  Electron density profiles of the Venus' ionosphere are inverted from the
Venus Express (VEX) one-way open-loop radio occultation experiments carried out
by Shanghai 25 m antenna from November 2011 to January 2012 at solar maximum
conditions and by New Norcia 35 m antenna from August 2006 to June 2008 at
solar intermediate conditions. The electron density profile (from 110 km to 400
km) retrieved from the X-band egress observation at Shanghai station, shows a
single peak near 147 km with a peak density of about $2 \times 10^4
\rm{cm}^{-3}$ at a solar zenith angle of 94$^{\circ}$. As a comparison, the VEX
radio science (VeRa) observations at New Norcia station were also examined,
including S-, X-band and dual-frequency data in the ingress mode. The results
show that the electron density profiles retrieved from the S-band data are more
analogous to the dual-frequency data in the profile shape, compared with the
X-band data. Generally, the S-band results slightly underestimate the magnitude
of the peak density, while the X-band results overestimate that. The
discrepancy in the X-band profile is probably due to the relatively larger
unmodeled orbital errors. It is also expected that the ionopause height is
sensitive to the solar wind dynamical pressure in high and intermediate solar
activities, usually in the range of 200 km - 1000 km on the dayside and much
higher on the nightside. Structural variations ("bulges" and fluctuations) can
be found in the electron density profiles in intermediate solar activity, which
may be caused by the interaction of the solar wind with the ionosphere.
Considerable ionizations can be observed in the Venus' nightside ionosphere,
which are unexpected for the Martian nightside ionosphere in most cases.
