We present the results of the analysis of annual variations in daily electron density (N) at heights 140–160 km for the last solar minimum (2007–2009) obtained from digisonde measurements at the ionospheric station Irkutsk (52 °N, 104 °E). New coefficients of the known semi-empirical model (SEM) describing the connection between N and thermospheric characteristics are calculated to identify regularities of these variations. We have revealed that a characteristic feature of the annual N variations during the solar minimum is a change in their phase by 180° in a relatively narrow altitude interval (170–180 km). These results and the new SEM coefficients are original and important for atmospheric and ionospheric physics.
semi-empirical model of electron density, annual variations, F1-layer heights
1. Picone J.M., Hedin A.E., Drob D.P., Aikin A.C. (GTD7-2000) NRLMSISE-00 Empirical model of the atmosphere: statistical comparisons and scientific issues. J. Geophys. Res. 2002, vol. 107, no. A12, pp. 1469. DOI: 10.1029/2002JA009430.
2. Shchepkin L.A., Klimov N.N. The Earth thermosphere. Moscow: Nauka Publ., 1980, 220 p.
3. Shchepkin L.A., Kushnarenko G.P., Freizon I.A., Kuznetsova G.M. The electron density connection with the thermospheric state in the middle ionosphere. Geomagnetizm i aeronomiya [Geomagnetism and Aeronomy]. 1997, vol. 37, no. 5, pp. 106–113. (In Russian).
4. Shchepkin L.A., Kushnarenko G.P., Kuznetsova G.M. Annual electron density variations in F1 region of ionosphere. Solnechno-zemnaya fizika [Solar-Terr. Phys]. 2005, vol. 7, pp. 62–65. (In Russian).
5. Shchepkin L.A., Kuznetsova G.M., Kushnarenko G.P., Ratovsky K.G. The interpretation of electron density measu-rements with using semiempirical model. Solnechno-zemnaya fizika [Solar-Terr. Phys]. 2007, vol. 10, pp. 89–92. (In Russian).
6. Shchepkin L.A., Kuznetsova G.M., Kushnarenko G.P., Ratovsky K.G. Approximation of electron density measurements data in middle ionosphere during the low solar activity. Solnechno-zemnaya fizika [Solar-Terr. Phys]. 2008, vol. 11, pp. 66–69. (In Russian).
7. Tobiska W.K., Eparvier F.G. EUV97: Improvements to EUV irradiance modeling in the soft x-rays and EUV. Solar Phys. 1998, vol. 147, no. 1, pp. 147–159.