The D-region ionosphere during the solar minimum as seen by the EISCAT Svalbard continuous 1-year IPY radar experiment

The EISCAT Svalbard Radar (ESR) was operated in a continuous mode during the International Polar Year (IPY), starting on 1 March 2007 and ending on 29 February 2008. The radar experiment was dedicated to ensure good coverage also in the ionospheric lower E and D regions, in addition to covering the more standard experiment target, the ionospheric F peak, and reaching into the topside ionosphere. By using this unique continuous dataset gathered during the solar minimum, we show that anomalous variability of the quiet daytime electron density in the D region can be explained by the varying NO concentration. Our focus is especially on September-October 2007, where the quiet daytime electron density suddenly increased by a factor of 7-10. This enhancement, most clearly seen at sunsets, lasted for several days. Using a detailed chemistry model we can show that an enhanced nitric oxide density explains the electron density at 92 km. This finding might provide aims to estimate NO from various quiet daytime electron density measurements in the upper mesophere-lower thermosphere.