Travel Time Shifts due to Amplitude Modulation in Time-Distance Helioseismology

Correct interpretation of acoustic travel times measured by time-distance helioseismology is essential to get an accurate understanding of the solar properties that are inferred from them. It has long been observed that sunspots suppress p-mode amplitude, but its implications on travel times has not been fully investigated so far. It has been found in test measurements using a 'masking' procedure, in which the solar Doppler signal in a localized quiet region of the Sun is artificially suppressed by a spatial function, and using numerical simulations that the amplitude modulations in combination with the phase-speed filtering may cause systematic shifts of acoustic travel times. To understand the properties of this procedure, we derive an analytical expression for the cross-covariance of a signal that has been modulated locally by a spatial function that has azimuthal symmetry, and then filtered by a phase speed filter typically used in time-distance helioseismology. Comparing this expression to the Gabor wavelet fitting formula without this effect, we find that there is a shift in the travel times, that is introduced by the amplitude modulation. The analytical model presented in this paper can be useful also for interpretation of travel time measurements for non-uniform distribution of oscillation amplitude due to observational effects.Comment: 17 pages, 1 figure, accepted for publication in Ap

Fast integral equation methods for the Laplace-Beltrami equation on the sphere

Integral equation methods for solving the Laplace-Beltrami equation on the unit sphere in the presence of multiple "islands" are presented. The surface of the sphere is first mapped to a multiply-connected region in the complex plane via a stereographic projection. After discretizing the integral equation, the resulting dense linear system is solved iteratively using the fast multipole method for the 2D Coulomb potential in order to calculate the matrix-vector products. This numerical scheme requires only O(N) operations, where $N$ is the number of nodes in the discretization of the boundary. The performance of the method is demonstrated on several examples

Changing health care quality paradigms: the rise of clinical guidelines and quality measures in American medicine

Clinical guidelines and quality measures are important new paradigms for conceptualizing and managing quality in the United States. Researchers have proposed that professional elites-including members of academic medicine-were an important cause of the shift to guidelines and measures. This paper draws on content analysis of abstracts focused on quality in major American medical journals between 1975 and 2009 to empirically assess whether and how paradigms for managing quality changed in academic medicine. The content analysis shows that guidelines- and measures-based approaches to quality increased in prominence. Individual expertise-based approaches to quality, however, remain important. Concurrent with changing paradigms in academic medicine, there was a reorientation of policy toward increased use of guidelines and measures the late 1980s and early 1990s in the United States. This policy reorientation was informed by earlier work by medical researchers proposing new approaches to quality. The policy reorientation was followed by an increase in the prominence of guidelines and measures in medical research