4. Results of Observations and Modeling: Data Analysis
[10] Traditionally, considerable deviation ( 
20% ) of the
F2 -layer
critical frequencies from the median values over a long period
of time (from half a day to 2-3 days) usually accompanying a
magnetic storm is called an ionospheric storm. It is known [see,
e.g.,
Buonsanto, 1999;
Prölss, 1995]
that the ionospheric and
thermospheric effects of a storm are closely coupled. In this section
we present a brief description of ionospheric and thermospheric
disturbances over Kharkov accompanying three magnetic storms
with a different character.
|
|
Figure 1
|
4.1. The magnetic storm on 25 September 1998
[11] The magnetic storm on 25 September 1998 was associated
with the M6/3B solar flare that occurred during 0644-1009 UT on
23 September 1998. The parameters of heliogeophysical situation
are shown in Figure 1 ( For this and other figures
the dates are shown at the horizontal axis, fluxes of protons are
taken with energies greater than 10, 50, and 100 MeV and of
electrons with energies greater than 2 MeV.)
(http://sec.noaa.gov,
http://swdcwww.kugi.kyoto-u.ac.jp/index.html). The first three
observation days on 21-23 September 1998 that preceded the storm
interval were weakly disturbed days (
Ap=14, 10, 14). A storm
began on 24 September (maximum index
Kp 
5,
Ap=28 ) (see
Figure 1). The disturbance rose to severe magnetic storm on 25
September. The storm was initiated by the arrival of interplanetary
shock associated with the M6/3B flare and registered with the ACE
satellite about 2300 UT on 24 September. The shock was followed
by the southward turning of the IMF
Bz component with a
maximum deflection of
-18 nT at 2333 UT. The solar wind
parameters changed (see Figure 1), the temperature T increased up
to about
7 
105 K and the speed from 440 to 880 km s-1.
The
dynamic pressure of solar wind reached 10 nPa value and the
energy
e,
transferred to the magnetosphere comprised 75-100 GJ s
-1.
The solar flare was accompanied by the ejections of energy
particles. A greater than 10 MeV energy proton flux reached its
maximal level after the midnight on 25 September. The maximum
precipitations of electrons were observed during the main storm
phase. The variations in the
Hp component of the geomagnetic
field were significant on 25 September. Index
Dst during the main
phase of the storm rapidly decreased to
-202 nT value at 0800 UT
and stayed at the level about
-200 nT until 1100 UT. The
Kp index
reached a maximum value of
8+ during 0600-0900 UT. The
recovery phase began after 1100 UT and continued at least to the
end of observation. Against a background of the storm the
sequence of the intense substorms with index
AE=1200-2000 nT
was registered at night 24-25 and on 25 September. During the
days under consideration, the solar activity was moderate with
F10.7=139 on 25 September, and 81-day average
F10.7a =130.
[12] The IS radar measurements were conducted from 1300 UT
on 21 September to 1500 UT on 25 September. The results of the
analysis of the ionosphere processes over Kharkov that
accompanied the geomagnetic storm on 25 September 1998 were
described in detail in some publications, for example, by
Chernogor et al. [2002a, 2002b],
Grigorenko et al. [2003a, 2003b],
Mishin et al. [2001, 2002],
Taran [2001], and
Taran et al. [1999].
Here we consider briefly the main results and their interpretation.
|
|
Figure 2
|
[13] The very strong negative ionosphere storm commenced
soon after the local midnight on 25 September and persisted at
least until the end of the measurements (here with
Kp 7 ). For the
Kharkov IS radar ( l=36.3 o E) local Daylight Saving time
corresponds to
LT
( UT + 0325).
The storm was accompanied by a
decrease in the peak of the electron density
NmF2 during the main
magnetic storm phase approximately by a factor up to 3-3.5 as
compared to the reference day, for which the data averaged over
the previous weakly disturbed days on 21-23 September were
selected (Figure 2a). The maximum decrease of
NmF2 was
observed in the morning time (approximately during 0130-0800
UT). Then
NmF2 gradually increased, reached and even exceeded
(during short time) the reference values
NmF2 near noon,
whereupon again began to decrease up to a factor of 1.4 about 1500
UT. The height of the electron density peak
hmF2 increased at
about 100 km at night and at 50 km near the noon (Figure 2b). In
the morning during the main storm phase, the main electron density
Ne peak descended to the
F1 region (below 200 km) where the
molecular ions dominate.
|
|
Figure 3
|
[14] To study peculiarities of the ionospheric storm, we consider
the vertical profiles of
Ne at successive moments of time (every 15 min)
during the main magnetic storm phase (Figure 3). Within the
time interval of 0430-0830 UT a significant deformation of the
Ne(h) profiles was observed: the electron density in the
F2-layer
maximum decreased, the height and the thickness of the layer
increased, and the shape of the profiles changed. These effects
could be caused by large-scale disturbance in the neutral
composition with a depletion of the ratio
p= N( O)/(N( N2)+N( O2)) and rebuilding in the global thermospheric circulation with an
increase of equatorward neutral wind velocity [see, e.g.,
Brunelly and Namgaladze, 1988;
Buonsanto, 1999].
It is known that such
events are associated with the high-latitude heating of the
thermosphere due to the enhancement of the auroral currents and
energetic particle precipitation during the magnetic disturbances
[e.g.,
Danilov and Morozova, 1985;
Serebryakov, 1982]
that were
observed during this storm (see Figure 1). Changes in the neutral
composition and thermospheric wind could be also transferred from
high to middle latitudes by the traveling atmospheric disturbances
(TADs) generated by enhancements of the auroral electrojets
during magnetospheric substorms [see, e.g.,
Buonsanto, 1999;
Prölss, 1993a, 1993b].
