Oscillations in the line-of-sight magnetic field strength in a pore observed by the GREGOR Infrared Spectrograph (GRIS)

Astrophysics Research Centre (ARC), School of Mathematics and Physics, Queen’s University, Belfast, BT7 1NN Northern Ireland, UK
e-mail: c.nelson@qub.ac.uk

Received: 21 May 2021
Accepted: 20 July 2021

Abstract

Context. Numerous magnetohydrodynamic oscillations have been reported within solar pores over the past few decades, including in line-of-sight (LOS) velocities, intensities, and magnetic field strengths.

Aims. Our aim is to identify whether high-amplitude oscillations in the LOS magnetic field strength can be detected within a pore located in Active Region 12748 and to investigate which physical mechanisms could be responsible for them.

Methods. A solar pore was observed on 1 September 2019 using the GREGOR Infrared Spectrograph instrument for around one hour. Full-Stokes vectors were sampled in a 37 Å window containing the Fe I 15 648.52 Å line (effective Landé g factor of 3). The LOS magnetic field strength was inferred using the strong-field approximation. Additionally, the Stokes Inversion based on Response functions code was used to gain a more complete understanding of the physical properties of the solar atmosphere at the locations of these oscillations.

Results. Oscillations of more than 100 G are observed in the LOS magnetic field in the period window between 600 and 1272 s at three localised (> 1″²) regions. These oscillations have coherence across individual regions, indicating that jitter cannot account for their occurrence. Longer-period amplitude variations, amplitudes over 200 G, are also detected, but they have periods outside of the cone-of-influence. Numerical inversions confirm both oscillations in the LOS magnetic field strength at optical depths of around log τ₅₀₀₀ = −0.5 (potentially caused by compression) and other effects (e.g. changes in the optical depth or the inclination of the magnetic field) may account for these changes.

Conclusions. The oscillations in the separations of the Stokes-V lobes of the 15 648.52 Å line appear to be solar in nature. Future work will be required to understand whether these are truly oscillations in the magnetic field strength at a specific depth in the solar atmosphere or whether other effects are responsible for these signatures.