  We analyze the ultraviolet spectrum of the cataclysmic variable GK Per at
maximum light. The flat ultraviolet spectrum in this system requires a
truncated inner accretion disk and an unusually flat radial temperature
profile. This requirement is not satisfied by any non-magnetic steady or
non-steady disk model. We consider a magnetized accretion disk model to explain
the ultraviolet spectrum. The available data on the white dwarf spin and
possible quasi-periodic oscillations constrain the magnetic field, $B_{*}$, and
the disk accretion rate, ${\dot M}$, to lie along a well-defined
spin-equilibrium condition $({\dot M}/10^{17} ~ g ~ s^{-1}) \sim
100(B_{*}/10^7G)^2$. Our self-consistent treatment of the magnetic torque on
the disk flattens the disk temperature distribution outside the disk truncation
radius. This modified temperature distribution is too steep to explain the UV
spectrum for reasonable field strengths. X-ray heating is a plausible
alternative to magnetic heating in GK Per. We estimate that the disk intercepts
$\sim$ 5% of the accretion energy in outburst, which results in an extra disk
luminosity of $\sim$ 5--10 $L_{\odot}$. Model spectra of optically thick disks
are too blue to match observations. The UV spectrum of an optically thick disk
with an optically thin, X-ray heated corona resembles the observed spectrum.
The X-ray luminosity observed during the outburst indicates ${\dot M}<10^{18} ~
g ~ s^{-1}$, which is a factor of 10 lower than that required to explain the
ultraviolet luminosity. Radiation drag on material flowing inward along the
accretion column lowers the shock temperature and reduces the X-ray luminosity.
Most of the accretion energy is then radiated at extreme ultraviolet
wavelengths.
