  We present the first self-consistent theoretical models of magnetar spectra
that take into account the combined effects of the stellar atmosphere and its
magnetosphere. We find that the proton cyclotron lines that are already
weakened by atmospheric effects become indistinguishable from the continuum for
moderate scattering optical depths in the magnetosphere. Furthermore, the hard
excess becomes more pronounced due to resonant scattering and the resulting
spectra closely resemble the observed magnetar spectra. We argue that while the
absence of proton cyclotron lines in the observed spectra are inconclusive
about the surface field strengths of magnetars, the continuum carries nearly
unique signatures of the field strength and can thus be used to infer this
quantity. We fit our theoretical spectra with a phenomenological two-blackbody
model and compare our findings to source spectra with existing two-blackbody
fits. The field strengths that we infer spectroscopically are in remarkable
agreement with the values inferred from the period derivative of the sources
assuming a dipole spindown. These detailed predictions of line energies and
equivalent widths may provide an optimal opportunity for measuring directly the
surface field strengths of magnetars with future X-ray telescopes such as {\em
Constellation}-X.
