MHD Seismology of a Coronal Loop System by the First Two Modes of Standing Kink Waves

TL;DR

This study analyzes the first two harmonics of standing kink waves in a coronal loop to infer physical parameters like magnetic field strength, density, and temperature, demonstrating the effects of stratification and magnetic variation on wave properties.

Contribution

It presents the first observation of the first two harmonics of kink waves in a coronal loop and uses these to estimate magnetic and plasma parameters with implications for coronal seismology.

Findings

Fundamental and first overtone periods measured with high precision.

Magnetic field strength estimated at 8.2 G, consistent with potential field models.

Density stratification and temperature differences significantly influence period ratios.

Abstract

We report the observation of the first two harmonics of the horizontally polarized kink waves excited in a coronal loop system lying at south-east of AR 11719 on 2013 April 11. The detected periods of the fundamental mode ($P_1$), its first overtone ($P_2$) in the northern half, and that in the southern one are $530.2 \pm 13.3$, $300.4 \pm 27.7$, and $334.7 \pm 22.1$ s, respectively. The periods of the first overtone in the two halves are the same considering uncertainties in the measurement. We estimate the average electron density, temperature, and length of the loop system as $(5.1 \pm 0.8) \times 10^8$ cm$^{-3}$, $0.65 \pm 0.06$ MK, and $203.8 \pm 13.8$ Mm, respectively. As a zeroth order estimation, the magnetic field strength, $B = 8.2 \pm 1.0$ G, derived by the coronal seismology using the fundamental kink mode matches with that derived by a potential field model. The extrapolation model also shows the asymmetric and nonuniform distribution of the magnetic field along the coronal loop. Using the amplitude profile distributions of both the fundamental mode and its first overtone, we observe that the antinode positions of both the fundamental mode and its first overtone shift towards the weak field region along the coronal loop. The results indicate that the density stratification and the temperature difference effects are larger than the magnetic field variation effect on the period ratio. On the other hand, the magnetic field variation has a greater effect on the eigenfunction of the first overtone than the density stratification does for this case.