Oscillations In The Line-of-Sight Magnetic Field Strength In A Pore Observed By The GREGOR Infrared Spectrograph (GRIS)

Abstract: Numerous magnetohydrodynamic oscillations have been reported within solar pores over the past decades, including in line-of-sight (LOS) velocities, intensities, and magnetic field strengths. 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. A solar pore was observed on the 1st September 2019 using the GREGOR Infrared Spectrograph for around one hour. Full-Stokes vectors were sampled in a 37 A window containing the Fe I 15648.52 A line (effective Lande g-factor of 3). The LOS magnetic field strength is inferred using the strong-field approximation. The Stokes Inversion based on Response functions code is used to gain a more complete understanding the properties of the solar atmosphere at the locations of these oscillations. Oscillations of more than 100 G are observed in the LOS magnetic field in the period window 600-1272 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 these 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-tau_5000=-0.5 (potentially caused by compression) and other effects (e.g., changes in the optical depth or the inclination of the field) may account for these changes. The oscillations in the separations of the Stokes-V lobes of the 15648.52 A 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.