Oscillations Above Sunspots and Faculae: Height Stratification and Relation to Coronal Fan Structure

TL;DR

This study investigates oscillation patterns above sunspots and faculae using multi-layer solar observations, revealing how different frequencies relate to magnetic structures and coronal loop dynamics.

Contribution

It provides new insights into the height stratification of oscillations and estimates magnetic-tube inclination angles using oscillation data, advancing understanding of wave propagation in solar magnetic features.

Findings

Low-frequency oscillations outline coronal fan structures.

High-frequency oscillations are concentrated near umbra boundaries.

Phase velocities in coronal loops reach up to 150 km/s.

Abstract

Oscillation properties in two sunspots and two facular regions are studied using Solar Dynamics Observatory (SDO) data and ground-based observations in the SiI 10827 and HeI 10830 lines. The aim is to study different-frequency spatial distribution characteristics above sunspots and faculae and their dependence on magnetic-field features and to detect the oscillations that reach the corona from the deep photosphere most effectively. We used Fast-Fourier-Transform and frequency filtration of the intensity and Doppler-velocity variations with Morlet wavelet to trace the wave propagating from the photosphere to the chromosphere and corona. Spatial distribution of low-frequency (1-2 mHz) oscillations outlines well the fan-loop structures in the corona (the Fe IX 171 line) above sunspots and faculae. High-frequency oscillations (5-7 mHz) are concentrated in fragments inside the photospheric umbra boundaries and close to facular-region centers. This implies that the upper parts of most coronal loops, which transfer low-frequency oscillations from the photosphere, sit in the Fe IX 171 line-formation layer. We used dominant frequency vs. distance from barycenter relations to estimate magnetic-tube inclination angle in the higher layers, which poses difficulties for direct magnetic-field measurements. According to our calculations, this angle is about 40 degrees in the transition region around umbra borders. Phase velocities measured in the coronal loops' upper parts in the Fe IX 171 line-formation layer reach 100-150 km/s for sunspots and 50-100 km/s for faculae.