  The purpose of this paper is to assess the feasibility of axion detection by
X-ray spectroscopy of the sun. We review the theory of axion-photon mode
conversion with special attention to axions emitted in the 14.4 keV M1 decay of
57Fe at the solar center. These then mode convert to photons in the outer
layers of the solar envelope, and may in principle be detected subsequently as
X-rays. For axion masses above about 10^-4 eV, resonant mode conversion at a
layer where the axion mass matches the local electron plasma frequency is
necessary. For axion masses above about 10^-2 eV, this mode conversion occurs
too deep in the solar atmosphere for the resulting photon to escape the solar
surface and be detected before Compton scattering obscures the line. At the
(detectable) axion masses below this, the flux of mode converted photons
predicted by axion models appears to be too low for detection to be feasible
with current instrumentation. Nonresonant mode conversion for axion masses
below 10^-4 eV is also plausible, but with still lower predicted fluxes, since
the axion coupling constant is related to it mass. Prospects for meaningful
constraints on massive axion parameters from X-ray observations of this
transition from the Sun do not appear to be promising. However parameters for
massless counterparts (e.g. the "arion") may still result from such
observations. It may mode convert in the outer layers of the solar atmosphere,
but is not restricted by this to have a small coupling constant.
