An Inside Look at Sunspot Oscillations with Higher Azimuthal Wavenumbers

Abstract

Solar chromospheric observations of sunspot umbrae offer an exceptional view of magneto-hydrodynamic wave phenomena. In recent years, a wealth of wave signatures related to propagating magneto-acoustic modes have been presented, which demonstrate complex spatial and temporal structuring of the wave components. Theoretical modelling has demonstrated how these ubiquitous waves are consistent with an m=0 slow magneto-acoustic mode, which are excited by trapped sub-photospheric acoustic (p-mode) waves. However, the spectrum of umbral waves is broad, suggesting that the observed signatures represent the superposition of numerous frequencies and/or modes. We apply Fourier filtering, in both spatial and temporal domains, to extract chromospheric umbral wave characteristics consistent with an m=1 slow magneto-acoustic mode. This identification has not been described before. Angular frequencies of 0.037 +/- 0.007 rad/s (2.1 +/- 0.4 deg/s), corresponding to a period approximately 170 s for the m=1 mode are uncovered for spatial wavenumbers in the range of 0.45<k<0.90 arcsec^-1 (5000-9000 km). Theoretical dispersion relations are solved, with corresponding eigenfunctions computed, which allows the density perturbations to be investigated and compared with our observations. Such magnetohydrodynamic modelling confirms our interpretation that the identified wave signatures are the first direct observations of an m=1 slow magneto-acoustic mode in the chromospheric umbra of a sunspot.

Accepted for publication in ApJ with 12 pages and 4 figures

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• Coronal heating by MHD waves
• Alfvenic Perturbations in a Sunspot Chromosphere Linked to Fractionated Plasma in the Corona
• The Frequency-dependent Damping of Slow Magnetoacoustic Waves in a Sunspot Umbral Atmosphere
• The Physical Nature of Spiral Wave Patterns in Sunspots
• Resonant absorption of surface sausage and surface kink modes under photospheric conditions
• An overall view of temperature oscillations in the solar chromosphere with ALMA
• Magnetohydrodynamic wave mode identification in circular and elliptical sunspot umbrae: evidence for high order modes
• Damping of slow surface kink modes in solar photospheric waveguides modeled by one-dimensional inhomogeneities
• The dynamics and observability of circularly polarized kink waves
• On the Description of Transverse Wave Propagation Along Thin Magnetic Flux Tubes