  We study the contradictory magnetic field strength distributions retrieved
from independent analyses of spectropolarimetric observations in the
near-infrared (1.56 micron) and in the visible (630 nm) at internetwork
regions. In order to solve this apparent controversy, we present simultaneous
and co-spatial 1.56 micron and 630 nm observations of an internetwork area. The
properties of the circular and linear polarization signals, as well as the
Stokes V area and amplitude asymmetries, are discussed. As a complement,
inversion techniques are also used to infer the physical parameters of the
solar atmosphere. As a first step, the infrared and visible observations are
analysed separately to check their compatibility. Finally, the simultaneous
inversion of the two data sets is performed. The magnetic flux densities
retrieved from the individual analysis of the infrared and visible data sets
are strongly correlated. The polarity of the Stokes V profiles is the same at
co-spatial pixels in both wavelength ranges. This indicates that both 1.56
micron and 630 nm observations trace the same magnetic structures on the solar
surface. The simultaneous inversion of the two pairs of lines reveals an
internetwork full of sub-kG structures that fill only 2 % of the resolution
element. A correlation is found between the magnetic field strength and the
continuum intensity: equipartition fields (B ~500 G) tend to be located in dark
intergranular lanes, whereas weaker field structures are found inside granules.
The most probable unsigned magnetic flux density is 10 Mx/cm2. The net magnetic
flux density in the whole field of view is nearly zero. This means that both
polarities cancel out almost exactly in our observed internetwork area.
