Modeling the F2 topside and plasmasphere for IRI using IMAGE/RPI and ISIS data B.W. Reinisch a, * , P. Nsumei a , X. Huang a , D.K. Bilitza b a Environmental, Earth and Atmospheric Sciences Department, Center for Atmospheric Research, University of Massachusetts, Lowell, MA 01854, USA b Raytheon ITSS, NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA Received 28 February 2006; received in revised form 25 April 2006; accepted 22 May 2006 Abstract 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 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 top- side electron density in the IRI model and to extend the IRI description into the plasmasphere. An a-Chapman function with a contin- uously varying scale height, dubbed a vary-Chap function, is used to describe the topside F2 vertical electron density profile N(h) that seamlessly connects the ionosphere with the plasmasphere. The Chapman scale height H(h) varies only slowly with height near hmF2 and increases rapidly at the O + to light–ion transition height. A hyperbolic tangent function suitably represents this variation. New plasma- sphere density profile data from the IMAGE/RPI measurements and topside profiles from the ISIS topside sounders are used to con- struct a continuous profile from hmF2 to several R E altitude. Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Topside ionosphere; Plasmasphere; Profiles; Chapman function; IRI 1. Introduction Comparisons with recently analyzed ionospheric topside sounder measurements from the ISIS satellites (Franklin and MacLean, 1969) and also with measured total electron content (TEC) data show shortcomings in the International Reference Ionosphere (IRI) representation of the topside electron density profile (Coisson et al., 2002; Triskova et al., 2002; Bilitza, 2004). Rawer et al. (1978) started the development of the IRI topside model based on the Bent et al. (1972) data compilation that divides the topside into three regions with different scale heights. Recent measure- ments from the radio plasma imager (RPI) on NASA’s IMAGE satellite (Reinisch et al., 2000, 2001; Burch et al., 2001) offer the possibility of extending the topside profile into the plasmasphere. Based on the measured RPI profiles Huang et al. (2004) proposed an empirical plasmasphere model with plasma density profiles extending from several Earth radii (R E ) down to h 0 = 3000 km or lower. Given the well-tested IRI bottomside profile model (Bilitza, 2001), the task at hand is to construct a new top- side profile model from hmF2 to h 0 that smoothly connects the topside profile to the plasmasphere model. This paper studies the feasibility of new techniques for the develop- ment of such a model. 1.1. Profile data and models The Bent et al. (1972) topside model was based on some 40,000 topside profiles obtained from Alouette 1 ionograms. More topside profiles from the Alouette/ISIS ionograms became available in later years from worldwide manual scaling efforts. Recognizing the importance of the Alouette/ISIS ionograms NASA has, in the framework of its AISRP program, supported the digitization of a large volume of analog Alouette/ISIS ionograms and the subsequent inversion to electron density profiles using 0273-1177/$30 Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2006.05.032 * Corresponding author. Tel.: +1 978 934 4903; fax: +1 978 459 7915. E-mail address: Bodo_Reinisch@uml.edu (B.W. Reinisch). www.elsevier.com/locate/asr Advances in Space Research 39 (2007) 731–738