Observations of the ion temperature in morning hours in the equatorial topside ionosphere from ICON and other satellites

An accurate description of solar activity variation in thermal plasma parameters including ion temperature (Ti) is an important task for empirical modelling. The recent ion temperature measurement from IVM-A ICON during very low solar activity (PF10.7 about 70 s.f.u.) shows a very pronounced morning peak over the equator at an altitude of about 600km with its maximum at about 7.25h local time (LT). Maximum of Ti values reaches 3000K. The average height of the peak (difference between peak maximum and nighttime temperature) during spring equinox 2020 was about 1700K (only IVM-A data with the highest quality flag was selected). Thus, the observed peak from IVM-A ICON is significantly higher than what Ti data show in the case of previous experiments. The data from earlier experiments were taken from our data-base which has been systematically built from almost all available satellite thermal plasma parameters measurements including Ti (e.g. Truhlik et al. 2019). From other comparable satellite missions comprised in the data-base, ROCSAT-1 data shows the height of the Ti peak about 600K, Indian SROSS C2 about 1100K, C/NOFS about 1000K (only C/NOFS Ti data for PF10.7>95 were considered) etc.. The solar activity variation of Ti at the peak maximum from all the Ti data (IVM-A ICON data plus data from the data-base) exhibits a decrease with increasing solar activity but the dependence is non-linear. The absolute value of the gradient is highest at lowest solar activity and decreases towards solar maximum (for PF10.7<100 the gradient is about -20K/s.f.u., for PF10.7 between 100 and 150 the gradient is about -4K/s.f.u. and for PF10.7 above 200 the gradient is near zero). In spite of that, it might be needed some calibration factor of the IVM-A Ti data (a preliminary comparison with linearly extrapolated trend of Ti in dependence on PF10.7 from the data from the data-base for nighttime suggests about 12% IVM-A Ti reduction), and thus the gradient at low solar activity can a little bit change, we discuss consequences of this non-linear dependence for future steps in empirical modelling of the ion temperature.

V. Truhlik, D. Bilitza, P. G. Richards, D. Kotov, L.Triskova, and M. Sulha, 'Global empirical modeling of ion temperature for the International Reference Ionosphere', Presentation at 2019 COSPAR-IRI Workshop, Nicosia, Cyprus, Sept. 2019.