Multi-height observations of magnetoacoustic cut-off frequency in a sunspot atmosphere

TL;DR

This study investigates the variation of magnetoacoustic cut-off frequencies at different heights in sunspots using imaging data, revealing how oscillation periods relate to magnetic field inclination and their possible origins.

Contribution

It introduces a method to measure the cut-off frequency across sunspot atmospheres and compares magnetic inclinations derived from oscillations with potential field extrapolations, highlighting discrepancies.

Findings

Shorter-period oscillations are confined to sunspot umbrae.

Longer periods form an annular pattern around the umbra.

Magnetic field inclination explains 60-80% of the cut-off frequency variation.

Abstract

We used the Pixelised Wavelet Filtering (PWF) method to compute narrow-band power maps of SDO/AIA imaging datasets in the 1700 \AA{}, 1600 \AA{} and 304 \AA{} bandpasses that correspond to different heights. The cut-off frequency was defined as contours where the spectral power droped to the median level. It was measured as a function of the spatial location. We inferred the magnetic field inclination according to the MAG wave theory in the low-$\beta$ limit and compared it with the potential field extrapolation.{We analysed intensity oscillations in a symmetric sunspot AR11131(08 Dec 2010) and an asymmetric sunspot AR11330 (27 Oct 2011). We reconstructed the magnetic field inclination in the radial direction for the symmetric sunspot and in both radial and azimuthal directions for the asymmetric sunspot. We observed 3D variation of the main oscillation periods in sunspots. We found that shorter-period oscillations were mostly constrained in sunspot umbrae, while longer-period oscillations formed an annular shape enclosing the umbra. Longer periods are found to be distributed further away from the sunspot centre. Our results indicate that 3-min oscillation are generated in the chromosphere, possibly by the acoustic resonator model, while 5-min and longer-period oscillations seemed to originate in a level under the photosphere. The reconstructed field inclinations gives the values of the field inclination that are systematically larger than the values obtained by the potential field extrapolation. The inclined magnetic field line can account for 60-80% of cut-offfrequency lowering only.