Variations of total electron content in the equatorial anomaly region in Thailand V. Rajesh Chowdhary a,⇑ , N.K. Tripathi a , Sanit Arunpold a , Durairaju Kumaran Raju b,a a Asian Institute of Technology, School of Engineering and Technology, RS&GIS Field of Study, P.O. Box 4, Klong Laung, Pathumthani 12120, Thailand b Tropical Marine Science Institute, National University of Singapore, S2S, 18 Kent Ridge Road, Singapore 119227, Singapore Received 28 February 2014; received in revised form 12 September 2014; accepted 15 September 2014 Available online 22 September 2014 Abstract This paper presents the first results of total electron content (TEC), derived by analyzing dual frequency Novatel GSV4004 GPS recei- ver’s data which were installed by the SCINDA project, located at the Asian Institute of Technology, Bangkok (AITB, 14.079N, 100.612E) and Chiang Mai University, Chiang Mai (CHGM, 18.480N, 98.570E) with magnetic latitude of 4.13°N and 8.61°N respec- tively in Thailand, for the year 2011. These two stations are separated by 657 km in the equatorial anomaly region. The highest TEC values occurred from 1500 to 1900 LT throughout the study period. The diurnal, monthly and seasonal GPS-TEC have been plotted and analyzed. The diurnal peaks in GPS-TEC is observed to be maximum during equinoctial months (March, April, September and October) and minimum in solstice months (January, February, June, July and December). These high TEC values have been attributed to the solar extreme ultra-violet ionization coupled with the upward vertical E  B drift. A comparison of both station’s TEC has been carried out and found that CHGM station experiences higher values of TEC than AITB station, due to formation of ionization crest over the CHGM station. Also, TEC values have shown increasing trend due to approaching solar maximum. These results from both stations were also compared with the TEC derived from the International Reference Ionosphere’s (IRI) recently released, IRI-2012 model. Results have shown positive correlation with IRI-2012 model. Although, IRI-model does not show any response to geomagnetic activity, the IRI model normally remains smooth and underestimates TEC during a storm. Ó 2014 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Equatorial ionization anomaly (EIA); GPS-TEC; IRI-2012; GPS SCINDA 1. Introduction The equatorial and low latitude ionosphere is always of a keen interest to many researchers across the world, because of its unique features, exhibited in its electron den- sity and temperature. The equatorial ionization anomaly (EIA), the plasma fountain and the equatorial electrojet among others are topic of common interests in the investi- gation of the equatorial and low latitude ionosphere. The geomagnetic field lines being horizontally orientated at the equator and the shift between the geomagnetic and geo- graphic equator is understood to be reason behind these observed features and their longitudinal variations. EIA is characterized by trough at the geomagnetic equator and two peaks (ionization crests) on either side of the equa- tor at about 15° magnetic latitudes (Appleton, 1946). Mitra (1946) suggested that the trough exists because plasma produced by photo ionization at great heights over the geomagnetic equator diffuses downwards and outwards on either side of the geomagnetic equator. This causes the depletion at the geomagnetic equator and formation of ionization crests. Martyn (1947) has briefed that the mutually perpendicular east–west electric field and north– south geomagnetic field give rise to an upward electrody- namic (E  B [E being the eastward electric field and B http://dx.doi.org/10.1016/j.asr.2014.09.024 0273-1177/Ó 2014 COSPAR. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. www.elsevier.com/locate/asr Available online at www.sciencedirect.com ScienceDirect Advances in Space Research 55 (2015) 231–242