  We adopt an isotropic representation of the Fourier-transformed two-point
correlation tensor of the magnetic field to estimate the magnetic energy and
helicity spectra as well as current helicity spectra of two individual active
regions (NOAA 11158 and NOAA 11515) and the change of the spectral indices
during their development as well as during the solar cycle. The departure of
the spectral indices of magnetic energy and current helicity from 5/3 are
analyzed, and it is found that it is lower than the spectral index of the
magnetic energy spectrum. Furthermore, the fractional magnetic helicity tends
to increase when the scale of the energy-carrying magnetic structures
increases. The magnetic helicity of NOAA 11515 violates the expected
hemispheric sign rule, which is interpreted as an effect of enhanced field
strengths at scales larger than 30-60Mm with opposite signs of helicity. This
is consistent with the general cycle dependence, which shows that around the
solar maximum the magnetic energy and helicity spectra are steeper, emphasizing
the large-scale field.
