  Data assimilation procedures have been developed for thermospheric models
using satellite density measurements as part of the EU Framework Package 7
ATMOP Project. Two models were studied; one a general circulation model,
TIEGCM, and the other a semi-empirical drag temperature model, DTM. Results of
runs using data assimilation with these models were compared with independent
density observations from CHAMP and GRACE satellites throughout solar cycles 23
and 24. Time periods of 60 days were examined at solar minimum and maximum,
including the 2003 Hallowe'en storms. The differences between the physical and
the semi-empirical models have been characterised. Results indicate that both
models tend to show similar behaviour; underestimating densities at solar
maximum, and overestimating them at solar minimum. DTM performed better at
solar minimum, with both models less accurate at solar maximum. A mean
improvement of ~4% was found using data assimilation with TIEGCM. With further
improvements, the use of general circulation models in operational space
weather forecasting (in addition to empirical methods currently used) is
plausible. Future work will allow near-real-time assimilation of thermospheric
data for improved forecasting.
