Local S-spectrum analysis of 1-D and 2-D data

The local changes of the spectrum with time are often more interesting than the spectrum of the whole time series. For example, there is an apparent drift in the nominal 28 day fluctuations of sunspot numbers over the period of the sunspot cycle, averaging similar to 11.1 years. This time-local change in spectrum is due to a combination of Sporer's Law and the differential rotation of the sun. Similarly, the space-local variations in the 2-D spectrum on an image conveys visual information on textures, boundaries and shapes. In this paper we use the recently developed S-transform to analyse two segments of the Wolf Sunspot series, a seismogram, and a synthetic 2-D image as examples of applications of the S-transform for time-local and space-local spectral analysis. (C) 1997 Elsevier Science B.V.