Ionosphere Response to Geospace Storm on 25 September 2016 over Kharkiv (Ukraine)

Instrumental observations of variations in the level of the geomagnetic field and ionospheric parameters were conducted during a weak geospace storm on September 25, 2016. It was found that a moderate magnetic storm was accompanied by variations in the level of the H component of the geomagnetic field up to 20-30 nT, as well as variations in the level of fluctuations of the horizontal compo-nents of the geomagnetic field up to +/-(2-3) nT. It was revealed that the ionospheric storm was three-phase, mostly moderate. The decrease in the electron density reached a factor of 1.5, and its increase reached a factor of 2.2. The increase in the electron temperature during the negative phase of ionospheric storm did not exceed 200-300 K, while its increase during the positive phase of ionospheric storm reached 400-500 K. The ion temperature variations were within +/- 100 K. During the negative phases of the ionospheric storm, individual deviations in the vertical plasma drift velocity from those on a geomagnetically quiet day were observed both in the direction of decreasing the downward drift velocity (increasing the upward drift velocity at high altitudes) and vice versa. Significant (17-53 m/s) deviations in velocity variations towards positive values (decrease of downward and increase of upward drift velocity), accompanied by the rise of the F2 layer, after significant changing in the zonal component of the magnetospheric electric field (from -2 to 3.5 mV/m), contributed to the increase in the electron density and to the appearance of the positive phase of the ionospheric storm. During the pos-itive phase, the increase in the velocity of downward plasma drift reached 9-20 m/s at altitudes above 450 km. The negative phases of the ionospheric storm were formed due to an increase in the electron and ion temperatures and a decrease in the O/N2 ratio during the storm. The generation of ionospheric storm positive phase at mid-latitudes apparently was caused by several mechanisms at once: a pen-etrating electric field, a meridional wind directed toward the equator, and downward plasma flows in the protonosphere. The phases of the ionospheric storm were accompanied by significant variations in the dynamic and thermal processes in the ionosphere.(c) 2023 COSPAR. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).