Normal modes and discovery of high-order cross-frequencies in the DBV white dwarf GD 358

We present a detailed mode identification performed on the 1994 Whole Earth Telescope (WET) run on GD 358. The results are compared with that obtained for the same star from the 1990 WET data. The two temporal spectra show ve ry few qualitative differences, although amplitude changes are seen in most modes, including the disappearance of the mode identified as k = 14 in the 1990 data. The excellent coverage and signal-to-noise ratio obtained durin g the 1994 run lead to the secure identification of combination frequencies up to fourth order, i.e. peaks that are sums or differences of up to four parent frequencies, including a virtually complete set of second-order freq uencies, as expected from harmonic distortion. We show how the third-order frequencies are expected to affect the triplet structure of the normal mode s by back-interacting with them. Finally, a search for l = 2 modes was unsu ccessful, not verifying the suspicion that such modes had been uncovered in the 1990 data set.