Variation of ionospheric total electron content in Indian low latitude region of the equatorial anomaly during May 2007–April 2008 Sanjay Kumar, A.K. Singh * Atmospheric Research Lab., Department of Physics, Banaras Hindu University, Lanka, Varanasi 221005, India Received 12 September 2008; received in revised form 27 January 2009; accepted 28 January 2009 Abstract The ionospheric total electron content (TEC), derived by analyzing dual frequency signals from the Global Positioning System (GPS) recorded near the Indian equatorial anomaly region, Varanasi (geomagnetic latitude 14°, 55 0 N, geomagnetic longitude 154°E) is studied. Specifically, we studied monthly, seasonal and annual variations as well as solar and geomagnetic effects on the equatorial ionospheric anomaly (EIA) during the solar minimum period from May 2007 to April 2008. It is found that the daily maximum TEC near equatorial anomaly crest yield their maximum values during the equinox months and their minimum values during the summer. Using monthly averaged peak magnitude of TEC, a clear semiannual variation is seen with two maxima occurring in both spring and autumn. Statistical studies indicate that the variation of EIA crest in TEC is poorly correlated with Dst-index (r = 0.03) but correlated well with Kp-index (r = 0.82). The EIA crest in TEC is found to be more developed around 12:30 LT. Ó 2009 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Total electron contents (TECs); Equatorial ionization anomaly (EIA); Global Positioning System (GPS) 1. Introduction The changes in the temporal and spatial features of total electron contents (TECs) at the equatorial and low latitude regions are significant compared to relatively small at mid latitude regions (Davies, 1980). The equatorial and low lat- itude ionospheres are very dynamic due to the various pro- cesses associated with the phenomenon of equatorial ionization anomaly (EIA) and equatorial spread-F (ESF) irregularities in these regions. The equatorial ionospheric anomaly (EIA) is character- ized, in terms of latitudinal distribution of ionization, by a trough at the magnetic equator and crests at about ±17° magnetic latitude (Appleton, 1946); the crest-to-trough ratio (about 1.5 in the day time peak electron density) and the position of the crests vary with various geophysical con- ditions. Many theories, like the diffusion theory (Mitra, 1946) and the electrodynamic drift theory (Martyn, 1955; Duncan, 1960), have been put forward to explain the anomaly. The diffusion theory has been shown to be important but not sufficient to explain the observations (Rishbeth et al., 1963). The electrodynamic drift theory, on the other hand, has been successful in explaining the observations (Bramley and Peart, 1965; Moffett and Hanson, 1965). According to the drift the- ory, the north–south geomagnetic field combined with the daytime east–west ionospheric electric field (both being paral- lel to the Earth’s surface at the equator) generates a plasma fountain which rises to several hundred kilometers at the equa- tor. When the upward drifting plasma loses its momentum, it diffuses under gravity along the geomagnetic field lines to higher latitudes forming the crests (Hanson and Moffett, 1966). The fountain and resulting anomaly can be cover more than 30° latitudes on either side of the magnetic equator (Balan and Bailey, 1995). Balan and Bailey (1995) further studied the plasma fountain including neutral wind also and shown that the plasma velocity turning more poleward in that hemisphere, where the wind is poleward. The formation of the EIA is seen in the total electron content (TEC), which is the integral of electron number 0273-1177/$36.00 Ó 2009 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2009.01.037 * Corresponding author. Tel.: +91 542 2313431. E-mail address: abhay_s@rediffmail.com (A.K. Singh). www.elsevier.com/locate/asr Available online at www.sciencedirect.com Advances in Space Research 43 (2009) 1555–1562