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This paper studies the response of a thin accretion disk to an externalradial magnetic field. Our focus is on protoplanetary disks (PPDs), which areexposed during their later evolution to an intense, magnetized wind from thecentral star. A radial magnetic field is mixed into a thin surface layer, iswound up by the disk shear, and is pushed downward by a combination ofturbulent mixing and ambipolar and Ohmic drift. The toroidal field reaches muchgreater strengths than the seed vertical field that is usually invoked in PPDmodels, even becoming superthermal. Linear stability analysis indicates thatthe disk experiences the magnetorotational instability (MRI) at a highermagnetization than a vertically magnetized disk when both the effects ofambipolar and Hall drift are taken into account. Steady vertical profiles ofdensity and magnetic field are obtained at several radii between 0.06 and 1 AUin response to a wind magnetic field $B_r \sim (10^{-4}$-$10^{-2})(r/{\rmAU})^{-2}$ G. Careful attention is given to the radial and vertical ionizationstructure resulting from irradiation by stellar X-rays. The disk is morestrongly magnetized closer to the star, where it can support a higher rate ofmass transfer. As a result, the inner $\sim 1$ AU of a PPD is found to evolvetoward lower surface density. Mass transfer rates around $10^{-8}\,M_\odot$yr$^{-1}$ are obtained under conservative assumptions about the MRI-generatedstress. The evolution of the disk, and the implications for planet migration,are investigated in the accompanying paper.

Radially Magnetized Protoplanetary Disk: Vertical Profile