Jupiter's
ionosphere peaks between ~600-2000 km altitude. The lower peak is better
suited for Chapman values, where the temperature would be about 600 K and
the neutral density is of order 10^17
m^-3.
Relevant references might be Yelle and Miller (2004; Jupiter book, chapter
9) and Barrow and Matcheva (2011; Icarus).
The
situation is similar at Saturn, with ionospheric peaks between ~1000-3000
km. Typical values near the lower peak might be 300 K for temperature and
~10^11
cm^-3
for neutral density. A good reference for Saturn would be Nagy et al
(2009; Saturn After Cassini-Huygens book).
Altitudes
in the giant planets are referenced to the 1 bar pressure level. Both
Jupiter and Saturn can be treated as being dominated by H2 in the neutrals
and H3+ in the ions.