A Semi-empirical Model of Ionospheric Electron Content at Low Latitudes for Solar Minimum

63-68An attempt has been made to develop a semi-empirical model based on a harmonic analysis approach for calculating the electron content at three locations in and around the northern equatorial anomaly crest for solar minimum conditions using the data collected during the ATS-6 period (Oct 1975-July 1976) in India. The harmonic coefficients show seasonal dependence at all the locations. The ratio of the first and second harmonic was found to be higher compared to those reported from midlatitudes indicating thereby a smoother diurnal variation at low latitudes. A set of 25 coefficients comprising the mean, first and second harmonic coefficients was found to be sufficient for calculating the electron content within ± 15%ofthe observed values under worst case conditions. The present model which requires only the precalculated set of coefficients as inputs and eliminates the necessity to integrate electron density profiles will enable the user to apply first order corrections to time delay (or refraction errors) with little demand on the computer