Empirical model of O + –H + transition height based on topside sounder data P. Marinov a , I. Kutiev b , S. Watanabe c, * a Central Laboratory for Parallel Processing, Bulgarian Academy of Sciences, Sofia, Bulgaria b Geophysical Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria c Graduate School of Science, Hokkaido University, Sapporo, Japan Received 28 January 2004; received in revised form 6 June 2004; accepted 8 July 2004 Abstract A new model of the O + –H + transition height (denoted as THM) is developed, based on vertical electron density profiles from topside ionosondes. The model provides the transition height as a function of month of the year, local time, geomagnetic latitude, longitude and solar flux F 107 . To define TH, the O + scale height is approximated by the lowest gradient in the measured profile and the O + profile is reconstructed. TH is taken at the height where O + density becomes half of total electron density. The model data base contains 170,033 TH values, sufficiently sampling all parameterÕs ranges. THM describes the transition height by a multivar- iable polynomial consisted with ChebishevÕs and trigonometric base functions, which is fitted to the data in the five-dimensional space. The model results are compared with other available models. The comparison shows that THM predictions agree in general with those of the other models, but THM variations along latitude, longitude and local time have larger amplitudes. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. 1. Introduction The height at which O + and H + densities are equal is an important characteristic of the ionospheric structure. This height is considered as the boundary dividing the O + dominated ionosphere from H + (or light ions) dom- inated plasmasphere. Several models has been developed to help various aeronomical studies and ionospheric applications, all of them based on satellite data. Tithe- ridge (1976) has extracted the transition height (TH) by fitting the topside electron density profiles from Alouette-1 sounder with theoretical models. Using 60,000 profiles measured between 1962 and 1968, he ob- tained transition height variations in various seasons and levels of solar activity. Miyazaki (1979) has con- structed simple model of TH, based on TAIYO satellite data. Kutiev et al. (1984) used 1400 direct encounters of TH from OGO 6 (1969–1970) to obtain a formula, rep- resenting TH within ±50° dip latitude and all longitude for 20 and 02 h local time. Kutiev et al. (1994) further developed the previous model by including data from Intercosmos-2 and limited portions of Alouette-1 and ISS b satellite data. This model (further denoted as K94), represented TH in five dimensions: sunspot num- ber, month of the year, local time, dipole latitude and longitude. It was based on a generalized multivariable polynomial, containing a system of linearly independent functions. The present model (further denoted as THM) is based of the vertical density profiles, inferred by the topside sounders on Alouette-1, -2 and -3 and ISIS-1 and -2 sat- ellites. It uses similar mathematical approach as in Ku- tiev et al. (1994). The input parameters are local time, month of the year, geomagnetic latitude (glat), solar flux F 10.7 and longitude (long). The gradient and transition height are extracted from each individual profile and accumulated in six-dimensional bins, defined by input 0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2004.07.012 * Corresponding author. Tel.: +81 011 706 2757. E-mail address: shw@ep.sci.hokudai.ac.jp (S. Watanabe). www.elsevier.com/locate/asr Advances in Space Research 34 (2004) 2021–2025