Ionospheric index of solar activity based on the data of measurements of the spacecraft signals characteristics

The method for estimating an ionospheric index of solar-activity (IISA) based on the processing of spacecraft radio signals is suggested. The IISA values have been obtained by comparison between the measured and calculated variations of radio-signal characteristics. To calculate the variations of radio-signal characteristics, the straight rays approximation and the solar-activity index (Wolf numbers W and/or values of F_10.7 solar flux) as a control parameter of the ionospheric model have been used. The suggested method was tested using spacecraft radio signals from the radio-navigation system “CIKADA”. The reduced differences of phases (ΔΦ) for frequencies 150 and 400 MHz were measured and the same characteristics were calculated by integration along the ray of radio-wave propagation between the receiver and the satellite. The IRI-95 has been used as a background ionospheric model. The satellite co-ordinates were determined using the orbit parameters recorded in the navigation messages. Minimization of the difference measured and calculated ΔΦ using arbitrary time steps, or during whole time intervals of observation, gives the IISA corresponding the satellite pass. Daily IISA values were obtained by averaging over all communication contacts during a day (20–30 passes). Testing this approach based on the measurements during March/April 1997, 1998, shows that on magnetically quiet days differences between IISA and the primary solar activity indices are about 5%.