  Based on the Meso-Structured Magnetic Atmosphere (MESMA) approximation,
recently introduced by Carroll & Kopf (2007), we present first results of an
inversion of spectropolarimetric observations obtained from solar internetwork
regions. To cope with the inherent complexity of the mostly unresolved magnetic
field in the solar photosphere the MESMA approach provides a statistical
description of the underlying atmosphere in terms of a random Markov field.
This statistical model allows us to derive a stochastic transfer equation for
polarized light. The stochastic transfer equation explicitly accounts for the
spatial correlation -- the characteristic length scale -- of the underlying
magnetic and non-magnetic structures. We use this new diagnostic parameter in
an inversion approach to demonstrate that the magnetic flux structures in the
solar internetwork possess a finite correlation length which is not compatible
with the classical flux tube picture.
