Density wavelet analysis. (a) Density along time measured at the central pixel of the B20 case. (b) Wavelet power spectrum. The black cross-lines show the part of the spectrum that is inside the cone of influence. The dominant period of ∼7 s is enclosed by the black contours with a 99% confidence level. (c) Fourier power spectrum (solid line) and the 99% confidence spectrum (dashed line).