ANNALS OF GEOPHYSICS, 58, 5, 2015, A0551; doi:10.4401/ag-6721 A0551 Evaluation of a global model of ionospheric slab thickness for foF2 estimation during geomagnetic storm Buldan Muslim 1,* , Haris Haralambous 2 , Christina Oikonomou 3 , Sefria Anggarani 1 1 Space Science Center, LAPAN, Bandung, West Java, Indonesia 2 Frederick University, Nicosia, Cyprus 3 Frederick Research Center, Frederick University, Cyprus ABSTRACT This study aims to evaluate the performance of the global model of ionos- pheric slab thickness (GMIST) in terms of F2 layer critical frequency (foF2) estimation during geomagnetic disturbed conditions. Hourly val- ues of foF2 as obtained from ionosonde stations located at equatorial, low- and mid-latitude regions are compared with the corresponding GMIST and IRI-STORM modeled values. For this purpose, the correla- tion coefficient, daily mean, root mean square error and improvement per- centage are calculated at different regions and geomagnetic disturbance levels. The results show that GMIST is more accurate than IRI-STORM model in terms of foF2 estimation at low- and mid-latitude regions, while at equatorial areas GMIST is less accurate during geomagnetically dis- turbed and quiet conditions. 1. Introduction The total electron content (TEC), measured by means of navigation satellite systems such as GPS, GLONASS, GALILEO, has turned into a key parameter in the characterization and monitoring of ionosphere on a global scale. However from the point of view of ionospheric radio propagation the most important characteristic is the critical frequency foF2, whose value directly defines an optimum working frequency for near veritcal incidence skywave (NVIS) propagation. The ionospheric density profile and its maximum elec- tron density (NmF2) is directly related to foF2 which is be regularly measured by ionosondes. The spatial cov- erage of foF2 ionosonde observations is not globally uniform due to the high cost of acquiring and main- taining ionosondes. This limitation can be surpassed considering the abundance of GPS receiver networks that provide TEC observations which can be used jointly with ionospheric slab thickness models to im- prove the foF2 predictability and therefore to provide support for various HF communication systems, radio amateurs and broadcaster operators. Houminer [1997] has reviewed the use of GPS data in relation to short-term foF2 prediction enhance- ment using one ionosonde station located in Cyprus and concluded that the high correlation between TEC GPS with foF2 (varying from 0.53 to 0.78) allows the use of GPS data for near real time foF2 map updating, both in temporal (short-term, medium and long-term) and in spatial (local, regional and global) terms. Kouris [2004] concluded that there is a high correlation be- tween the TEC and the square of foF2 (around 0.8), al- though ionospheric hysteresis is detected before and after noon time. Ma and Maruyama [2002], based on GPS TEC data and Kokubunji ionosonde foF2 data, reported that sea- sonal variations of ionospheric slab thickness during a daytime exhibits opposite phase with respect to the nighttime variation. Ionospheric slab thickness depends on three parameters; latitude, season, and solar activity. In the summer, ionospheric slab thickness is greater during daytime than in the night. The ionospheric slab thickness depends linearly on the solar activity. Jaya- candran et al. [2004] concluded that the diurnal varia- tion of the ionospheric slab thickness during low solar activity period is generally characterized by higher value in the night than during the day, while the situa- tion is reversed during high solar activity at the non-au- roral region. In auroral areas, the nighttime slab thickness is higher than the daytime one during both low and high solar activity periods. Variability of ionospheric slab thickness is higher in the night than during the day in the two phases of solar activity for mid-, low- and high-latitude ionosphere. Suvannasang et al. [2008] con- Article history Received January 13, 2015; accepted September 3, 2015. Subject classification: Ionosphere, Model, TEC, Slab thickness, foF2, Ionospheric storm.