  The average unsigned magnetic flux density in magnetograms of the quiet Sun
is generally dominated by instrumental noise. Due to the entirely different
scaling behavior of the noise and the solar magnetic pattern it has been
possible to determine the standard deviation of the Gaussian noise distribution
and remove the noise contribution from the average unsigned flux density for
the whole 15-yr SOHO/MDI data set and for a selection of SDO/HMI magnetograms.
There is a very close correlation between the MDI disk-averaged unsigned
vertical flux density and the sunspot number, and regression analysis gives a
residual level of 2.7 G when the sunspot number is zero. The selected set of
HMI magnetograms, which spans the most quiet phase of solar activity, has a
lower limit of 3.0 G to the noise-corrected average flux density. These
apparently cycle-independent levels may be identified as a basal flux density,
which represents an upper limit to the possible flux contribution from a local
dynamo, but not evidence for its existence. The 3.0 G HMI level, when scaled to
the Hinode spatial resolution, translates to 3.5 G, which means that the much
higher average flux densities always found by Hinode in quiet regions do not
originate from a local dynamo. The contributions to the average unsigned flux
density come almost exclusively from the extended wings of the probability
density function (PDF), also in the case of HMI magnetograms with only
basal-level magnetic flux. These wings represent intermittent magnetic flux.
While the global dynamo appears to dominate the magnetic energy spectrum at all
the resolved spatial scales, there are indications from the observed Hanle
depolarization in atomic lines that the local dynamo may dominate the spectrum
at scales of order 1-10 km and below.
