COSTTEC - The electron content monthly median map adopted by COST 251

COSTTEC is a monthly median regional map for ionospheric electron content. The data used for the mapping effort are monthly and bi-hourly medians from Differential Doppler observations on the signals of the Navy Navigation Sa tellites (NNSS) made at Graz/Austria and Lindau/Harz, Germany. COSTTEC uses linear dependences of electron content on the solar activity index R12, on geographic latitude and on geographic longitude. For the local time depend ence and for the seasonal dependence it uses Fourier decomposition. The dec omposition converts the 12x12 time domain medians into 12x12 Fourier coeffi cients. For the maps only 5x5 coefficients are kept (time independent term, diurnal and semi-diurnal variation, annual and semi-annual variations and all combinations). Finally the maps consist of 2x3 sets of 5x5 Fourier coef ficients: 3 sets for high and 3 sets for low solar activity. At each solar activity level one set is for a central point, one for the latitudinal and one for the longitudinal gradients. Since no TEC data could be used to deri ve the longitude dependence it was necessary to incorporate experience from F2 peak density behaviour. We report on the reasons for the chosen mapping approach, demonstrate the m ost important map properties and show comparisons between observations and TEC map data. (C) 2000 Elsevier Science Ltd. All rights reserved.