The Observational Evidence for the Internal Excitation of Sunspot Oscillations Inferred from the Fe i 5435 Å Line

Abstract

The umbral oscillations of velocity are commonly observed in the chromosphere of a sunspot. Their sources are considered to be either the external p-mode driving or the internal excitation by magnetoconvection. Even though the possibility of the p-mode driving has been often considered, the internal excitation has been rarely investigated. We report the identification of the oscillation patterns that may be closely related to the events of internal excitation from the observations of velocity oscillations in the temperature minimum region of two sunspots. The velocities were determined from the spectral data of the Fe i 5435Å line, a magnetically insensitive line, taken with the Fast Imaging Solar Spectrograph of the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory. As a result, we identified 4 oscillation patterns of 2.0 × 10 3 km coherent size that were clearly identified for about 7.9 minutes with oscillation amplitude of 9.3 × 10 −2 km s −1 . The power of the oscillations in these centers was concentrated in the 3 minute band. All the oscillation centers were located above the umbral dots undergoing noticeable morphological and dynamical changes that may be regarded as an observable signature of small-scale magnetoconvection inside the umbrae. Our results support the notion that magnetoconvection associated with umbral dots inside sunspots can drive the 3 minute umbral oscillations.

Doppler shift oscillations of a sunspot detected by CYRA and IRIS

“…In other words, the three-minute oscillation can be regarded as the upwardly propagating slow magnetoacoustic waves (e.g., Su et al 2016;Chae et al 2019;Cho & Chae 2020;Feng et al 2020b). Finally, Chae et al (2017) found that the threeminute oscillation in the light bridge or umbral dots of a sunspot originates from the photosphere (see aslo Cho et al 2019), which is consistent with our results and further suggests that the CYRA data is reliable.…”

“…They are thought to be resonant modes of sunspot oscillations, with cavities that may be located at sunspot umbrae in the solar layers of sub-photospheres (Scheuer & Thomas 1981;Thomas 1984Thomas , 1985 or chromospheres (Uexkuell et al 1983;Gurman 1987;Khomenko & Collados 2015). The typical three-minute oscillation above the sunspot can be simultaneously detected from the photosphere through the chromosphere and transition region to the corona, supporting the interpretation of propagating waves at sunspots (De Moortel et al 2002;O'Shea et al 2002;Brynildsen et al 2004;Su et al 2016;Chae et al 2019), for instance trans-sunspot waves (Tziotziou et al 2006) or slow magnetoacoustic waves (Bloomfield et al 2007;Krishna Prasad et al 2015;Wang et al 2018;Cho et al 2019;Cho & Chae 2020).…”

“…At photospheric layers the velocity fluctuations at both umbra and penumbra are similar to their surrounding photosphere (Khomenko & Collados 2015). However, the oscillation amplitudes at sunspot umbra and penumbra are significantly reduced when compared to the surroundings, i.e., no more than 1 km s −1 (Howard et al 1968;Soltau et al 1976;Lites 1988;Chae et al 2017;Cho et al 2019). The Doppler shift oscillations of sunspots are much more apparent and easier to be detected in chromosphere, such as the spectral lines of Ca ii H & K, He i 10830 Å, Hα off-band, Mg ii h & k, and C ii (Lites 1986;Solanki et al 1996;Bogdan 2000;Khomenko & Collados 2015).…”

“…8. Torrence & Compo 1998;Chae et al 2017;Cho et al 2019) on the detrended time series of Doppler velocities at the sunspot umbra, as indicated by the cyan line in Figs. 3 and 5.…”

Linear Acoustic Waves in a Nonisothermal Atmosphere. II. Photospheric Resonator Model of Three-minute Umbral Oscillations

“…Recent observations (Chae et al 2017;Cho et al 2019;Kang et al 2019) rather support the notion that the sources of excitation are located below the atmosphere, as has commonly been modeled by the piston motion at the bottom of the atmosphere in previous theoretical studies (e.g., Fleck & Schmitz 1991;Kalkofen et al 1994;Sutmann & Ulmschneider 1995). These studies found that regardless of the specific form of the piston driving, the oscillations with a frequency close to the cutoff prevail in high altitudes of the atmosphere.…”