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According to the study by Mayer and Jakowski (2009), periods of E layerdominated ionosphere (ELDI) are defined as being characterized by verticalelectron density profiles having a maximum density at E layer altitudes. Inthis paper, characteristics of ELDI intervals have been investigated,focusing on their temporal variations, using field-aligned measurements fromthe EISCAT and ESR radars during the interval 2009–2011. ELDI events wereidentified using simple but reasonable criteria, in which a minimal durationwas required to exclude possible "fake" events induced by random errors inmeasurements. It was found that ELDIs were observed more often in winter andearlier spring than other seasons, especially in the auroral zone. Theoccurrence of ELDI intervals peaks around geomagnetic midnight at aurorallatitudes, while it reaches a maximum around geomagnetic local noon at thelatitude of the ESR. Our results imply that ELDI intervals appear to be asporadic rather than a regular phenomenon, in disagreement with previousresults inferred from radio occultation measurements. The discrepancy betweenthe typical durations of ELDI events observed by the two radars isremarkable, being 30 min on average at Tromsø but about a half of this atSvalbard. During intervals of ELDI, the mean thicknesses of the E layer arequite close at the two sites, as are the values of HmE and theratio of NmE / NmF2. Case studies confirmthat either extra E layer ionization or F layer density depletion alone couldlead to the presence of ELDIs. Based on a careful check on ELDI intervals ofvarious types, however, we suggest that both of them play a critical role inELDI formation

E layer dominated ionosphere observed by EISCAT/ESR radars during solar minimum