Such substorms with the increase of
index
AE up to about 2000 nT were registered during the main
phase of the storm in consideration (see Figure 1, where hourly
AE index values are taken). For example, at night on 24-25 September
in the absence of ionization production sudden rises in the height of
the
F2 layer were observed at about 2230, 0100, and 0200 UT (see
Figure 2b). One can suppose that they are associated with pulse-like
increases of the
AE index that reached the values of 1400,
2000, and 1600 nT at about 2145, 2345, and 0030 UT, respectively
(see (http://swdcwww.kugi.kyoto-u.ac.jp/index.html), and
AE index
values with 1-min time resolution). A similar case was considered,
for example, by
Prölss [1993a].
From the observed time delay in
the disturbances of
hmF2 height (of about 45, 75, and 90 min in
these cases) and under assumption that the maximum of the source
of high-latitude heating occurs near 70o, i.e., at a distance of
approximately 2000 km from the Kharkov radar, one can obtain the
velocities of TAD propagating toward the equator of about 740,
440 and 370 m s-1, respectively. Such values are typical for the
horizontal phase velocity component of the internal gravity waves
(IGWs) related to the large-scale TADs.
[15] In the course of the
F2 -layer deformation, the density
Ne in
the
F1 layer varied slightly. As a result, the
NmF2/NmF1 ratio
became less than 1 over 0630-0730 UT. The so-called
G condition
happened, when in ionograms the
F2 layer was shielded by the
F1 layer ( foF2 
foF1 ).
Similar effects were described, for example, by
Buonsanto [1995a] and
Mikhailov and Foster [1997].
|
|
Figure 4
|
[16] The simulation using the NRLMSISE-00 empirical model
of the atmosphere
[Picone et al., 2002]
showed that the
development of a deep
Ne depression in the ionospheric
F2 layer
and the
F2-layer decay (the
G condition), only partly could be
explained by the changes in the neutral composition. For example,
at a height of 300 km in the daytime (around 0730 UT) the
parameter
p from the NRLMSISE-00 model decreased by a factor
of 1.1 as compared, for example, to the quiet day on 22 September
(Figure 4), whereas the depletion of
Ne was by a factor of 3.8 (see
Figure 2a). Thus it is required to attract some additional factors to
explain the
Ne depression. Among these factors, there can be: a
correction of the neutral composition taken from the NRLMSISE-00
model for the conditions of geomagnetic disturbances and
taking into account possible contribution to the increase of the O+
ions loss rate of such factors as the atmosphere heating,
enhancement of the electric fields, and the excitation of vibrational
levels of N2 and O2 molecules
[Buonsanto, 1995a;
Mikhailov and Foster, 1997;
Mikhailov and Förster, 1999;
Pavlov, 1998;
Pavlov and Buonsanto, 1996;
Pavlov et al., 1999;
Richards et al., 1994;
Schlesier and Buonsanto, 1999].
It is known that the
character of a magnetic storm is determined by the complicated
interaction of a complex of processes in the near-Earth
environment. Therefore one should expect that the observed
features in the behavior of the disturbed ionosphere are the result of
superposition of the effects caused by different disturbance sources,
and their contribution changes during the storm. One of such
sources (together with the mentioned above) could be the
equatorward shift of the main ionospheric trough caused by the
enhancement of the electric field of the magnetospheric convection
during the main phase of the magnetic storm. The trough shift is
confirmed by the analysis of the maps of global distribution of the
total electron content (TEC) obtained from the GPS navigation
system data
[Afraimovich et al., 2002].
It follows from this analysis
that on 25 September during the main phase of the storm, the
low-latitude wall of the trough in the European region reached a
geographic latitude
j=50-40o and the Kharkov radar could enter
into the trough (in the night and dawn sectors).
|
|
Figure 5
|
[17] The time variations of electron
Te and ion
Ti temperatures at
heights of 250-500 km are shown in the Figure 5. Under quiet
conditions the electron temperature at midlatitudes is determined
by a balance between heating by photoelectrons, thermal
conduction along the magnetic field lines, and cooling due to
collisions with ions and neutrals. After the storm commencement
the increase in
Te
was noticed. The enhancement in
Te increased
with altitude and at
0200 UT and 500 km altitude it reached
700 K as compared to a quiet day on September 23 (see Figure 5).
Increase in
Te could be due to Joule heating associated with
penetration of magnetospheric electric fields to midlatitudes and
energetic particle precipitations, and also decreased cooling due to
very low electron density in the morning time [see, e.g.,
Buonsanto, 1995a].
The shift toward equator of the precipitation zone could be
indirectly confirmed by the maximum values of the POES auroral
activity level equal to 10, which were registered on board the
NOAA POES 12, 14 and 15 satellites on 24-25 September during
2138 UT-1255 UT interval
[http://www.sec.noaa.gov/ftpdir/lists/hpi/power_1998.txt].
The statistical pattern describing
the auroral oval is appropriate to the Auroral Activity Level
determined from the particle power flux observed during the most
recent polar satellite pass. The value 10 of this parameter confirms
that the equatorward boundary of the auroral oval could shift to
geomagnetic latitudes
f 51-45o [http://www.sec.noaa.gov/Aurora/index.html].
Thus the Kharkov radar ( f = 45.7o ) that was
close to the midnight sector during the storm main phase could be
situated within the trough (see above) and not far from the
equatorward boundary of the auroral oval.
[18] The peculiarity of the electron temperature behavior was a
decrease in
Te after the sunrise (for the Kharkov radar the sunrise
was nearly 0325 UT on the Earth's surface) that was by about of
500 K at a height of 250 km around 0600 UT and decreased with
the altitude growth. This decrease could be caused by several
factors, including the intensification of the cooling of the electron
gas due to the started morning increase in
Ne (see Figure 2a), etc.
