Astrophys Space Sci (2017) 362:65 DOI 10.1007/s10509-017-3043-x ORIGINAL ARTICLE Analysis of ionospheric TEC from GNSS observables over the Turkish region and predictability of IRI and SPIM models Kutubuddin Ansari 1 · Ozsen Corumluoglu 1 · Sampad Kumar Panda 2 Received: 9 December 2016 / Accepted: 21 February 2017 © Springer Science+Business Media Dordrecht 2017 Abstract The present study investigates the ionospheric To- tal Electron Content (TEC) variations in the lower mid- latitude Turkish region from the Turkish Permanent GNSS Network (TPGN) and International GNSS Services (IGS) observations during the period from January 2015 to De- cember 2015. The corresponding TEC predicted by the In- ternational Reference Ionosphere (IRI 2012) and Standard Plasmasphere-Ionosphere Model (SPIM), and interpolated from Global Ionosphere Maps (GIMs) are evaluated to re- alize their reliability over the region. We studied the diur- nal and monthly behavior of TEC and the relative TEC de- viations along with the upper and lower quartiles to rep- resent its spatio-temporal variability. The diurnal variation of GNSS-derived TEC indicates its maximum peak value around 10.00 UT which decreases gradually to attain min- imum value after midnight. The monthly maximum value of TEC is observed in March followed by May and August, and the lowest value is seen during September. Studies show that the monthly relative deviation of TEC variability lies in the range of −1 to 4 units for all stations with the maxi- mum difference between positive and negative variability re- maining around 5. The studies also cover seasonal variation, grand-mean of ionospheric TEC and TEC intensity from the TPGN. The seasonal ionospheric VTEC pattern over all sta- tions depicts slight increment in VTEC distribution during March equinox compared to September equinox. The De- cember solstice perceived relatively higher VTEC than June solstice. The overall of VTEC values enhanced at all sta- tions towards end of the year 2015 compare to mid of year B K. Ansari kdansarix@gmail.com 1 Department of Geomatics Engineering, Izmir Katip Celebi University, Izmir, Turkey 2 Department of Electronics and Communication Engineering, KL University, Guntur, India due the high solar activity. The maximum grand-mean of VTEC is registered in March equinox while the lowest value is seen in September irrespective of all stations. The mea- sured grand-mean intensity variations of VTEC values are in ascending phase during March, May, August and Novem- ber months, but in descending phase during February, April, June and September months. The latitudinal study shows daytime TEC slowly decreasing with latitudes with a latitu- dinal gradient range of 0.1–0.2 TECU/degree. Additionally, the TEC analysis during the strong geomagnetic storm pe- riod (07–11 September 2015; SYM-H −120 nT) infers rel- atively better predictability of the SPIM model compared to the IRI 2012 model. The outputs of this study would com- plement towards a complete understanding of the lower mid- latitude ionospheric dynamics and its effects on radio prop- agations, particularly over the Turkish region. Keywords TPGN · TEC · IRI · SPIM · GIM 1 Introduction The ionosphere is the part of atmosphere extending between altitudes of 50–1000 km above the Earth. It is a dominant layer of atmosphere challenging the Communication, Nav- igation and Surveillance (CNS) applications (Ratnam et al. 2016). The molecules and atoms in the ionosphere undergo dissociation mainly due to the solar energy; the process is called ionization (Razin et al. 2016). The physical and chem- ical parameters of the ionosphere such as number of ions and electrons, temperature and composition change with re- spect to solar and magnetic activity, geographic location, local time of the day and season of the year (Rees 1989; Hong 2008). However, largest variations of ionospheric electron density are mostly associated with solar activity and