  We derive a first order linear differential equation describing the shape of
the probability density function of magnetic field strengths in the quiet Sun
(PDF). The modeling is very schematic. It considers convective motions which
continuously supply and withdraw magnetic structures. In addition, a magnetic
amplification mechanism increases the field strength up to a threshold that
cannot be exceeded. These three basic ingredients provide PDFs in good
agreement with the PDFs produced by realistic numerical simulations of magneto
convection, as well as with quiet Sun PDFs inferred from observations. In
particular, the distribution is approximately lognormal, and it produces an
excess of magnetic fields (i.e., a 'hump' in the distribution) right before the
maximum field strength. The success of this simple model may indicate that only
a few basic ingredients shape the quiet Sun PDF. Our approach provides a
concise parametric representation of the PDF, as required to develop automatic
methods of diagnostics.
