[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 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
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 5). The magnetic storm began in the morning (0445 UT), the
main phase developed slowly
(|Dst/dt|
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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