[19] Ion temperature under quiet conditions depends on a
balance between heating due to collisions with electrons and
cooling via collisions with the neutrals. At night, when the heating
of electrons is reduced, electron, ion and neutral temperatures tend
toward the common value. During this storm the increase in the ion
temperature
Ti (approximately by 300 K about 0600 UT at 300 km
as compared to a quiet day on 23 September) till the end of the
measurements (see Figure 5) could be the result of the Joule and
the frictional heating associated with the intensification of the
ionospheric currents (see below)
[Buonsanto, 1995a;
Richards et al., 1994].
|
|
Figure 6
|
[20] The temperature
Tn of the neutral gas was derived from the
IS data by solving the heat balance equation
[Salah and Evans, 1973;
Salah et al., 1976]
and using the NRLMSISE-00 model of
the atmosphere. The calculations showed that during magnetic
storm the increase in
Tn (Figure 6) was on the average nearly 200 K
at a height of 300 km. The heating of neutrals could be related to
both, nonlocal source of the heat transported from the region of the
high-latitude heating of the thermosphere
[Danilov and Morozova, 1985;
Mikhailov and Foster, 1997;
Serebryakov, 1982]
and local
Joule heating due to the penetration of the magnetospheric electric
fields to midlatitude (see below)
[Mikhailov and Foster, 1997].
Probably both phenomena could occur in the ionosphere over
Kharkov during the main phase of the storm. As the obtained
results showed (see Figure 6 and
Grigorenko et al. [2003b]),
the
nonlocal heating apparently dominated, because the
Tn disturbances
were propagating from the above with a velocity of about 50 m s
-1 (the time delay in the 400-250 km altitude range was about 50 min)
and this value could correspond to the vertical component of the
IGWs velocity related to TADs. The friction heating due to the ion
drift with respect to the neutral gas could provide also some
contribution to the increase of
Tn during the electric field
enhancement over radar
[Buonsanto, 1995a].
It should be also
noted that the NRLMSISE-00 model (see Figure 6) and MSIS 86
model
[see Grigorenko et al., 2003b]
give underestimated values of
Tn (at a height of 300 km by 450 and 350 K, respectively) in
disturbed conditions and require correction. This disagreement
(with MSIS 86 model) has been also noted by other authors [see,
e.g., Buonsanto, 1995a;
Mikhailov and Förster, 1999;
Richards et al., 1994].
[21] It should be noted that during quiet days
Tn at the heights of
250 and 300 km, apparently, reached the exospheric temperature
value and changed slightly, whereas above 300 km the calculated
Tn
value decreased with the height growth (see Figure 6). This
could manifest that the method applied for calculation of
Tn at large
heights is incorrect and requires taking into account thermal
conductivities of ion and neutral gases. At the same time on the
disturbed day 25 September thermal conductivity effects can be
neglected at least up to the height of 450 km (see Figure 6), that
probably can be explained by the disturbance in the neutral
composition (increase in N2 and O2 concentrations)
[Prölss, 1993a, 1993b]
(see Figure 4) and in collision frequencies of charged and
neutral gas species.
[22] As was mentioned above, the technique we applied to
calculate
Ti and
Te insufficiently reflects changes in ion
composition.
Mikhailov and Schlegel [1997] and
Mikhailov and Foster [1997]
indicated that incorrect consideration of ion
composition could result, e.g., in underestimation of
Ti and
Te up to
50% during geomagnetic disturbances. Therefore we should
anticipate that the calculated values of
Tn (as well as other derived
parameters, e.g., energy input rate to the electron gas
Q/Ne, the heat
flux density
PT transferred by electrons from the plasmasphere,
etc.) only qualitatively describe the behavior of the disturbed
atmosphere.
|
|
Figure 7
|
[23] The mechanisms of the ionospheric disturbance considered
above can be attracted for explanation of the observed reversal in
the vertical plasma drift velocity and plasma flux during the main
phase of the magnetic storm (Figure 7). In the morning hours (near
0400 UT) of the disturbed day, for example, at altitudes 250-350 km,
the vertical velocity and flux density of plasma were
Vz 50-35 m s-1 and
F p (4-3) 1012 m-2 s-1,
respectively, whereas on the
quiet day (on 23 September) these values were
Vz -(25-20) m s
-1 and
F p -(8-4) 1012 m-2 s-1,
respectively.
[24] It is known that at midlatitudes the ion drift velocity is
determined by the influence of three mechanisms: ambipolar
diffusion along the geomagnetic field lines, neutral wind and
E B drift of ions. Near the peak of the
F2 layer where the O+ ions
dominate the vertical ion velocity may be written as [see, e.g.,
Brunelly and Namgaladze, 1988]
 | (2) |
where
(Vd|)z ,
(Vn|)z ,
(V
)z are the components of the vertical ion
velocity due to ambipolar diffusion, neutral wind and
electromagnetic drift, respectively (subscript parallels and
perpendiculars relate to the ion velocities parallel and perpendicular
to the geomagnetic field induction vector). Substituting the value of
each term in the expression (2), we obtain the velocity
Vz in the
form
 | (3) |
where
 | (4) |
is the vertical velocity component due to ambipolar diffusion,
Da= kTp/mi Snin is the ambipolar diffusion coefficient,
Vnx,
Vny,
Vnz are the meridional, zonal and vertical components of the neutral
wind velocity positive in the north hemisphere when they are
directed toward the geographic south, the east and at zenith,
respectively,
nin is the ion-neutral collision frequency,
Hp=kTp/mi g is the scale height of plasma,
Tp=Te+Ti is the
plasma temperature,
I and
D are the geomagnetic field inclination
and declination (for Kharkov
I = 66.4o,
D = 6.7o );
Ex and
Ey are the
components of electric field intensity vector, directed toward the
geographic south and the east in the north hemisphere,
B is the
absolute value of the geomagnetic field induction vector,
mi is the
ion mass (near the peak of the
F2 layer, predominant ion is O+ ).
|
|
Figure 8
|
[25] From the radar measurements of
Vz and calculation of the
diffusion velocity
Vdz, the velocity
W that includes both electric
field and neutral wind effects can be detected:
 | (5) |
If the declination
D effects in the expression of
Vz (equation (3))
are neglected due to their smallness the velocities
Vn|z and
V z may
be written as
 | (6) |
 | (7) |
In the morning hours (near 0400 UT) on the disturbed day 25
September
W =100 m s-1 at a height of 300 km, whereas on a quiet
day
W 0 (Figure 8). One of the reasons of this disturbance
W and
Vz could be equatorward surge in the neutral wind
Vnx caused by the
high-latitude heating and TAD
[Buonsanto, 1995a;
Buonsanto et al., 1999;
Prölss, 1993a, 1993b;
Richards et al., 1994].
