This paper reports the results of ionosphere and plasmasphere observations with the Kharkiv incoherent scatter radar and ionosonde, Defense Meteorological Satellite Program, and Arase (ERG) satellites and simulations with field line interhemispheric plasma model during the equinoxes and solstices of solar minimum 24. The results reveal the need to increase NRLMSISE‐00 thermospheric hydrogen density by a factor of ~2. For the first time, it is shown that the measured plasmaspheric density can be reproduced with doubled NRLMSISE‐00 hydrogen density only. A factor of ~2 decrease of plasmaspheric density in deep inner magnetosphere (L ≈ 2.1) caused by very weak magnetic disturbance (Dst > −22 nT) of 24 December 2017 was observed in the morning of 25 December 2017. During the next night, prominent effects of partially depleted flux tube were observed in the topside ionosphere (~50% reduced H+ ion density) and at the F2‐layer peak (~50% decreased electron density). The likely physical mechanisms are discussed.

Abstract. This paper describes the stages of the development of IS radars, the formation and development of the Institute of Ionosphere of the National Academy of Sciences of Ukraine and Ministry of Education and Science, Youth and Sport of Ukraine (IION) from the first steps in the implementation of ionospheric radar equipment near Kharkiv (Kharkov) up to the current state. The paper discusses the main trends in the scientific and technical activities of the Institute, its relations with scientific communities, and demonstrates major scientific achievements.

A comprehensive study of the response of the ionosphere-plasmasphere system at mid-latitudes to weak (Dst min > −50 nT) magnetic storms is presented. For the first time, it is shown that weak magnetic disturbances can lead to significant modulation of ionosphere-plasmasphere H + ion fluxes. It is found that this modulation is caused by the enhancements/reductions of the topside O + ion density, which is induced by F2-layer peak height rise and fall during the storms. The F2-layer motion is caused by thermospheric wind changes and by a penetration electric field. Both drivers are closely related to the changes in the B z component of interplanetary magnetic field. The most prominent manifestation of the H + ion flux modulation is strong changes in H + ion fraction in the topside ionosphere. This study also indicates that the NRLMSISE-00 model provides the correct relative changes of neutral H density during weak magnetic storms and also that there is a compelling need to include geomagnetic activity indices, in addition to solar activity (F 10.7 ), as input parameters to empirical topside ionosphere models.

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