  We present calculations of the heliospheric SWCX emission spectra and their
contributions in the ROSAT 1/4 keV band. We compare our results with the soft
X-ray diffuse background (SXRB) emission detected in front of 378 identified
shadowing regions during the ROSAT All-Sky Survey (Snowden et al. 2000). This
foreground component is principally attributed to the hot gas of the so-called
Local Bubble (LB), an irregularly shaped cavity of ~50-150 pc around the Sun,
which is supposed to contain ~10^6 K plasma. Our results suggest that the SWCX
emission from the heliosphere is bright enough to account for most of the
foreground emission towards the majority of low galactic latitude directions,
where the LB is the least extended. In a large part of directions with galactic
latitude above 30deg the heliospheric SWCX intensity is significantly smaller
than the measured one. However, the SWCX R2/R1 band ratio differs slightly from
the data in the galactic center direction, and more significantly in the
galactic anti-centre direction where the observed ratio is the smallest.
Assuming that both SWCX and hot gas emission are present and their relative
contributions vary with direction, we tested a series of thermal plasma spectra
for temperatures ranging from 10^5 to 10^6.5 K and searched for a combination
of SWCX spectra and thermal emission matching the observed intensities and band
ratios, while simultaneously being compatible with O VI emission measurements.
In the frame of collisional equilibrium models and for solar abundances, the
range we derive for hot gas temperature and emission measure cannot reproduce
the Wisconsin C/B band ratio. We emphasize the need for additional atomic data,
describing consistently EUV and X-ray photon spectra of the charge-exchange
emission of heavier solar wind ions.
