Variations in the mid-latitude ionospheric F2-layer during magnetically quiet periods close to the solar cycle 25 maximum: observation results and comparative analysis

The study of seasonal and temporal variations in ionospheric parameters and their dependence on space weather is an important problem in geophysics. This article presents the results of ionosonde observations of the ionospheric F2-layer peak electron density ( N m F2) and peak height ( hm F2) over Kharkiv, Ukraine (49.6° N, 36.3° E), during the high solar activity period of solar cycle 25 under geomagnetically quiet conditions. Temporal variations of these parameters were analyzed for the characteristic geophysical periods of 2023, namely the March and September equinoxes and the June and December solstices. The diurnal variations of N m F2 were compared with those observed during the maximum (2014), early declining phase (2015), and minimum (2019) of solar cycle 24. The results were also compared with data from mid-latitude ionosondes operating in the eastern and western hemispheres at Pruhonice (50.0° N, 14.6° E), Dourbes (50.1° N, 4.6° E), and Millstone Hill (42.6° N, 288.5° E). It was found that the N m F2 data from Pruhonice are closest to those from Kharkiv, whereas the largest differences were observed between the Millstone Hill and Kharkiv stations, which can be explained by differences in geomagnetic latitude. The h m F2 variations were generally similar at all stations. A comparison with the IRI-2016 and NeQuick2 models was also performed. Both IRI-2016 options (CCIR and URSI) and NeQuick2 underestimate N m F2 during the March equinox and overestimate it during the June solstice and September equinox, whereas the December solstice values predicted by IRI-2016 agree well with the observations. All models show good agreement with nighttime observations in all seasons. For spring, summer, and winter, the AMTB-2013 h m F2 option of IRI-2016 shows the best agreement with the experimental data, while the SHU-2015 option best reproduces the autumn variations.