On the sensitivity of total electron content (TEC) to upper atmospheric/ionospheric parameters

The total electron content (TEC) is a key ionospheric parameter for various space weather applications. Over the last decade an extensive database of TEC measurements has become available from both space- and ground-based observations, and these measurements have established the general morphology of the global TEC distributions. In particular, the TOPEX TEC measurements have shown strong longitudinal variations of TEC in addition to the observed day-to-day variabilities. To better understand the observed TEC variations and to better guide its modeling, we have studied the sensitivity of quiet-time TEC to the following key atmospheric and ionospheric parameters: neutral density, neutral wind, plasma temperatures, plasmaspheric flux, and the O+-O collision frequency. These parameters are often only roughly known and can cause large uncertainties in model results. For this study, we have developed a numerical mid-latitude ionospheric model, which solves the momentum and continuity equations for the O+ density and a simplified set of equations for the H+ density. To obtain TEC, the calculated ion densities have been integrated from the bottom altitude (100 km) to the altitude of the TOPEX satellite (1336 km). Our study shows that during the day the neutral wind and the neutral composition have the most important effect on TEC. In particular, the zonal component of the neutral wind can have a large effect on TEC in the southern hemisphere where the magnetic declination angle is large. During the night, most of the above-mentioned parameters can play a significant role in the TEC morphology, except for the plasma temperature, which has only a small effect on TEC. Finally, the TEC varies roughly linearly with respect to all of the parameters except for the neutral wind.