Statistical ionospheric E layer properties measured with the Cyprus Digisonde and comparisons with IRI predictions
This paper reports the diurnal, seasonal, and long term variability of the E layer critical frequency (foE) and peak height (hmE) derived from Digisonde measurements from 2009 to 2016 at the low-middle latitude European station of Nicosia, Cyprus (geographical coordinates: 35°N, 33°E, geomagnetic lat. 29.38°N, I = 51.7°). Manually scaled monthly median values of foE and hmE are compared with IRI-2012 predictions with a view to assess the predictability of IRI. Results show that in general, IRI slightly overestimates foE values both at low and high solar activity. At low solar activity, overestimations are mostly limited to 0.25 MHz (equivalent electron density, 0.775 × 103 el/m-3) but can go as high as 0.5 MHz (equivalent electron density, 3.1 × 103 el/m-3, during noon) around equinox. In some months, underestimations, though sporadic in nature, up to 0.25 MHz are noted (mostly during sunrise and sunset). At high solar activity, a similar pattern of over-/underestimation is evident. During the entire period of study, over-/under estimations are mostly limited to 0.25 MHz. In very few cases, these exceed 0.25 MHz but are limited to 0.5 MHz. Analysis of hmE reveals that: (1) hmE remains almost constant during ±2 to ±4 h around local noon, (2) hmE values are higher in winter than in spring, summer and autumn, (3) there are two maxima near sunrise and sunset with a noontime minimum in between. During the entire period of study, significant differences between observed hmE and the IRI predictions have been noted. IRI fails to predict hmE and outputs a constant value of 110 km, which is higher than most of the observed values. Over- and under estimations range from 3 to 13 km and from 0 to 3 km respectively.