Characteristics Of ELDI Varying With Solar Activity In Polar Region From Observations Of EISCAT/ESR Radars

Due to the ionization of the neutral atmosphere,there are plenty of free electrons and ions in the polar regions,which will influence the the radio propagation in the ionosphere.The E-Layer Dominated Ionosphere is defined as the ionosphere where the maximum electron density in the E-layer is larger than that of the F-layer.It is of great importance in space weather broadcast and in radio communication to investigate this newly discovered anomaly.Characteristics of polar ELDI?E-Layer Dominated Ionosphere,ELDI?events under various solar activities have been investigated in this paper based on the field-aligned measurements from EISCAT?EISCAT Troms???UHF?and ESR?EISCAT Svalbard Radar?radars during 1999-2011.It is found that:?1?ELDI occurrences exhibit clear reliance on solar activities.During solar maximum,ELDIs appear more frequently at the EISCAT site?at the auroral latitude?than at the ESR site?at cusp latitude?and vice versa during solar minimum.?2?ELDIs are rarely observed by both radars in summer,which shows few effects of solar activities.Seasonal variations of ELDI occurrences at both sites during solar maximum are in accordance with that during solar minimum reported by Shen et al:in winter and early spring,the occurrences of ELDIs are higher than that in other seasons,especially in summer.?3?On the occurrence of ELDIs,the duration of the event and thickness of ionospheric E-layer show little variations under various solar activity conditions:the duration is shorter and the thickness is thicker during solar maximum;whilst N_mE/N_mF₂ and H_mE seem to be less variable.?4?Diurnal variations have no reliance on solar activities,however,at both radar sites,ELDI events show different diurnal variations.At the ESR radar site,ELDI events mainly occur at mid-day other than at mid-night observed by occualtation.?5?The long-duration ELDI events in the auroral oval region exhibit no reliance on solar activities.