Modeling the F2 Topside and Plasmasphere for IRI

P. Nsumei, B.W. Reinisch, X. Huang, D.K. Bilitza, and D.L. Gallagher, 


Empirical models are an important tool for the study of the different geospace
regions from Earth to sun, providing the user with easy access to a synthesis of
most or all reliable measurements from ground and space for specific parameters
and regions. This paper describes a new effort to develop a coherent model of
the topside F2 layer and the plasmasphere with the goal to improve the
representation of the topside electron density in the IRI model and to extend
the IRI description into the plasmasphere. An _-Chapman function with a
continuously varying scale-height [Rishbeth and Garriott, 1969], which we refer
to as a vari-Chap function, is used to describe the topside F2 vertical electron
density profile N(h) that seamlessly connects the ionosphere with the
plasmasphere. While the Chapman (neutral) scale-height H(h) varies only slowly
near hmF2, it increases rapidly at the O+ to light-ion transition height. A
hyperbolic tangent function is suitably representing this variation. New
plasmasphere density profile data from the IMAGE/RPI measurements and topside
profiles from the ISIS topside sounders are used to construct a continuous
profile from hmF2 to several RE altitude. A large number of ISIS-2 topside
profiles have recently become available through the TOPIST-processing of the
digitized ISIS-2 ionograms. We represent these profiles in terms of vari-Chap
functions using a scale-height function that is specified by three parameters:
the height hT and scale-height HT of the O+ to H+ transition and a shape factor
that controls the gradient of H(h) at the transition height:

H(h) = HT + [(HmÐHT)/tanh(b)]  tanh [(h-hT)/(hmF2-hT)]  hmF2 <= h <= 3000km

The parameters show a systematic variation with latitude. The task of matching
the topside profiles to the RPI measured profiles in the plasmasphere and polar
cap is under development.

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Presented at the URSI National Radio Science Meeting, University of Colorado at 
Boulder, January 4-7, 2006