If the electric fields are neglected, that is correct for the
magnetic quiet conditions (as on 21-23 September when
Ap= 14,
10, 14), the meridional (equatorward positive) component
Vnx
of
the thermospheric wind velocity is presented in the form
 | (8) |
This velocity should have a value of
270 m s
-1 (see Figure 8).
The other reason could be a penetration into midlatitudes of the
nonstationary magnetospheric electric field
[Buonsanto et al., 1999;
Foster and Rich, 1998;
Foster et al., 1998a]
with the zonal
component
Ey = 12-17 mV m-1 (determination of
Ey from radar
measurement of
hmF2 see below) capable also to provide
W V z =100-130 m s-1.
This case is the limiting one that neglects the
neutral wind effects and gives the upper estimations of
Ey and
V z.
In expression (7),
B 5 10-5 T
was taken for Kharkov. Probably,
both factors contribute to the increase in
W and
Vz. The high
substorm activity (in the auroral region the
AE index reached
800-1200 nT at 0300-0340 UT interval) and also the upward
propagation of the disturbance in
Vz
with a velocity of about
100 m s-1 (the delay of the disturbance in
Vz at 250-500 km altitude range
was about 40 min; see
Grigorenko et al. [2003b] and Figure 7)
could testify to the predominance of the electric field pulse effects
over Kharkov. The disturbance in
Vz could be caused by the local
Joule heating of the atmosphere at the dynamo-region heights
(100-110 km), related to the disturbance in the electric field over
Kharkov, and upward motion of the gas (similarly to the
high-latitude heating source
[Mikhailov and Förster, 1999]).
The
decrease of
Vz with the height growth is likely associated with the
dissipation of kinetic energy of gas due to viscosity and thermal
conductivity.
[26] The effects of the electric field penetration (together with
the enhancement of the equatorward meridional winds due to the
intensification of the high-latitude thermosphere heating source and
with the trough shift to midlatitudes) could be one of the reasons of
the long-lasting increase in the height of the electron density peak
hmF2 by about 100 km at night and 50 km around the noon as
compared to the quiet day on 23 September (see Figure 2b).
However, these effects are not related to the nonstationary
magnetospheric electric fields as in the case considered above.
These effects could be caused by long-lasting precipitation of
energetic particles registered during the storm (see Figure 1).
Particle precipitations lead to an increase in the conductivity of the
underlying auroral ionosphere and short circuiting of the shielding
polarization field, and promote a penetration of the magnetospheric
electric fields to midlatitudes [see, e.g.,
Brunelly and Namgaladze, 1988;
Gonzales et al., 1983].
Estimations of
Ey were derived
from
hmF2 deviations during magnetic disturbances according to
simplified empirical dependence given by
Brunelly and Namgaladze [1988].
They showed that over Kharkov such fields
should be eastward directed, should have the values of
Ey 17 and
12 mV m
-1 at night and in the daytime, respectively, and can
contribute into the observed uplifting of the
F2 layer.
|
|
Figure 9
|
4.2. The magnetic storm on 29-30 May 2003
[27] The magnetic storm on 29-30 May 2003 was caused by the
arrival of two interplanetary shocks from the X1.3 and X3.6 flares
on 27-28 May. The main parameters of heliogeophysical situation
are presented in Figure 9. The first shock passed the NASA/ACE
spacecraft on 29 May at 1150 UT. The second and stronger shock
passed ACE at 1830 UT with 125 km s-1 increase in the solar wind
speed up to over 800 km s-1 and
Bz deflections that ranged between
-20 nT and
+25 nT. The solar wind temperature increased, the
dynamic pressure exceeded 15 nPa,
e energy reached the value of
50 GJ s-1 at night on 29-30 May.
Hp component of geomagnetic
field varied in the 0-200 nT range. The greater than 10 MeV proton
fluxes were observed during several days. The maximum
precipitations of electrons began in the premidnight sector on 29
May and continued till local noon on 31 May. Magnetospheric
substorms with index
AE value greater than 2000 nT were
registered during storm. The geomagnetic response to these events
was severe storm with maximum indices
Ap=89,
Kp = 8+. The
storm commenced suddenly on 29 May about 1225 UT. The main
phase developed slowly. The
Dst index rapidly decreased to
-108 nT at 2300 UT on 29 May and remained at the level of
-(116-131) nT until 0300 UT on 30 May, which was followed by the recovery
phase related to northward turn of the IMF
Bz component. For this
storm solar activity was moderate with
F10.7=138 and 117 on 29
and 30 May, respectively, and
F10.7 a=124. The Kharkov IS radar
was operated on 30-31 May according to the international program
Low/High Latitude.
|
|
Figure 10
|
|
|
Figure 11
|
|
|
Figure 12
|
[28] The magnetic storm was accompanied by a strong
ionospheric storm. Detailed description of the results of
observation obtained by the Kharkov IS radar was published by
Grigorenko et al. [2005a, 2005c].
Substantial effects of a negative
ionospheric disturbance were revealed (Figure 10). Among them
there were: a depletion of
NmF2 by a factor up to 4 during the storm
main phase (Figures 10 and 11a); unusual plasma heating at night
on 29-30 May when the ion and electron temperatures at altitudes
of about 300-800 km increased up to the daytime values of 1200-2400 K
and 2000-3200 K, respectively, whereas during quiet
conditions the values of these temperatures at night were about 800 K
(Figures 10 and 12). One of the reasons of these disturbances
could be the shift of the main ionospheric trough to midlatitudes
and also the shift of the hot zone together with the plasmapause to
lower
L shells
[Buonsanto, 1995a, 1995b;
Richards et al., 1994].
