Abstract
Electron density profiles measured by the EISCAT incoherent scatter radar are used to determine the undisturbed behaviour of the E region at 69.6° N, 19.2° E. Results are presented showing the solar zenith dependence for both low (F10.7 < 80) and high (F10.7 > 180) solar activity levels. The results are compared with aeronomical calculations based on the MSIS86 neutral atmosphere model and measurements of solar EUV fluxes, and with the International Reference Ionosphere. Fairly good agreement is found between IRI and the measurements for solar zenith angles less than 80°. The aeronomical calculations show good agreement with the measurements of the bottomside and peak of the E region also for solar zenith angles less than 80°. For zenith angles from 85° to 90° neither model gives a good representation of the true electron density profile, particularly in winter when a strong, narrow layer peaking at about 100 km altitude is observed. A pronounced valley above the E region peak is predicted by the aeronomical calculations at all solar zenith angles but is not observed.
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