E layer dominated ionosphere observed by EISCAT/ESR radars during solar minimum
According to the study by Mayer and Jakowski (2009), periods of E layer dominated ionosphere (ELDI) are defined as being characterized by vertical electron density profiles having a maximum density at E layer altitudes. In this paper, characteristics of ELDI intervals have been investigated, focusing on their temporal variations, using field-aligned measurements from the EISCAT and ESR radars during the interval 2009–2011. ELDI events were identified using simple but reasonable criteria, in which a minimal duration was required to exclude possible "fake" events induced by random errors in measurements. It was found that ELDIs were observed more often in winter and earlier spring than other seasons, especially in the auroral zone. The occurrence of ELDI intervals peaks around geomagnetic midnight at auroral latitudes, while it reaches a maximum around geomagnetic local noon at the latitude of the ESR. Our results imply that ELDI intervals appear to be a sporadic rather than a regular phenomenon, in disagreement with previous results inferred from radio occultation measurements. The discrepancy between the typical durations of ELDI events observed by the two radars is remarkable, being 30 min on average at Tromsø but about a half of this at Svalbard. During intervals of ELDI, the mean thicknesses of the E layer are quite close at the two sites, as are the values of H mE and the ratio of N mE / N mF 2. Case studies confirm that either extra E layer ionization or F layer density depletion alone could lead to the presence of ELDIs. Based on a careful check on ELDI intervals of various types, however, we suggest that both of them play a critical role in ELDI formation.