Such phenomena in the ionosphere related to the inner
plasmasphere (for Kharkov
L 1.9 ) occur rarely. The trough
equatorward shift was indirectly confirmed by the maximum values
of the POES Auroral Activity Level equal to 10, which were
registered on board the NOAA POES 14, 15, 16, and 17
satellites during the storm main phase (approximately from 2223 UT
on 29 May till 0233 UT on 30 May) and could manifest the
shift of the auroral oval equatorward boundary toward geomagnetic
latitudes
f 51-45o (similar to magnetic storm on 25 September
1998). Thus the oval was able to approach the Kharkov radar
latitude near local midnight.
[29] It should be noted that we took as a reference data of the
averaged values of
foF2 on quiet days 19 and 20 May 2003
obtained by the ionosonde at San Vito (the geographic and
geomagnetic coordinates are 40o N, 17o E and 39.7o, 96.4o,
respectively), and also the results of ionosphere measurements on
26-27 May 1998 and 23-24 June 1998 (due to the absence of closer quiet periods). The
latter periods were on the rising branch of the current solar cycle 23
but similar to the considered period with respect to the
parameters of the heliogeophysical conditions (summer, moderate
solar activity).
[30] An increase in the
hmF2 height by about 160 km during the
storm main phase (at night) and by 70 km near noon as compared
to reference day on 26-27 May 1998 was
registered (Figures 10 and 11b). The lifting of the
F2 layer is probably explained by the
joint action of several factors. They include expansion of the
thermosphere, the increase in the equatorward meridional velocity
of the thermospheric wind, and the trough shift to middle latitudes.
Along with the above reasons, the penetration of magnetospheric
electric fields to midlatitudes, similar to the magnetic storm on 25
September 1998, could contribute to an increase in
hmF2 due to
long-lasting (for more than a day) precipitation of energetic protons
and electrons. The latter fact could be manifested in the increase in
the flux density of these particles registered on board the GOES 8
and GOES 12 satellites (see Figure 9). In the case when the effects
of electric fields are predominant, the maximal values of the
eastward zonal field component over Kharkov (determined from
the upper
Ey estimates obtained based on a change in
hmF2 ) are
approximately equal to 25 and 20 mV m
-1 at night and in daytime,
respectively.
|
|
Figure 13
|
[31] It should be referred to an unusual phenomenon that was
observed near the sunrise on 30 May. It included a quasiperiodic
disturbance in the velocity
Vz
for about
0200-0400 UT (Figure 13) against a background of the unusual morning decrease
in
NmF2
(see Figure 11a), a sharp decrease and the following increase in
hmF2 by
about 160 km (see Figure 11b, the time of these perturbations is
shown by horizontal segments), and deformation of the
Ne profile
|
|
Figure 14
|
(Figure 14) (see
Grigorenko et al. [2005a, 2005c]
for details). One
should note that such phenomena also could result from the
superposition of the effects of different disturbance sources [see,
e.g.,
Buonsanto, 1999].
One of these sources could be the
penetration to midlatitudes of the nonstationary magnetospheric
electric fields
[Gonzales et al., 1983;
Foster and Rich, 1998;
Foster et al., 1998].
Thus an increase in
hmF2 by about 90 km at 0330-0400 UT
and later (ignoring
hmF2 changes due to
Ne profile
stratification, see Figures 11b and 14) could be caused by a pulse of
the electric field in the ionosphere over Kharkov with the eastward
zonal component
Ey 
20 mV m-1. The sharp turn of the IMF
Bz component from the south to the north and change in the dynamical
pressure of the solar wind after midnight (see Figure 9) could be
the sources of the electric field pulse in the magnetosphere.
|
|
Figure 15
|
[32] The decrease in the relative density of hydrogen ions
N( H+)/ Ne at altitudes of 1000-1500 km more than by an order of
magnitude during the storm main phase (at night on 29-30 May),
as compared to a reference day on 26-27 May 1998, with its
following increase in the daytime on 30 May during the recovery
phase (Figure 15) points out the processes of emptying and further
filling of the magnetic flux tube over Kharkov radar
[Bailey et al., 1979;
Brunelly and Namgaladze, 1988;
Krinberg and Tashchilin, 1984;
Naghmoosh and Murphy, 1983].
The tube is located in the
inner plasmasphere and usually is slightly influenced by magnetic
disturbances. These effects could be related to the equatorward
shift of the main ionospheric trough and the light ion trough and,
probably, are accompanied by a change of the processes of
ionosphere-magnetosphere interaction. The development of the
process of filling in of the magnetic flux tube is defined by a
decrease of the height
ht, where
N( O+) = N( H+)
|
|
Figure 16
|
(Figure 16). At
night on 30 May,
ht exceeded 1500 km and on 31 May
ht decreased
down to near 1000 km. During quiet conditions
ht 700-850 km,
that is, the
ht height did not reach the level preceding the beginning
of the storm. It should have been expected, because the process of
filling in of magnetic flux tubes proceeds slowly with a time
constant proportional to
L4 [Brunelly and Namgaladze, 1988;
Krinberg and Tashchilin, 1984;
Saenko et al., 1982].
|
|
Figure 17
|
[33] The magnetic storm was also accompanied by
thermospheric disturbances. The calculations using the
NRLMSISE-00 model showed that on 30 May, for example, in the
daytime about 0800 UT, the changes in the neutral composition led
to the depletion of the
p parameter by a factor of 1.4 as compared
with quiet days on 19-20 May (Figure 17). However, it could not
provide the observed for this time depletion of
NmF2 by a factor of
2.5. In the same way as in the case of the 25 September 1998
storm, a correction of the model, or attraction of other factors
considered above were required. The distinction of the neutral
composition and parameter
p during the reference days on 19 and
20 May and before the storm commencement on 29 May
(at 0000-1200 UT) can be explained by the fact that before the reference day
the conditions were quiet (for 16-18 May
Ap= 10, 9, and 9,
maximal indices
Kp 3 ), and for 29 May the conditions were
disturbed (on 26-28 May
Ap = 18, 26, and 36, maximal indices
Kp 4, 5, and 6, respectively). It is known that in the model
NRLMSISE-00 the values of the 3 hours
Ap indices are taken with
the "history" (within 72 hours).
