An Inside Look at Sunspot Oscillations with Higher Azimuthal Wavenumbers

David B Jess,Damian J Christian,Samuel D T Grant,Peter H Keys,Viktor Fedun,Han Uitenbroek,Robert Erdélyi,S Krishna Prasad,Gary Verth,Tom Van Doorsselaere

doi:10.3847/1538-4357/aa73d6

Abstract

Abstract Solar chromospheric observations of sunspot umbrae offer an exceptional view of magnetohydrodynamic 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 modeling has demonstrated how these ubiquitous waves are consistent with an m = 0 slow magneto-acoustic mode, which is 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 − 1 ( 2.1 ± 0.4 deg s − 1 , corresponding to a period ≈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 modeling 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.

Highlights

Since the early pioneering work by Beckers & Tallant (1969), Wittmann (1969), and Havnes (1970), to name but a few, oscillations and propagating waves tied to sunspot atmospheres have remained a challenging research area within solar physics
The interplay between various plasma measurements has allowed researchers to verify that the majority of visible wave signatures in sunspot umbrae are synonymous with the m = 0 slow magneto-acoustic mode
We consider the sunspot to be a cylindrical structure in the polar coordinate system (r, f, z), with the z-axis aligned with the umbral magnetic field, using the associated wavenumbers m and kz

Summary

Introduction

Since the early pioneering work by Beckers & Tallant (1969), Wittmann (1969), and Havnes (1970), to name but a few, oscillations and propagating waves tied to sunspot atmospheres have remained a challenging research area within solar physics. The interplay between various plasma measurements (e.g., the magnetic field strength, the line-of-sight velocity, the intensity perturbations, etc., Fujimura & Tsuneta 2009; Freij et al 2014; Moreels et al 2015b) has allowed researchers to verify that the majority of visible wave signatures in sunspot umbrae are synonymous with the m = 0 slow magneto-acoustic mode Such activity can readily be identified in chromospheric (e.g., Bloomfield et al 2007; Vecchio et al 2007; Kobanov et al 2011; Jess et al 2013; Löhner-Böttcher & Bello González 2015; Moreels et al 2015a) and coronal (e.g., De Moortel 2006; McEwan & De Moortel 2006; Jess et al 2012a, 2016; Krishna Prasad et al 2012, 2015) sunspot-related studies involving both imaging and spectroscopic capabilities. In this article, we employ modern processing techniques to extract, interpret, and model, for the first time, higher-order wave modes found within a sunspot umbral atmosphere

Observations and Processing

Analysis and Discussion

Conclusions