| Summary: | 博士 === 國立清華大學 === 物理系 === 100 === As solar acoustic and surface-gravity waves propagate through a sunspot, they interact with the sunspot and carry information about the sunspot structure. In this dissertation, phenomenological parameters describing aspects of interaction between waves and sunspots are defined and are then determined from observation. These measured parameters help investigate the interaction between waves and sunspots and the sunspot structures in that they give constraints to theories and models on the interaction and the sunspot structures.
This work consists of two parts. First, the intensity of solar oscillation inside and near a magnetic region has been observed to be lower than that in quiet region. In this work, the mechanisms causing this reduction of intensity are divided into three categories -- absorption, emissivity reduction, and local suppression, and a method is developed to measure the respective contributions of these three categories. In this method, an energy budget model is devised to describe the modification of the power of a wave packet propagating through a magnetic region. Three coefficients associated respectively with the three categories of mechanisms are defined in this model. Using the property that the signals of waves emitted along the wave path between two points do not correlate with the signal at the starting point, we can determine the values of the three coefficients from the magnitudes of cross-correlation functions. From the measured cross-correlation functions of various pairs of points, spatial distributions (2-dimensional) of the three coefficients are obtained for active regions NOAA 9055 and 9057. The maps of spatial distribution of absorption coefficient and local-suppression coefficient correlate with the map of magnetic field, including in plage regions. However, the maps of emissivity-reduction coefficient are noisier and do not have significant signal at the sunspot location in most cases.
In the second part, the scattering of solar acoustic or surface-gravity waves by sunspots is studied. In this work, the scattered wave is defined as the wave in the presence of a sunspot minus the wave as if the sunspot were not present. The surface part of the scattered wave of specific radial order and frequency is expressed as a convolution of the 2-dimensional Green funcion and the product of the wave function (on the surface) and the 2-dimensional interaction parameter. The 2-dimensional interaction parameter, having complex value, is a distribution over the magnetic (sunspot) area on the surface, and it describes the wave-sunspot interaction and the sunspot structure. We assume a simple model for the distribution of the complex two-dimensional interaction parameter: its absolute value is axisymmetric with a Gaussian profile, and its phase is constant. The measured scattered waves of various radial orders and frequencies for sunspots NOAA 11084 and 11092 are fitted to the scattered waves computed with the above model using high-order Born approximations, yielding the values of model parameters (the 'magnitude', 'Gaussian radius', and 'phase'). The values of the three model-parameters converge at high orders of Born approximations, with the convergence taking place more rapidly for the smaller sunspot and longer wavelengths. The result of the 1st-order Born approximation significantly differs from that of higher orders. The 'magnitude' increases with frequency and wavenumber for each radial order. The 'Gaussian radius' is insensitive to frequency or wavenumber. The spatial extent of the interaction parameter is greater than that of the continuum-intensity deficit, but is smaller than that of the acoustic-power deficit by the sunspot. The 'phase' falls within small range for fixed phase speed, suggesting that the 'phase' might be a function of phase speed. The 2 sunspots NOAA 11084 and 11092 have similar 'magnitude' and 'phase', although the ratio of their sizes is about $3:4$.
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