|
|
Figure 18
|
[34] The neutral temperature
Tn, as calculations showed (see
Grigorenko et al. [2005a, 2005c]
and Figure 18), during the storm
main phase ( Kp 8 ), when an unusual plasma heating was
observed against a background of deep
Ne depression in the
F region, was about 1000-1350 K at altitudes of 220-470 km,
respectively. For comparison, we note that the
Tn values decreased
by approximately 200-350 K at the same altitudes on the next night
during the recovery phase ( Kp 5 ). The heating of the atmosphere
led to the increase of the thermopause height during the storm main
phase at least up to 400 km, whereas on a quiet day its height was
about 300 km.
|
|
Figure 19
|
[35] Considerable variations in the thermal regime of plasma
accompanied this magnetic storm. The calculations showed that on
the disturbed day about noon, the rate of the energy input to electron gas
Q/Ne decreased as compared with a quiet day by a factor
of 1.6 (Figure 19). This energy is determined from the thermal balance
equation for electrons. Thermal electrons are heated in the process of
thermalization of suprathermal electrons, and this process in the
lower ionosphere ( h 300-350 km) is local because the mean free
paths of these electrons are small. The main mechanisms of
electron gas cooling at these altitudes are Coulomb collisions of
electrons with ions and excitation of the fine structure levels of
oxygen atoms
[Banks, 1966;
Shunk and Nagy, 1978].
In such a
case, the electron energy balance equation in the SI system can be
written in the following form for stationary conditions
[Banks, 1966;
Dalgarno and Degges, 1968]:
 | (9) |
 | (10) |
 | (11) |
where
Q is the rate of the energy transfer to thermal electrons during
Coulomb collisions with suprathermal electrons;
Lei is the energy
loss rate during electron-ion collisions; and
Le is the rate of the energy loss by
excitation of the fine structure of oxygen atoms (
Q,
Lei, and
Le are the corresponding energy values per unit time reduced to unit
volume). Figure 19 presents results of calculating the
Q/Ne energy
transferred to electron per unit time, as well as the components of
the electron gas energy loss during heat exchange with ions
Lei/Ne and neutrals
Le/Ne. Figure 19 indicates that under the quiet and
disturbed conditions the contributions of
Lei/Ne and
Le/Ne components to the process of electron gas cooling were different.
|
|
Figure 20
|
[36] The decrease in
Q/Ne on the disturbed day was accompanied
by an increase in the heat flux density
PT, transferred from the
plasmasphere due to electron thermal conductivity by a factor of
1.2 (Figure 20). The vertical component of the heat flux density is
where
ke=2.082 k Ne Te/ m nei is the coefficient of the electron gas
thermal conductivity,
k is the Boltzmann constant,
m is the electron
mass, and
nei is the electron-ion collision frequency. In the SI
system
[Ginzburg, 1967]:
Substantial values of the energy input rate
Q/Ne and the heat flux density
PT during
the storm main phase (at night on 29-30 May) became the specific
feature of the thermal regime of the ionosphere rarely observed at
midlatitudes. Under quiet conditions, these nighttime values were
close to zero. These effects manifest the change in the processes
controlling the heat balance in the ionosphere-plasmasphere
system during a storm
[Banks, 1966;
Shunk and Nagy, 1978].
|
|
Figure 21
|
4.3. The magnetic storm on 20-21 March 2003
[37] The magnetic storm on 20-21 March 2003 proceeded
against a background of high flare activity on the Sun. However,
the geoefficiency of the flares was low, and the flares resulted in a
minor magnetic storm on 20-21 March 2003 (maximal index
Kp =5 ). Variations in heliogeophysical parameters are presented in
Figure 21. After a sudden storm commencement at 0445 UT on 20
March with
Dst increasing up to 15 nT at 0600 UT,
Dst decreased to
-57 nT at 2000 UT. The recovery phase began after 2100 UT and
continued till the end of the observations. Solar wind parameters:
the temperature and the speed changed weakly, the dynamic
pressure did not exceed 4 nT value, the value of the Akasofu
function
e was less than 30 GJ s
-1.
The energetic proton fluxes and
precipitations of electrons were not practically registered. The
variations of the
Hp component of geomagnetic field were
insignificant. The high substorm activity with the values of index
AE=1000-1500 nT was observed during the sunset period on 20
March. The storm occurred during moderate solar activity, with the
values of index
F10.7=97 and 91 on 20 and 21 March, respectively,
and an 81-day average value of index
F10.7 a =132. The
measurements were conducted on 19-23 March according to the
Storms/TIMED/LTCS program. The radar operated in two-pulse
sounding mode in the 100-550 height range with the altitude
resolution of about 10 km. The effects of this geomagnetic storm in
the ionospheric
F region and upper thermosphere over Kharkov
were described in detail by
Grigorenko et al. [2005b, 2005d].
Here we consider briefly the main results of the study.
|
|
Figure 22
|
[38] The magnetic storm was accompanied by a two-phase
ionospheric storm (Figure 22a). A peculiarity of the latter was that
its strong negative phase occurred against a background of a minor
disturbance of the geomagnetic field ( Kp 5).
