Waves and oscillations are important solar phenomena not only because they can propagate and dissipate energy in the chromosphere, but also because they carry information about the structure of the atmosphere in which they propagate. Among these phenomena, the one of the most interesting ones occurs in the sunspot umbra. In this area, continuously propagating magnetohydrodynamic (MHD) waves generated from below the photosphere create the famous 3-minute sunspot umbral oscillations that affect the line profile of spectral lines due to temperature, density, and velocity changes of the plasma in the region. In the past decades, numerous observations and models have been carried out about the nature of the 3-minute oscillation and its relation with the coronal heating problem, but the lack of direct observations of the temperature variation in the chromosphere has made it hard to answer these questions.
The need for a better understanding of the fine structure of the 3-minute oscillation and its time evolution in sunspots has intensified with the development of better observing tools. Among modern observatories, the Atacama Large Millimeter/submillimeter Array (ALMA) opens up a new era of solar radio observation due to its high spatial and temporal resolution and image quality. When combined with other cutting-edge instruments, such as the Goode Solar Telescope (GST) at the Big Bear Solar Observatory (BBSO), the Atmospheric Imaging Assembly (AIA) on board the Solar DynamiCST Observatory, and Interface Region Imaging Spectrograph (IRIS), ALMA can provide unique electron temperature diagnostics that clarify the behavior of the solar chromosphere’s response to propagating waves.
In this dissertation, a study is carried out about sunspot oscillations and wave propagation in NOAA active region 12470 using an approximately 1 hr long dataset acquired on 2015 December 17 by the instruments listed above. The discovery of 3-minute sunspot oscillations seen in the mm-wave band is reported for the first time. The 2 s cadence of ALMA images makes it possible to well resolve the typical 3-minute period sunspot oscillation in the chromosphere. Fourier analysis is applied to the ALMA 3 mm continuum and CST Hα data sets to obtain the power spectra as well as phase information of the oscillations. The properties of the wave propagation are analyzed by combining multiple wavelengths that probe physical parameters of the solar atmosphere at different heights.
The chromospheric radiation is synthesized in 1-D using a radiation transfer code which uses the Solar Irradiance Physical Modeling (SRPM) as an input. A good correlation of the phase relationship between the observed and modeled oscillations of Hα and temperature fluctuations has been found and it is consistent with the result from a possible physical model for impulse-driven acoustic waves propagating in the gravitationally stratified medium. An asymmetry in the time profile of the temperature fluctuations discovered in the ALMA data is found to require a nonlinear wave solution, which is applied to several atmospheric models in an attempt to match the asymmetry and the absolute brightness temperature of the simulations to the observations. The asymmetry is successfully reproduced using the nonlinear wave scenario, although the absolute brightness of the simulated atmosphere remains lower than observed. These results demonstrate the capability of ALMA mm observations to provide new insight into what is needed for improving such atmospheric models in the future.
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