5. Discussion

[50]  The considered features of three magnetic storms and of the processes in the ionosphere over Kharkov that accompanied these storms make it possible to formulate the principal regularities in development of these processes. Conventionally the presented ionospheric storms may be split into two groups.

[51]  The ionospheric storms accompanying the severe magnetic storms ( Kp ge 8 ) form the first group. These magnetic storms occurred on 25 September 1998 and 29-30 May 2003. They had long-lasting (6-9 hours) periods of high geomagnetic activity ( Kp ge 8 ), the active period of the main phase of the storms developed quickly with the maximum values |Dst/dt| = 35-65 nT h -1 and fell on the time interval when the Kharkov radar was in the midnight-predawn sectors. The ionospheric storms accompanying these magnetic storms are characterized by the considerable disturbances: the decrease in electron density by a factor of up to 3-4, increase in the height of the electron density peak hmF2 by 100-160 km, nighttime heating of the plasma up to 2400-3200 K, increase in the neutral temperature by 200-350 K, increase in the thermopause height not less than to 400 km, infringement of the processes controlling thermal balance of the ionosphere and plasmasphere during a storm, and depletion by more than an order of magnitude of the relative density of hydrogen ions N( H+)/Ne during the storm main phase with its following increase during the recovery phase. One of the reasons of these disturbances could be the shift to midlatitudes of the main ionospheric trough, light ion trough, and hot zone to the geomagnetic shells L located deep within the inner plasmasphere. The nonstationary disturbances of magnetospheric electric fields accompanying the intensification of the auroral electrojets during a substorm on the background of a storm and also energetic particle precipitations from the magnetosphere could lead to a penetration of the magnetospheric electric fields into middle latitudes and destabilize the state of the ionosphere.

[52]  The second group includes the ionospheric storm, which accompanied the minor magnetic storm on 20-21 March 2003 ( Kp approx 5). The magnetic storm began in the morning (0445 UT), the main phase developed slowly (|Dst/dt| approx 5 nT h -1 ) and reached minimum value of index Dst = -57 nT at 2000 UT. The ionospheric storm had a two-phase character and began with a positive phase. The prominent feature of this storm was that its negative phase occurring on the background of weak geomagnetic activity was accompanied by very strong ionospheric disturbances with a depletion in NmF2 by a factor of up to 5, electron temperature increase up to 2400-3500 K at heights of 300-500 km, and uplifting in the F2 layer by more than 100 km during the night on 20-21 March and around sunrise. The reversal of the storm phase occurred during less than a hour in the dusk period was, apparently, caused by a superposition of the effects of two destabilizing factors generated by magnetospheric substorms: the pulse of the electric field in the ionosphere over Kharkov (with the Ey component changing the direction from the westward to the eastward and having the values of -10 and +15 mV m-1 ) and passage of TAD.


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