An increase in
NmF2 by about a factor of 1.5 during the positive phase of the storm and a
decrease in
NmF2 by a factor up to 5 during the negative phase (in
the morning hours) as compared to the reference day were
registered (Figure 22b). As reference data the
foF2 values were
taken during the quiet day on 19 May 2003. They were obtained by
the ionospheric stations at Kharkov within 1230-2400 UT and at
San Vito within 0000-0730 UT. The information from other
|
|
Figure 23
|
(closer) stations was absent for this period. Figure 23 illustrates the
behavior of the electron density
Ne and other ionospheric
parameters at altitudes of
100-550 km during the storm. A
significant distinction of the daytime electron temperatures
Te during the positive (1300 K at an altitude of 300 km) and negative
(2400 K) storm phases from the value
Te=1700 K on a quiet day
was detected. The distinction in
Te values during considered days is
explained by the different cooling rates of the electron gas in the
process of elastic heat exchange with ions, the cooling being
proportional to
Ne2. The ion temperature
Ti increase in the daytime
at altitudes of 250-300 km was about 50 K and 100-150 K on 20
and 21 March, respectively. Apparently, the
Ti growth was related
to a high-latitude source of atmospheric heating which was not
considerable during the minor magnetic storm and also to the
increased electron-ion heat exchange due to the substantial
difference in the temperatures of electrons and ions on 21 March.
|
|
Figure 24
|
[39] The positive storm phase on 20 March had a character of a
long-duration disturbance and lasted approximately for 6 hours. It
could be caused by the enhanced equatorward meridional wind
Vnx,
related to the high-latitude thermosphere heating
[Buonsanto, 1998;
Danilov and Morozova, 1985;
Danilov et al., 1985a].
A comparison
with the quiet period (22 and 23 September 1998 which were
chosen as reference days due to absent of any other, more suitable
period) indicated that the downward velocity
Vz at altitude of 300 km
decreased on average by approximately 10 m s
-1 near local
noon on 20 March 2003 (Figure 24). For Kharkov LT
(UT + 0225)
in March. If we neglect the contribution of additional electric
fields during minor storm and the change of diffusion velocity, we
obtain the upper estimate of the
DW 10 m s-1
that lead to the
observed increase in
hmF2 by about 20 km. In this case an
additional velocity value
DVnx should be about 25 m s-1.
[40] Let us consider more in detail the processes accompanied
the ionospheric storm phase reversal. It occurred in the sunset
period and was accompanied by a decrease in
hmF2
by 50 km
during 1700-1800 UT with a subsequent ascent of the layer by
almost 200 km from 1800 to 1900 UT (Figure 22c). On the
reference days of 22-23 September 1998
[see
Grigorenko et al., 2003b]
and 19 March 2003
(see Figure 22c) a change in the
hmF2 at
the sunset was much smaller (about 50 km). At the same time on 20
March,
hmF2 rapidly (for not more than 30 min) decreased by 40 km.
Fifteen minutes later,
hmF2 increased by about 90 km during
one record of measurements, i.e., within 15 min (the data
processing was performed at a 15-min signal integration).
[41] It is known [see, e.g.,
Foster and Rich, 1998;
Foster et al., 1998]
that rapid changes in
hmF2 (together with
Vz,
see Figure 24) can be caused by nonstationary disturbances of magnetospheric
electric fields and by the penetration of these fields to midlatitudes.
Another mechanism explaining such variations can be related to
propagation of TADs generated by an enhancement of auroral
electrojets during a substorm [see, e.g.,
Prölss, 1993a, 1993b].
Such a substorm with a maximal
AE value of 1500 nT at
auroral latitudes was registered about 1800 UT on the considered
day (see Figure 21), i.e., during the ionospheric storm phase
reversal. However, TAD effects cannot explain the initial decrease
of
hmF2 by 50 km since TADs are characterized by the transport of
the equatorward meridional wind, as a result of which the
F2 layer
ascends and
hmF2 increases both in daytime and at night.
Moreover, TAD (which propagates from the auroral region with a
velocity of 400-700 m s
-1 ) appears at midlatitudes with a delay of
about 1.5-1 hours. In our case this delay relative to the time of the
substorm intensity maximum was almost absent.
[42] Proceeding from the aforesaid, we can assume that an initial
decrease in
hmF2 could be caused by the penetration into the
ionosphere over Kharkov of the magnetospheric electric field with
a westward zonal component of
Ey - 10 mV m
-1. The estimation
of
Ey value was obtained from the
hmF2 variations
[Brunelly and Namgaladze, 1988].
Such cases of the
hmF2 decrease are
considered, e.g., by
Reddy and Nishida [1992] and
Prölss [1993b]
and are explained by increased magnetospheric convection before a
substorm, which generates the westward electric field.
[43] The following increase of
hmF2 could be caused by the
regular equatorward turning of
Vnx during the sunset, when
DhmF2 50 km
as on quiet days, and by the additional change in
Vnx (DVnx 25 m s
-1 ),
which occurred in the preceding hours and was one of
the reasons of the initial positive phase (see above). This additional
change in
Vnx could explain the
F2 -layer lifting by about 20 km
more.
[44] Now we consider the mechanisms that can explain the
remaining increase of
hmF2 by about 130 km. One of these
mechanisms could be the eastward electric field. If we assume that
the registered uplifting of the
F2 layer by 90 km during 15 min was
caused by this electric field effects, we obtain
Ey 15 mV m-1. The
cases of such rapid eastward-westward switching of the electric
field related to changes in the electrodynamic conditions during a
magnetospheric substorm (gradients of conductivity, electric fields
and currents at the edges of the westward auroral electrojet,
nonstationary magnetospheric convection, etc.) were discussed,
e.g., by
Reddy and Nishida [1992].
One of the reasons of
nonstationary magnetospheric convection could be the abrupt
change of the solar wind parameters: the speed
V sw,
IMF
Bz component, the dynamic pressure
p sw around 1800 UT (see Figure 21).
[45] The second mechanism could be related to TAD, which
results in an increase in the
hmF2 after switching off of the
previously acting westward electric field. TAD was able to provide
the remaining 40-km lifting of the
F2 layer. Such examples of a
successive action of the electric field and TAD on the ionosphere
were described, for example, by
Reddy and Nishida [1992] and
Prölss [1993b].
