imaginary part describes wave attenuation, due to lateral energy leakage.

We investigate the eigen-value and initial value problem and determine the conditions of transition from contact to tangential discontinuity.

Finally, I will present an investigation into the effect of magnetic field inclination on magnetic Rayleigh-Taylor instability.