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This study investigates the causes of nighttime enhancements in ionospheric density that are observed in winter by the incoherent scatter radar at Kharkiv, Ukraine. Calculations with a comprehensive physical model reveal that large downward ion fluxes from the plasmasphere are the main cause of the enhancements. These large fluxes are enabled by large upward H+ fluxes into the plasmasphere from the conjugate summer hemisphere during the daytime. The nighttime downward H+ flux at Kharkiv is sensitive to the thermosphere model H density, which had to be increased by factors of 2 to 3 to obtain model‐data agreement for the topside H+ density. Other studies support the need for increasing the thermosphere model H density for all seasons at solar minimum. It was found that neutral winds are less effective than plasmaspheric fluxes for maintaining the nighttime ionosphere. This is partly because increased equatorward winds simultaneously oppose the downward H+ flux. The model calculations also reveal the need for a modest additional heat flow from the plasmasphere in the afternoon. This source could be the quiet time ring current.

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.

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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.