GPS TEC near the crest of the EIA at 95°E during the ascending half of solar cycle 24 and comparison with IRI simulations Pradip Kumar Bhuyan ⇑ , Rumajyoti Hazarika Centre for Atmospheric Studies, Dibrugarh University, Dibrugarh 786004, India Received 5 April 2013; received in revised form 25 June 2013; accepted 28 June 2013 Available online 6 July 2013 Abstract Total electron content (TEC) data obtained from GPS dual frequency measurements during the ascending half of the solar cycle 24 from 2009 to 2012 over Dibrugarh (27.5°N, 94.9°E; 17.6°N MLAT) have been used to study the diurnal, seasonal, annual and solar cycle variation of TEC. The measurements reported here are for the first time from the location situated at the poleward edge of the northern equatorial ionization anomaly (EIA) and within the peak region of the longitudinal wave number 4 (WN4) structure in EIA crest TEC. TEC exhibits a minimum around 0600 LT and diurnal maximum around 1300–1600 LT. In the low and moderate solar activity years 2009–2010 and 2010–2011, average daytime (1000–1600 LT) TEC in summer was higher (25.4 and 36.6 TECU) compared to that in win- ter (21.5 and 26.1 TECU). However, at the peak of the solar cycle in 2011–2012, reversal in the level of ionization between winter and summer takes place and winter TEC becomes higher (50.6 TECU) than that in summer (45.0 TECU). Further, TEC in spring (34.1, 49.9 and 63.3 TECU respectively in 2009–10, 2010–11 and 2011–12) is higher than that in autumn (24.2, 32.3 and 51.9 TECU respectively) thus showing equinoctial asymmetry in all the years of observation. The winter anomaly in high solar activity years and equinoctial asymmetry all throughout may be largely attributed to changes in the thermospheric O/N 2 density ratio. A winter to summer delay of 1 h in the time of occurrence of the diurnal maximum has also been observed. Daytime maximum TEC bears a nonlinear relation- ship with F 10.7 cm solar flux. TEC increases linearly with F 10.7 cm solar flux initially up to about 140 sfu (1 sfu = 10 22 Wm 2 Hz 1 ) after which it tends to saturate. On the contrary, TEC increases linearly with solar EUV flux (photons cm 2 s 1 , 0.5–50 nm) during the same period. TEC predicted by the IRI 2012 are lower than the measured TEC for nearly 90% of the time. Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Ionosphere; Equatorial ionosphere; GPS; TEC; IRI 1. Introduction The ionosphere induced errors in any transionospheric satellite radio communication is proportional to the total electron content, TEC along the ray path. TEC provides an overall description of the ionosphere with high temporal resolution. Therefore, measurement of TEC under different geophysical conditions and having high accuracy and tem- poral resolution has become important because of the increasing demand on satellite, aircraft or ground based navigation based on transionospheric communication sys- tems. The GPS is a satellite based positioning system widely used for navigation, relative positioning and time transfer. The dispersive ionosphere introduces time delay in the 1.57542 GHz (L1) and 1.22760 GHz (L2) simulta- neous transmissions from GPS satellites orbiting at 20,200 km. The relative ionsopheric time delay of the two signals can be converted to TEC along the ray path. Thus although GPS was originally developed to serve as a radio navigation satellite system, it is now widely used for many other scientific and technological applications. As the GPS signals traverse the ionosphere carrying signatures of the dynamic medium, GPS TEC measurements offer unique opportunity for studying the temporal and spatial distribu- tion. Temporal and spatial variation of TEC has been 0273-1177/$36.00 Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2013.06.029 ⇑ Corresponding author. Tel.: +91 3732370224; fax: +91 3732370323. E-mail address: pkbhuyan@gmail.com (P.K. Bhuyan). www.elsevier.com/locate/asr Available online at www.sciencedirect.com Advances in Space Research 52 (2013) 1247–1260