  We analyze the power spectrum of a QSO's Ly_alpha transmitted flux in the
discrete wavelet transform (DWT) representation. Although the mean DWT power
spectrum is consistent with its counterpart in Fourier representation, the
spatial distribution of the local power varies greatly, i.e. the local DWT
power spectra show remarkably spiky structures on small scales. To measure
these spiky features, we introduce the quantities, roughness, of the local
power spectrum, and the correlation between spikes on different scales. We then
test the predictions made by the correlation hierarchy model on the roughness
and the scale-scale correlations of the local power spectrum. Using the
Ly_alpha transmitted flux of the QSO HS1700, we find that the underlying cosmic
mass field of the transmitted flux at redshift around z ~ 2.2 can be described
by the hierarchical clustering model on physical scales from 2.5 h^{-1} Mpc to
few tens h^{-1} kpc in an Einstein-de Sitter universe. However, the non-linear
features of the clustering show differences on different scale ranges; 1. On
physical scales larger than ~ 1.3 h^{-1} Mpc, the field is almost Gaussian. 2.
On scales 1.3 h^{-1} Mpc - 0.3 h^{-1} Mpc, the field is consistent with the
correlation hierarchy with a constant value for the coefficient Q_4. 3. On
scales less than 300 h^{-1} kpc, the field is no longer Gaussian, but
essentially intermittent. In this case, the field can still be fitted by the
correlation hierarchy, but the coefficient, Q_4, should be scale-dependent.
These three points are strongly supported by the following result: the scale
dependencies of Q_4 given by two statistically independent measures, i.e. Q_4^R
by the roughness and Q_4^C by scale-scale correlation, are the same in the
entire scale range considered.
