  Non-thermal X-ray emission in compact accretion engines can be interpreted to
result from magnetic dissipation in an optically thin magnetized corona above
an optically thick accretion disk. If coronal magnetic field originates in the
disk and the disk is turbulent, then only magnetic structures large enough for
their turbulent shredding time to exceed their buoyant rise time survive the
journey to the corona. We use this concept and a physical model to constrain
the minimum fraction of magnetic energy above the critical scale for buoyancy
as a function of the observed coronal to bolometric emission. Our results
suggest that a significant fraction of the magnetic energy in accretion disks
resides in large scale fields, which in turn provides circumstantial evidence
for significant non-local transport phenomena and the need for large scale
magnetic field generation. For the example of Seyfert AGN, for which of order
30 per cent of the bolometric flux is in the X-ray band, we find that more than
20 per cent of the magnetic energy must be of large enough scale to rise and
dissipate in the corona.
