  A common assumption used in the study of accretion disks is that the magnetic
energy density and the kinetic energy density should be in equipartition. This
assumption relies on the faster growth rate of the magnetic field strength
against the kinetic energy of the particles in the flow, for decreasing radius,
combined with a dissipation mechanism that tends towards equipartition. In this
paper, we examine this assumption by modeling the radio, mm and optical spectra
of several black hole binaries in their quiescent state. We use a standard
two-component disk model, consisting of an inner geometrically thick and
optically thin disk, emitting thermal synchrotron radiation, along with an
outer, thin disk, which radiates as a multicolor blackbody. We find that at the
low accretion rates typical of the quiescent state, the spectral shape is
qualitatively reproduced using magnetic fields that are between 0.1% and 1% of
the equipartition value, considerably smaller than previously thought. We
discuss our findings in view of (1) the launching of jets in these objects,
which is commonly believed to rely on the presence of a strong magnetic field
in the central region of the disk; and (2) the role of magnetic dissipation in
the structure of the inflow.