However, TAD effects should have been related to
the earlier substorms (see Figure 21) with smaller intensity
( AE =600-900 nT). Nevertheless, the TAD effects were found out. They
were observed as a delay of the velocity
Vz disturbance that
propagated from top to bottom (see Figure 24b). The delay was 80 min
in altitude range of 400-200 km. This corresponded to a
velocity of about 40 m s
-1, which is typical of the vertical
component of IGW velocity. Besides, the
Vz disturbance amplitude
increased up to an altitude of 350 km and then began to decrease.
Such features of the wave energy dissipation with an increase of
height are also typical of IGWs. From this we can conclude that an
increase of
hmF2 by about 130 km could result from the
superposition of the effects of two sources: the eastward electric
field, which is related to the intense substorm that occurred at 1800
UT ( AE=1500 nT), and TAD generated by the earlier substorm,
e.g., after 1600 UT ( AE=600-900 nT).
[46] Thus an unusual behavior of
hmF2 during the sunset on 20
March can be explained in the following way. The 50-km decrease
and the subsequent 90-km rapid increase in
hmF2 (within the total
approximately 200-km increase) were probably caused by the
nonstationary electric field, with the zonal component changing its
direction from westward to eastward ( Ey = -10 and
+15 mV m
-1 ),
which penetrated to midlatitudes. It should be mentioned that the
hmF2 variations correlated with the
Vz changes from
+10 to
-35 m s
-1 and, later on, to
+20 m s-1 at an altitude of 300 km
(see Figures 22c and 24b) in the time interval approximately 1700-1900 UT
with a delay of about 20 min, which is typical for the effects caused
by electric field disturbances in the ionosphere
[Foster and Rich, 1998;
Foster et al., 1998a].
These disturbances were probably
related to the intense substorm that occurred at 1800 UT (see
Figure 21) when at auroral latitudes the electric field strength could
reach
70-100 m V m-1 at the values of index
AE=1000-1500 nT
[Krinberg and Tashchilin, 1984;
Serebryakov, 1982].
Penetrating to
midlatitudes, such a field could reach the calculated value
|Ey| (10-15) mV m
-1 and destabilize the behavior of the ionospheric
F2 layer.
Besides, TAD, which could be caused by a less intensive
substorm occurred after 1600 UT, also could contribute to an
additional increase in
hmF2 by about 40 km.
|
|
Figure 25
|
[47] At the ionospheric storm phase reversal, the
NmF2 decrease
approximately by a factor of 2 (whereas on the quiet day such
decrease is by about 20%) became a beginning of a deep negative
disturbance. The estimates showed
[Grigorenko et al., 2005b, 2005d]
that the decrease in
NmF2 could be caused by the increase in
the downward plasma drift velocity
Vz (see Figure 24b) and the
change in the velocity
W by
-25 m s
-1 near
F2 -layer peak against a
background, of
b( O+) loss coefficient increase by a factor of almost
5 at a decrease in
hmF2 from 280 to 230 km (see Figure 22c). The
further development in the depression in
Ne (by a factor of 4-5 in
the morning on 21 March), could be related to the change of neutral
composition and increase in the
N( N2) and
N( O2) concentrations
that should be maximum in the dawn sector. However, the
NRLMSISE-00 model data did not confirm this assumption:
N( N2)
and
N( O2) concentrations about noon on 21 March, on the contrary,
decreased by a factor of 1.7 and
p parameter increased by a factor
of nearly 1.3 as compared to a quiet day on
19 March (Figure 25). Possibly, model requires the correction of the values of
N( N2) and
N( O2) as it was shown, e.g., by
Pavlov et al. [2004].
Besides, one
more mechanism (vibrational excitation of
N2 and O2 molecules
[Buonsanto, 1999;
Pavlov, 1998;
Pavlov et al., 1999
Richards et al., 1994])
could begin to operate at the sunrise when
plasma rapidly warmed up ( Te increased to 2000-3500 K) against a
background of low
Ne values. This mechanism becomes substantial
at
Te 2000 K
[Banks, 1969;
Brunelly and Namgaladze, 1988;
Pavlov et al., 1999;
Shunk and Nagy, 1978].
The contribution of excited N2 and O2 molecules also accelerates the loss of O+ ions.
|
|
Figure 26
|
[48] The considered minor magnetic storm did not cause
considerable thermospheric disturbances. The daytime neutral
temperature
Tn as indicated calculations based on the radar data and
NRLMSISE-00 model increased by approximately 50 and 100 K
during the positive and negative ionospheric storm phases,
respectively, as compared to quiet days on 19 and 23 March and
the height of the thermopause where the atmosphere becomes
isothermal was about 300 km (Figure 26). We established that the
daytime
Tn values obtained from NRLMSISE-00 model are lower
than the calculated values. Under slightly disturbed conditions on
22 March, the differences were about 80 K at an altitude of 300 km,
whereas during the storm, they were up to 130 K on 20 March,
and about 180 K on 21 March.
|
|
Figure 27
|
[49] At the same time, the plasma thermal regime appeared to be
sensitive to ionospheric disturbances. The calculations
demonstrated that the rate of the energy input to the electron gas
(per one electron)
Q/Ne near noon at an altitude of 300 km during
the storm negative phase (21 March) was factors of 2.5-4 and up to
2 higher than the
Q/Ne value during the positive phase (20 March)
and on the slightly disturbed day (22 March), respectively
(Figure 27). Simultaneously the absolute value of the heat flux density
PT transferred by electrons from the plasmasphere also reached the
|
|
Figure 28
|
maximal value during the negative phase (Figure 28). This is
explained by the fact that
PT depends strongly on
Te ( PT Te5/2 dTe/dz ).
The observed effects could manifest the
changes in the ionosphere-plasmasphere thermal balance during the
storm.
Powered by TeXWeb (Win32, v.2.0).
