We present a step by step guide of a refined magneto-seismological technique for plasma diagnostics in the Sun's corona. Recently developed MHD wave theory which models a coronal loop as an expanding magnetic flux tube with an arbitrary longitudinal plasma density profile is applied to TRACE observations of fast kink oscillations. The theory predicts that using the observed ratio of the first overtone and fundamental mode to predict the plasma density scale height and not taking account of loop expansion will lead to an overestimation of scale height. For the first time, the size of this correction is quantified directly from observation and for the particular case study presented here, it is found that the overestimation is approximately by a factor of 2.

Authors: G. Verth, R. Erdelyi & D.B. Jess
Projects: TRACE

Publication Status: ApJ (accepted 17 September 2008, in press)
Last Modified: 2008-09-17 13:53

We present a step by step guide of a refined magneto-seismological technique for plasma diagnostics in the Sun's corona. Recently developed MHD wave theory which models a coronal loop as an expanding magnetic flux tube with an arbitrary longitudinal plasma density profile is applied to TRACE observations of fast kink oscillations. The theory predicts that using the observed ratio of the first overtone and fundamental mode to predict the plasma density scale height and not taking account of loop expansion will lead to an overestimation of scale height. For the first time, the size of this correction is quantified directly from observation and for the particular case study presented here, it is found that the overestimation is approximately by a factor of 2.

Authors: G. Verth, R. Erdelyi & D.B. Jess
Projects: TRACE

Publication Status: ApJ (accepted 17 September 2008, in press)
Last Modified: 2008-09-17 04:15

CONTEXT. Observations of post-flare transversal coronal loop oscillations by TRACE have given us an excellent opportunity to implement magneto-seismological techniques to probe the plasma fine structure of the Sun's upper atmosphere. AIMS. To investigate the effect of magnetic stratification combined with density stratification on transversal coronal loop oscillations. METHODS. A coronal loop will be modelled as an expanding magnetic flux tube with arbitrary longitudinal plasma density. The governing equation of the fast kink body mode is derived and solved by analytical approximation and numerical methods. RESULTS. It is found that even a relatively small coronal loop expansion can have a significant and pronounced effect on the accuracy of plasma density scale height measurements derived from observation of loop oscillations. CONCLUSIONS. To conduct more accurate and realistic magneto-seismology of coronal loops, the magnetic field divergence should be taken into account.

Authors: G. Verth & R. Erdelyi
Projects: None

Publication Status: A&A (submitted)
Last Modified: 2008-09-23 21:11

Spatial magneto-seismology: effect of density stratification on first harmonic amplitude profile of transversal coronal loop oscillations

CONTEXT: The new generation of extreme-ultraviolet (EUV) imagers onboard missions such as the Solar Dynamics Observatory (SDO) and Solar Orbiter (SO) will provide the most accurate spatial measurements of post-flare coronal loop oscillations yet. The amplitude profiles of these loop oscillations contain important information about plasma fine structure in the corona.
AIMS: To show that the position of the anti-nodes of the amplitude profile of the first harmonic of the standing fast kink wave of a coronal loop relate to the plasma density stratification of that loop.
METHODS: The MHD kink transversal waves of coronal loops are modelled both numerically and analytically. The numerical model implements the implicit finite element code POLLUX. Analytically, dispersion relations are derived and solved. The results of the two methods are compared and verified.
RESULTS: Density stratification causes the anti-nodes of the first harmonic to shift towards the loop footpoints. The greater the density stratification, the larger the shift. The anti-node shift of the first harmonic of a semi-circular coronal loop with a density scale height, H=50 Mm and loop half length L=100 Mm is approximately 5.6 Mm. Shifts in the Mm range are measureable quantities providing valuable information about the subresolution structure of coronal loops.
CONCLUSIONS: The measurement of the anti-node shift of the first harmonic of the standing fast kink wave of coronal loops is potentially a new tool in the field of solar magneto-seismology, providing a novel complementary method of probing plasma fine structure in the corona.

Authors: G. Verth, T. Van Doorsselaere, R. Erdelyi and M. Goossens
Projects: None

Publication Status: A&A (submitted)
Last Modified: 2007-06-19 01:06

The effect of density stratification on the amplitude profile of transversal coronal loop oscillations

CONTEXT. Measuring how the density of the plasma in the Sun's corona varies along fine structures such as coronal loops using emission measure is very difficult as the exact value of the line of sight filling factor and the correct amount of background subtraction are unknown.
AIMS. To investigate if MHD wave theory can be used to complement existing observational methods to obtain diagnostic information about the density structure of coronal loops by measuring the amplitude profile of the fundamental standing mode of the fast kink wave.
METHODS. Analytical and numerical approaches are used to show how the amplitude profile of the fundamental kink mode changes by varying the amount of density stratification in a coronal loop.
RESULTS. Increasing the amount of density stratification in a coronal loop will increase the difference in amplitude profile from one of constant longitudinal density. For a semi-circular TRACE 171 Å temperature loop of length 200 Mm and density scale height, H=50 Mm, the maximum change in amplitude profile is of the order 50 km. It is also found that the fixed footpoints for these oscillations are probably at the transition region.
CONCLUSIONS. The effect of gravitational density stratification on the amplitude profile of the fundamental standing kink mode of a TRACE 171 Å temperature loop may be very subtle and well below the resolution of its EUV imager (approximately 714 km on the Sun's surface). Only loops with homogeneous magnetic fields have been addressed here, but inhomogeneous magnetic fields may well dominate over the effect of density stratification on the amplitude profile.

Authors: Erdelyi, R. & Verth, G.
Projects: None

Publication Status: A&A (submitted)
Last Modified: 2006-06-29 10:36