[53] The results of the studies of disturbances in the midlatitude ionosphere over Kharkov during three geomagnetic storms distinguished by their intensity and occurred under moderate level of solar activity are presented.
[54] The obtained data showed that intense geomagnetic
disturbances (on 25 September 1998 and 29-30 May 2003,
Kp 
8) could be accompanied by phenomena rare for middle latitudes and
related to the equatorward shift of the main ionospheric trough,
light ion trough, and hot zone to the geomagnetic
L shells located
within the inner plasmasphere. The disturbances could lead to
considerable changes in the structure of the ionospheric
F region
and thermal and dynamical regimes of the charged and neutral
components of the Earth's upper atmosphere.
[55] The results of observations and modeling of the dynamical
processes in the ionosphere showed that a geomagnetic storm of
even a modest intensity (on 20-21 March 2003,
Kp 5) is capable
of causing at middle latitudes a strong negative ionospheric storm
accompanied by considerable variations in ionospheric parameters.
The reversal of the storm phases can be caused by a superposition
of two destabilizing factors: electric field pulse and traveling
atmospheric disturbance, both factors being generated by
magnetospheric substorms.
[56] The obtained results are used in a study and modeling of disturbed ionosphere (including the processes accompanying the storm phases reversal) in the central Europe region.
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