New approach to an IRI topside profile Bodo W. Reinisch, Patrick Nsumei, and Xueqin Huang The topside electron density profile can be modeled as an a-Chapman function with a varying scale height H(h) [Rishbeth and Garriott, 1969]: The value of H at the F2 layer peak, Hm = H(hmF2), can be calculated from the bottomside Ne-profile [Reinisch and Huang, 2001] as defined by the IRI-2000 [Bilitza, 2001]. Assuming a constant scale height H = Hm would lead to too low densities for altitudes higher than ~300 km above hmF2, but the profiles up to this altitude compare well with measured incoherent scatter radar profiles [Reinisch et al., 2001; 2004). Instead we have used Ne(h) measurements by the Radio Plasma Imager (RPI) on NASA's IMAGE satellite [Nsumei et al., 2003, Huang et al., 2004] in the polar cap and the plasmasphere for altitudes from ~2000 Ð7000 km to determine the function H(h) that assures a seamless connection from the IRI F2 peak to the plasmasphere. Integration over different sections of these profiles reveals the contribution of the bottom side, topside, and plasmasphere to the total vertical electron content. We will show how the modeled profiles compare with profiles derived from topside ionograms. Some 20,000 ISIS II ionograms have recently been processed by a new software algorithm, TOPIST [Huang et al., 2002], which automatically generates topside profiles up to 1400 km altitude. _______________ IRI 2005 Workshop, Observatori de l'Ebre Roquetes/Tortosa, Spain, 27 June - 1 July 2005