Global ionospheric maps from GPS observations using modip latitude

In recent years, the new possibility of estimating the global distribution of free electrons in the ionosphere by means of GPS has opened a very active and promising field of research. Nowadays, there is a variety of approaches for processing dual-frequency GPS observations to produce global ionospheric maps of vertical total electron content (vTEC) with temporal resolution of 2 h or less. At the University of La Plata, we have developed the so-called La Plata Ionospheric Model (LPIM), based on a spherical harmonic expansion. For a better modelling of the variability of the vTEC, a solar-fixed coordinate system and the geomagnetic latitude are usually adopted. Early investigations by Rawer, K. [Encyclopedia of Physics, Geophysics III, Part VII. Springer-Verlag, pp. 389-391, 1984.] demonstrated the benefit of using the modified dip latitude (modip) to describe free electron distribution on the F2 layer at mid and low latitude. We have updated the LPIM to use modip latitude instead of the geomagnetic one. Exhaustive comparisons against Topex vTEC measurements provided by Aviso - performed for 90 days of 1999 - have demonstrated that the use of modip significantly improved the agreement between GPS and Topex. (C) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.