Seasonal and UT variations of modelled density gradient (polar model) in 2015 above the geomagnetic pole in the northern hemisphere (NP, 83.0011° N; 83.7272°W – a) and geomagnetic pole in the southern hemisphere (SP, 74.8207° S; 124.5879°E – b). Seasonal and UT variation of the averaged plasma density gradient at 100 km in the polar cap ionosphere (poleward of ±75° MLAT) binned by 1 h in UT and 5 days in the day of the year, for Northern (c) and Southern (d) hemispheres. The solar zenith angle near the altitude of Swarm A above geomagnetic poles in the northern and southern hemispheres are shown in contours. The horizontal black lines show solstices and equinoxes.