  Using a set of 28 high resolution, high signal to noise ratio (S/N) QSO
Ly-alpha absorption spectra, we investigate the non-Gaussian features of the
transmitted flux fluctuations, and their effect upon the power spectrum of this
field. We find that the spatial distribution of the local power of the
transmitted flux on scales k >= 0.05 s/km is highly spiky or intermittent. The
probability distribution functions (PDFs) of the local power are long-tailed.
The power on small scales is dominated by small probability events, and
consequently, the uncertainty in the power spectrum of the transmitted flux
field is generally large. This uncertainty arises due to the slow convergence
of an intermittent field to a Gaussian limit required by the central limit
theorem (CLT). To reduce this uncertainty, it is common to estimate the error
of the power spectrum by selecting subsamples with an "optimal" size. We show
that this conventional method actually does not calculate the variance of the
original intermittent field but of a Gaussian field. Based on the analysis of
intermittency, we propose an algorithm to calculate the error. It is based on a
bootstrap re-sampling among all independent local power modes. This estimation
doesn't require any extra parameter like the size of the subsamples, and is
sensitive to the intermittency of the fields. This method effectively reduces
the uncertainty in the power spectrum when the number of independent modes
matches the condition of the CLT convergence.
