The observations obtained using the Rapid Oscillations in the Solar Atmosphere instrument (ROSA) show variations in both cross-sectional area and intensity for magnetic pores in the photosphere. We aim to determine the wave mode by looking at the phase difference between the cross-sectional area and intensity variations. We used a straight cylinder as a model for the flux tube. The plasma is uniform both inside and outside the flux tube with a possible jump in the equilibrium values at the boundary, the magnetic field is directed along the flux tube. We derived analytic expressions for the cross-sectional area variation and the total intensity variation. Using these analytic expressions, we calculated the phase differences between the cross-sectional area and the intensity variations. These phase differences were then used to identify the wave mode. We found that for slow sausage modes the cross-sectional area and intensity variations are always in phase, while for fast sausage modes the variations are in antiphase.

Authors: M.G. Moreels, M. Goossens, and T. Van Doorsselaere
Projects: None

Publication Status: Published
Last Modified: 2013-07-10 13:33

Phase relations for seismology of photospheric flux tubes

We present a mathematical framework for the seismology of photospheric flux tubes using a uniform straight cylinder as a flux-tube model. In contrast to the earlier model of Fujimura & Tsuneta (2009), we also include a non-zero gas pressure; we do not use the thin tube approximation and we use an underdense region inside the flux tube. We used the linearised ideal magnetohydrodynamic equations to describe
different wave modes in the photosphere. Using the wave mode
polarisations we then obtained phase relations which represent different observables. Those phase relations were used to calculate phase differences and amplitude ratios. Finally we inverted these amplitude ratios to obtain plasma parameters which are not directly observable. The mathematical framework results in phase differences that can be conveniently compared with observational data to distinguish between different wave modes. Once the wave mode has been identified, the inverted amplitude ratios can be used to either analytically or numerically estimate the magnitude of plasma parameters which are not directly observable, such as the vertical wavenumber. Artificial
observations of different wave modes have shown that the framework mostly succeeds in identifying the correct wave mode and in reproducing the correct plasma parameters using the inverted amplitude ratios.

Authors: M. G. Moreels and T. Van Doorsselaere
Projects: Hinode/SOT

Publication Status: Accepted
Last Modified: 2013-03-07 06:54