Abstract
We present a geomagnetic quiet time (Dst > −50 nT) empirical model of ionospheric total electron content (TEC) for the northern equatorial ionization anomaly (EIA) crest over Calcutta, India. The model is based on the 1980–1990 TEC measurements from the geostationary Engineering Test Satellite-2 (ETS-2) at the Haringhata (University of Calcutta, India: 22.58° N, 88.38° E geographic; 12.09° N, 160.46° E geomagnetic) ionospheric field station using the technique of Faraday rotation of plane polarized VHF (136.11 MHz) signals. The ground station is situated virtually underneath the northern EIA crest. The monthly mean TEC increases linearly with F10.7 solar ionizing flux, with a significantly high correlation coefficient (r = 0.89–0.99) between the two. For the same solar flux level, the TEC values are found to be significantly different between the descending and ascending phases of the solar cycle. This ionospheric hysteresis effect depends on the local time as well as on the solar flux level. On an annual scale, TEC exhibits semiannual variations with maximum TEC values occurring during the two equinoxes and minimum at summer solstice. The semiannual variation is strongest during local noon with a summer-to-equinox variability of ~50–100 TEC units. The diurnal pattern of TEC is characterized by a pre-sunrise (0400–0500 LT) minimum and near-noon (1300–1400 LT) maximum. Equatorial electrodynamics is dominated by the equatorial electrojet which in turn controls the daytime TEC variation and its maximum. We combine these long-term analyses to develop an empirical model of monthly mean TEC. The model is validated using both ETS-2 measurements and recent GNSS measurements. It is found that the present model efficiently estimates the TEC values within a 1-σ range from the observed mean values.
Highlights
Radio waves traversing the ionosphere experience effects such as group path delay, radio frequency carrier phase advance, Faraday polarization rotation, angular refraction, frequency Doppler shift, scintillation, etc. (e.g., Budden 1961)
Most of the above effects exhibited by the signals propagating through the ionosphere are directly proportional, at least to the first order, to the number of free electrons encountered along the path of the signal between the satellite and ground receiver, i.e., the total electron content (TEC) or its time derivative (Ezquer et al 2004)
The majority of the models may be categorized into three groups: (i) empirical models (Bent et al 1972; Ching & Chiu 1973; Rawer et al 1978; Anderson et al 1987, 1989; Nisbet & Divany 1987; Tascione et al 1988; Daniell et al 1995; Batista et al 1996; Abdu et al 2008; Brum et al 2011, 2012; Bilitza et al 2012), (ii) theoretical or mathematical models (Bailey et al 1978; Anderson & Klobuchar 1983; Anderson et al 1996; Schunk & Sojka 1996; Brum et al 2006), and (iii) parameterized models (Daniell et al 1995; Souza et al 2010)
Summary
Radio waves traversing the ionosphere experience effects such as group path delay, radio frequency carrier phase advance, Faraday polarization rotation, angular refraction, frequency Doppler shift, scintillation, etc. (e.g., Budden 1961). No currently available ionospheric model can accurately predict the TEC near the anomaly crest or low latitude zone. The region of most concern to the users of satellite-based communication and navigation systems is the region near the anomaly crest. We will analyze the TEC variation using a long (1980–1990) database taken at the ionosphere field station Haringhata (University of Calcutta, India: 22.58° N, 88.38° E, geographic; 12.09° N, 160.46° E geomagnetic). This station is situated virtually underneath the statistical location of the EIA crest. Development of station-specific local models is necessary for application to satellite communication and navigation systems
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