Variations in the Mid-Latitude Ionosphere Parameters over Ukraine during the Very Moderate Magnetic Storm on December 18, 2019

Multiyear researches show that weak and moderate magnetic storms may induce considerable and unpredictable changes in the ionosphere state. The problems of predicting the ionosphere response in a certain region to space weather changes currently remain topical since the physical processes occurring in the ionospheric plasma are variable and complicated. Particular interest is attracted by ionospheric disturbances with variable phases at middle latitudes and their propagation to low latitudes and the occurrence of strong ionospheric storms as a result of moderate or weak magnetic storms. The objective of this study is to perform the experimental studies of variations in the ionospheric plasma parameters over Ukraine during the very moderate magnetic storm on December 18, 2019. The study was carried out by using the incoherent scatter of radio waves as providing the most complete diagnostic capabilities and the vertical sounding method. Observations were performed in the Ionospheric Observatory of the Institute of Ionosphere (National Academy of Sciences of Ukraine, Ministry of Education and Science of Ukraine, Kharkiv) with an incoherent scatter radar. The critical frequencies were measured with a portable ionosonde. In addition, the geophysical information about the space weather and magnetosphere parameters was used. The ionosphere response to the geospace storm on December 18, 2019, over Kharkiv was analyzed. The very moderate magnetic storm (K-p = 4) was established to induce positive ionospheric disturbance. An increase in the critical frequency (up to 1.6 times) and a corresponding increase in the ionospheric F2 peak electron density (up to 2.6 times) was accompanied by a sequence of changes in the variations of principal ionospheric plasma parameters, such as the F2 layer peak height (a decrease by 30 km), the electron density throughout the entire range of studied altitudes (200-450 km), the electron and ion temperatures, and the vertical ionospheric plasma velocity component (with a decrease in the downward plasma drift velocity V-z at the noon after the magnetic storm began with further velocity recovery, the occurrence of fluctuations in the variations V-z with a quasi-period of 1 h 50 min at 15:40 UT (Universal Time) at altitudes of 360-420 km, and weakening of the evening extremum effect in the V-z variations and a maximum decrease in the velocity to 40-70 m/s at these altitudes). A substantiation was given for the following mechanism of the formation of a positive ionospheric storm: the downward plasma drift is weakened in the mid-latitude ionosphere during the winter daylight due to the fact that normal circulation is weakened by reverse storm-induced circulation. The very moderate magnetic storm on December 18, 2019, induced appreciable changes in the ionospheric plasma parameters throughout the entire range of studied altitudes. The measured data provided additional information for solar-terrestrial relationships study and the ionosphere state prediction.