Spectra [of sequences]
The spectra shown are given by Abs[Fourier[data]], where the symmetrical second half of this list is dropped in the pictures. Also of relevance are intensity or power spectra, obtained as the square of these spectra. These are related to the autocorrelation function according to
Fourier[list]2 Fourier[ListConvolve[list, list, {1, 1}]]/Sqrt[Length[list]]
(See also page 1074.)