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Astronomy Colloquium (2019 Fall Semester)

* Date
Sept. 26
* Speaker
Prof. Jongchul Chae (SNU)
* Title
Three-minute Umbral Oscillations in Sunspots [태양흑점의 3분 암부진동]
* Abstract

Three-minute umbral oscillations represent the only known detectible MHD
waves in sunspots -- slow magnetoacoustic waves. They are important in
the upper atmosphere because they nonlinearly develop and contribute to
the heating and accceleration of plasma there. They are also useful in
the investigation of the interior and the low atmosphere because they
carry informtion on the source region in the interior as well as the
atmosphere through which they propagate.

We studied the physical properties and investigated their origin, based
on the high resoution imaging spectral data taken by the Fast Imaging
Solar Spectrograph of the Goode Solar Telescope, and the analytical
solutions of acoustic waves in an atmosphere. As a result, we showed
that the three-minute oscillations correspond to dispersive slow waves
of frequencies just above the acoustic cutoff. Moreover, we found the
observational results strongly suggesting that the three-minute
oscillations are driven by the fast waves that are originally produced
by impulsive disturbances in the interior like magnetoconvection events.
Finally, we propose that the single power peak of frequency just above
the acoustic cutoff can be successfuly explained by the model of a
photospheric resonator formed between the driving location and the
temperature minimum region. Our results paved the way not only for the
further investigation of slow waves, but also for the future detection
of fast waves and Alfven waves in the sunspot atmosphere.