Observing MHD oscillations in sunspot

Abstract

Attempts to detect magnetohydrodynamic waves in the solar photosphere by identifying oscillations in the magnetic field have proved problematic due to suspected contributions from systematic temperature and density fluctuations causing the spectral line formation height to vary, which in turn samples a vertical gradient in the magnetic field strength. We investigate this effect in sunspot umbrae and penumbrae through the analysis of data obtained with the Advanced Stokes Polarimeter in spectral lines with notoriously different temperature sensitivities. The temporal behavior of the magnetic field strength in sunspot is presented with special consideration to line formation physics occurring in the dynamic solar atmosphere. These results are compared to forward modeling of Stokes profiles with a radiative transfer code given a sunspot atmosphere perturbed by an MHD oscillation.