Local eigenfrequency and its uncertainty inferred from fundamental spheroidal mode frequency shifts

Abstract

Summary 6297 observations of fundamental spheroidal mode apparent peak frequencies assembled from 968 records of the IDA network have been inverted for δωlocal, a measure of local dispersion, using a scheme which takes into account coupling between the singlets of an isolated normal mode multiplet and which does not rely upon the ‘pure path’ approximation. Rotational coupling with fundamental toroidal modes contaminates the data set below 0S23, but inversions of peak shifts between 2.2 and 4.7 mHz show the consistency expected from long wavelength velocity structure. In addition to the strong degree-two signal reported in earlier work, a degree-six signal is consistently evident. Structure at degrees eight and four has less power, and the results for these portions of the fit are more ambiguous. Maps of the great circular average of δωlocal derived from fits up to degree eight agree well with the even degree parts of model M84A, and the degree-six part compares very well with M84A. Formal error analysis suggests the uncertainty is almost uniform globally but that the magnitude of the error estimate is not sufficient to explain fluctuations remaining in the observations after the modelled signal has been removed.