Kyuhyoun Cho & Jongchul Chae
Astronomy Program, Department of Physics and Astronomy, Seoul National University

Oscillations are commonly observed in sunspot umbrae. Umbral oscillations refer to the fluctuations of intensity or velocity with 3-minute period well observed in the chromospheric level. They are usually thought to represent upward propagating slow waves affected by the acoustic cutoff.

What is the exciting origin of the umbral oscillations? Since they are observed to propagate upward in the atmosphere, their origin is expected to exist in the photosphere or in the interior. The p-mode oscillations are a potential external driver of the umbral oscillations because they are found to exist everywhere in the photosphere outside sunspots. Some previous studies presented observational results supporting this external p-mode driving^[1]. Another possible driver is the internal excitation. Although the convective motions are much inhibited by strong magnetic fields inside umbrae, observations suggest that small-scale magnetoconvection can nevertheless occur in the vicinity of umbral dots and light bridges inside umbrae^[2]. It is theoretically expected that acoustic waves can be generated in such environment. However, there have been few observational reports supporting the internal excitation so far, except a recent study^[3] which reported that strong 3-minute oscillation powers are spatially associated with the positions of umbral dots and light bridges in a sunspot.

Figure 1| Time series of velocity maps showing one of the identified oscillation patterns. Black contours mark the umbral-penumbral boundary and the position of the umbral dots. Black-cross symbols mark the position of the oscillation center. Each white ellipse indicates the shape of the propagating blueshift oscillation pattern at a specific time. The last image presents all the ellipses.

We found observational evidence for the internal excitation of the umbral oscillations from observations of two sunspots near the solar disk center in 2015 and 2017 using the Fast Imaging Solar Spectrograph (FISS^[4]) of the Goode Solar Telescope (GST) at the Big Bear Solar Observatory (BBSO). The umbral velocity oscillations were investigated by analyzing the imaging spectral data of the FeI 5435 Å line, which is a magnetically-insensitive strong absorption line and is formed near the temperature minimum region. We used the 1-4 minutes bandpass filtering data to reduce the effect of noises and 5-minute oscillations.

We have found 4 oscillation patterns inside the umbrae that may be the result of internal excitation events (Figure 1). In each pattern, the oscillation centers were determined, and a specified velocity pattern propagated out of the center on the image plane. The oscillations center is thus similar to the epicenter of the seismic waves in earthquake. We regard each oscillations pattern as the result of excitation events in the interior below the center. What is important is that all the oscillation centers are located inside the sunspot umbrae, which is a strong observational support of the internal excitation of the umbral oscillations.

Figure 2| Left: TiO intensity image at the beginning of the oscillation patterns shown in Figure 1. The yellow circle and the yellow cross represent the coherent size of the oscillation patterns and the oscillation centers, respectively. The yellow arrow marks the slit position of the time-distance map. The perpendicular tick in the middle of the arrow represents the zero distance that is the nearest point from the oscillation center. Right: TiO intensity time-distance map. The time between red dashed lines indicates the duration of the oscillation patterns. The moving umbral dots are marked by the yellow dashed lines with their horizontal speed.

We found umbral dots nearby the oscillation centers at the photospheric level (Fig 2). The umbral dots underwent considerable morphological and dynamical changes while the oscillation patterns persisted. The mean speed of horizontal motion was about 1 km/s, which is much higher than the typical speed of umbral dots of about 0.4 km/s. These changes suggest that small-scale magnetoconvection may have proceeded in the vicinity of the umbral dots. Our results support the notion that the 3-minute umbral oscillations are internally excited by small-scale magnetoconvection associated with umbral dots.

For more information, please refer to Cho et al.^[5].

References

[1] Khomenko, E., & Collados, M. 2015, LRSP, 12, 6
[2] Ortiz, A., Bellot Rubio, L. R., & Rouppe van der Voort, L. 2010, ApJ, 713, 1282
[3] Chae, J., Lee, J., Cho, K., et al. 2017, ApJ, 836, 18
[4] Chae, J., Park, H.-M., Ahn, K., et al. 2013, SoPh, 288, 1
[5] Cho et al., 2019, ApJ, 879, 67