Quoted from: https://ccmc.gsfc.nasa.gov/modelweb/ionos/ursi.html

Parameter: F2-peak plasma frequency, foF2

Brief Description:
The URSI set of model coefficients is the result of efforts undertaken by the International Union of Radio Science (URSI) Working Group G.5. The numerical mapping method is the same as that explained for the CCIR foF2 model. It has long been known that the CCIR model has its shortcomings above the oceans and in the southern hemisphere, where ionosonde measurements do not exist or are sparse. Rush et al. (1983, 1984) used aeronomic theory to fill the data gaps before applying the spherical harmonics mapping procedure. Fox and McNamara (1988) established the final URSI coefficients for the combined data base of Rush's values and about 45,000 station-months of ionosonde data (about 180 stations worldwide).

Availability: This data set is included in the International Reference Ionosphere.

References:
C. M. Rush, M. PoKempner, D. N. Anderson, F. G. Stewart, and J. Perry, Improving Ionospheric Maps Using Theoretically Derived Values of foF2, Radio Sci. 18, 95, 1983.

C. M. Rush, M. PoKempner, D. N. Anderson, J. Perry, F. G. Stewart, and R. Reasoner, Maps of foF2 Derived from Observations and Theoretical Data, Radio Sci. 19, 1083, 1984.

D. Bilitza, K. Rawer, S. Pallaschke, C. M. Rush, N. Matuura, and W. R. Hoegy, Progress in Modeling the Ionospheric Peak and Topside Electron Density, Adv. Space Res. 7, #6, 5, 1987.

K. Davies (Chairman), Report of URSI Working Group G.5 on Mapping of Characteristics at the Peak of the F2-Layer, URSI Information Bulletin 243, 93, 1987.

M. W. Fox and L. F. McNamara, Improved World-Wide Maps of Monthly Median foF2, J. Atmos. Terr. Phys. 50, 1077, 1988.

C. M. Rush, M. Fox, D. Bilitza, K. Davies, L. McNamara, F. Stewart, and M. PoKempner, Ionospheric Mapping: An Update of foF2 Coefficients, Telecomm. J. 56, 179, 1989.