D-region electron density and effective recombination coefficients during twilight - experimental data and modelling during solar proton events

Accurate measurements of electron density in the lower D-region (below 70 km altitude) are rarely made. This applies both with regard to measurements by ground-based facilities and by sounding rockets, and during both quiet conditions and conditions of energetic electron precipitation. Deep penetration into the atmosphere of high-energy solar proton fluxes (during solar proton events, SPE) produces extra ionisation in the whole D-region, including the lower altitudes, which gives favourable conditions for accurate measurements using ground-based facilities. In this study we show that electron densities measured with two ground-based facilities at almost the same latitude but slightly different longitudes, provide a valuable tool for validation of model computations. The two techniques used are incoherent scatter of radio waves (by the EISCAT 224MHz radar in Tromso, Norway, 69.6 degrees N, 19.3 degrees E), and partial reflection of radio-waves (by the 2.8MHz radar near Murmansk, Russia, 69.0 degrees N, 35.7 degrees E). Both radars give accurate electron density values during SPE, from heights 57-60 km and upward with the EISCAT radar and between 55-70 km with the partial reflection technique. Near noon, there is little difference in the solar zenith angle between the two locations and both methods give approximately the same values of electron density at the overlapping heights. During twilight, when the difference in solar zenith angles increases, electron density values diverge. When both radars are in night conditions (solar zenith angle >99 degrees) electron densities at the overlapping altitudes again become equal. We use the joint measurements to validate model computations of the ionospheric parameters f(+), lambda, alpha(eff) and their variations during solar proton events. These parameters are important characteristics of the lower ionosphere structure which cannot be determined by other methods.