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We suggest that magnetic fields in the accretion disks of AGN reach into thecoronae above and have a profound effect on the mass flow rate in the corona.This strongly affects the location where the accretion flow changes from ageometrically thin disk to a pure vertically extended coronal oradvection-dominated accretion flow (ADAF). We show that this can explain thedifferent disk truncation radii in elliptical galaxies and low luminosity AGNwith about the same mass flow rate, a discrepancy pointed out by Quataert etal. (1999). Without disk magnetic activity the disk truncation is expected tobe uniquely related to the mass flow rate (Meyer et al. 2000b). Whether dynamoaction occurs depends on whether the electrical conductivity measured by amagnetic Reynolds number surpasses a critical value (Gammie and Menou 1998).Inelliptical galaxies the disk is self-gravitating at the radii where thetruncation should occur. It is plausible that instead of a cool disk a ``layerof clouds'' may form (Shlosman et al. 1990, Gammie 2001) for which no dynamoaction is expected. For low luminosity AGN the magnetic Reynolds number is wellabove critical. Simple model calculations show that magnetic fields in theunderlying disks reduce the strength of the coronal flow and shift thetruncation radius significantly inward.Comment: 4 pages, 1 figure; accepted for publication in A&A Letter

The effect of disk magnetic fields on the truncation of geometrically thin disks in AGN