Empirical ionospheric models have been developed from long-term datasets of seven incoherent scatter radars spanning invariant latitudes from 25 to 75 degress in American, European and Asian longitudes at Svalbard, Tromso, Sondrestrom, Millstone Hill, St. Santin, Arecibo and Shigaraki. These local models represent electron density, ion and electron temperatures, and ion drifts in the E and F regions (100 - 650 km). Using a variety of pulse lengths, these radars provide typically 25-50 km height resolution for the F region observations, and 10-15 km height resolution for the E region observations. The data for each site are binned by month and local time with 3-month and 1-hour bin sizes. Least squares fits for model profiles are then computed for each of the 12 (monthly bins) X 24 (hourly bins) = 288 bins. Assuming a linear variation between any two consecutive altitude nodes (piecewise-linear function), coefficients are obtained for the height variation. These coefficients are assumed to be linear in the solar activity index F10.7 and magnetic activity index 3-hourly ap, where F10.7 is for the previous day and ap is for the previous 3 hours. A sequential least squares fit to the solar activity dependence and 6 piecewise-linear altitude dependence functions is performed, generating three sets of coefficients (for the constant, F10.7 and ap terms) for a given month, local time, and altitude node. To further smooth diurnal and seasonal variations, a 3 (month) X 3 (hour) median filter is applied in season and local time to the coefficients. Lastly, a cubic B-spline is fitted to give twice-differentiable height variations.
Availability: The models are available from the authors or on-line at http://www.openmadrigal.org .
S.-R. Zhang and J. M. Holt, Ionospheric plasma temperatures during 1976-2001 over Millstone Hill, Adv. Space Res., 33, 963-969, 2004.
J.M. Holt, S.-R. Zhang, and M. J. Buonsanto (2002), Regional and local ionospheric models based on Millstone Hill incoherent scatter radar data, Geophys. Res. Lett., 29, 1207, doi:10.1029/2002GL014678.
S.-R. Zhang, J. M. Holt, T. van Eyken, M. McCready, C. Amory-Mazaudier, S. Fukao, and M. Sulzer, Ionospheric model and climatology from long-term databases of worldwide incoherent scatter radars, submitted to Geophys. Res. Lett. 2005.