  We present an XMM-Newton spectrum of diffuse X-ray emission from within the
solar system. The spectrum is dominated by probable C VI lines at 0.37 keV and
0.46 keV, an O VII line at 0.56 keV, O VIII lines at 0.65 keV and ~0.8 keV, Ne
IX lines at ~0.92 keV, and Mg XI lines at ~1.35 keV. This spectrum is
consistent with that expected from charge exchange emission between the highly
ionized solar wind and either interstellar neutrals in the heliosphere or
material from Earth's exosphere. The emission is clearly seen as a low-energy
(E<1.5 keV) spectral enhancement in one of a series of four observations of the
Hubble Deep Field North. The X-ray enhancement is concurrent with an
enhancement in the solar wind measured by ACE, Wind, and SoHO spacecraft. The
solar wind enhancement reaches a flux level an order of magnitude more intense
than typical fluxes at 1 AU, and has a significantly enhanced O^{+7}/O^{+6}
ratio. Besides being of interest in its own right for studies of the solar
system, this emission can have significant consequences for observations of
cosmological objects. It can provide emission lines at zero redshift which are
of particular interest in studies of diffuse thermal emission (e.g., O VII and
O VIII), and which can therefore act as contamination in the spectra of objects
which cover the entire detector field of view. We propose the use of solar wind
monitoring data as a diagnostic to screen for such possibilities.
