F-REGION CLIMATOLOGY DURING THE SUNDIAL ATLAS-1 CAMPAIGN OF MARCH 1992 - MODEL-MEASUREMENT COMPARISONS AND CAUSE-EFFECT RELATIONSHIPS/

We present the first joint comparison of global measurements of F regi on characteristics with three models used widely in the specification of the ionospheric-thermospheric system. The models, the International Reference Ionosphere (IRI), the field line interhemispheric plasma (F LIP) model, and the Thermospheric-Ionospheric General Circulation Mode l (TIGCM), represent a unique set of capabilities with major differenc es in approaches to the prevailing physics and different levels of com putational complexity. The database was developed by a global network of 53 ionosonde stations operating around-the-clock for the period Mar ch 22 through April 4, 1992 in collaboration with the ATLAS 1 mission. The emphasis is on the F region characteristics of peak heights (h(m) F(2)) and densities (NmF2), their climatological (i.e., average) behav ior during the ATLAS 1 period, and associated cause-effect relationshi ps. We explore latitudinal and local time variations with attention to the influences of meridional winds and plasmaspheric fluxes in the ma intenance of different domains in the ionospheric-thermospheric system . We find that all three models tend to underestimate the values of h( m)F(2) and NmF2 with the largest discrepancies in NmF2 resulting in th e FLIP and TIGCM representations at night. These discrepancies can gro w to levels as large as 110% near 0400 LT, a ''rediscovery'' of the ol d but unsettled issue of maintenance of the nighttime ionosphere. This nighttime discrepancy is traceable in first order to model underestim ates of prevailing meridional winds. The contributions of plasmaspheri c fluxes are also considered, with the conclusion that they are of sec ondary importance, but substantially more work is necessary to uniquel y quantify their role. In contrast to their nighttime characteristics, the FLIP and TIGCM generally have excellent agreement (i.e., 6 +/- 6% ) with daytime observations of NmF2, and the IRI tends to underestimat e the observed values of NmF2 by a nominally LT-insensitive level of 2 8 +/- 6%. Other campaign results are reviewed in this issue, with a fo cus on regional responses to the prevailing conditions and their chara cterization in terms of latitudinal distributions off region heights a nd densities.