A. N. Fahrutdinova, O. N. Sherstyukov, and S. V. Maksyutin
Kazan State University, Kazan, Tatarstan, Russia
Received December 17, 1998
The intense studies of the atmospheric physics and solar-terrestrial relations carried out during the recent decade [Fahrutdinova and Berdunov, 1994; Fahrutdinova et al., 1994; Kazimirovsky and Vergasova, 1991; Vergasova and Kazimirovsky, 1992] made it possible to discover solar and geomagnetic impacts on the state of the Earth's atmosphere. For example, Kazimirovsky and Vergasova [1991] discovered a reverse of the wind direction during intense magnetic storms accompanied by a dispersion of the wind velocity. A considerable change of the phase of the semi-diurnal tidal component under small amplitude changes was also discovered. Performing a correlation analysis of the prevailing wind components with the F10.7 indices of the solar radioemission at l = 10.7 cm, Vergasova and Kazimirovsky [1992] obtained that a significant correlation is mainly observed in summer and winter changing the sign from a year to year. The correlation of the prevailing wind components with the Ap geomagnetic activity indices is small and a considerable negative correlation of the zonal prevailing wind with the Ap indices was found only for some years for the Irkutsk station. The temporal change of the correlation coefficients depends on seasonal peculiarities of the prevailing wind and is mainly determined by the presence of the annual oscillations.
Developing a methodical approach for discovering of solar and geomagnetic effects one should bear in mind that they appear on the background of well pronounced height-temporal variations of the atmospheric thermodynamic parameters.
In this paper the problem is considered of detection of geomagnetic disturbance effects in the behavior of the following dynamic processes: prevailing, tidal, and mesoscale turbulence motions at heights of the upper mesosphere and lower thermosphere (80-110 km).
The dynamical process parameters in the 80-110 km region were obtained from radiometeor measurements in Kazan during 1986-1995 at the radar with an altimeter. The technical characteristics of the latter were described by Sidorov and Fahrutdinova [1991]. Because there are pronounced annual and semi-annual variations of the dynamical processes and their vertical variations, we studied geomagnetic effects as functions of the altitude and season.
The data on the geomagnetic activity Kp indices for 1986-1990 and 1993-1995 and the prevailing wind in the neutral atmosphere measured in the height range 80-110 km with the help of the radar station with the phase altimeter in Kazan (56o N, 49o E) [Sidorov and Fahrutdinova, 1991] for the same time period were used in the study.
The influence of geomagnetic conditions on the wind in the upper mesosphere and lower thermosphere was studied for various parameters. The changes of prevailing wind, amplitudes, and phases of the diurnal and semi-diurnal tides separately for zonal and meridional components as well as the parameter B characterizing the irregular wind structure in the region of mesoscale disturbances was considered. The value of the latter parameter was obtained from the expression:
|
where
sl2U and
sl2V are the
root-mean squares of
the zonal and meridional components of the hourly mean wind
velocity for corresponding hour
i. It was assumed that the vertical
component of the wind velocity is small as compared with the
horizontal component. The
B value is determined by the wind
irregularities in the region of spatial averaging in the process of
meteor trail radiolocation. In our case the horizontal dimensions
of the region of averaging were
700 
700 km. As it follows from the
method of
sl2U and
sl2V determination,
the parameter
B is equal to the daily mean total kinetic energy of irregular
(with the temporal scales of one hour and less) wind motions
referred to a unit mass.
While forming the experiment temporal series of dynamic parameters the vertical scale of averaging was chosen to be 3 km to provide a statistical significance of the estimates obtained.
The daily mean value of the geomagnetic activity index was used as a criterion for estimation of the geomagnetic activity level. The geomagnetic situation was considered quiet or disturbed under Kp < 2.5 or Kp > 2.5, respectively. From the data on various components of the neutral wind two samples were made for various states of geomagnetic activity. Further the geomagnetic activity influence on wind parameters was estimated for various seasons.
The analysis of the height dependence of the geomagnetic activity influence on the value and direction of the daily mean zonal and meridional prevailing wind, amplitude and phase of the diurnal and semi-diurnal tidal wind, and also the parameter B characterizing the intensity of the wind field disturbances with the temporal scales of 1 hour and less was carried out.
At the first stage, the analysis of geomagnetic activity influence on the components of the prevailing wind averaged over all the height region considered was carried out. Tables 1 and 2 show the results of this analysis. In Tables 1-7, N is the volume of the samples considered. A degree of difference of the mean values for the samples considered was evaluated checking the hypotheses on equality of the means and excess over the mean under various conditions. The significance of changes of the standard deviation (SD) of the samples was estimated using the Fisher criterion [Leman, 1964]. In this case the SD characterizes a disturbance degree of the corresponding wind components. All checks of statistical significance were performed for a significance level of 0.05. In the tables presented the significant changes are underlined.
It follows from the data shown in the tables that for the zonal component of the prevailing wind a decrease of its value with increasing geomagnetic activity is mainly observed. In autumn no impact of the geomagnetic activity on the seasonal mean values of the zonal component of the prevailing wind is observed. The meridional component of the prevailing wind demonstrates the following reaction to variations of the geomagnetic activity level: in all seasons there appears a meridional wind component directed northward that results in an increase of the northward wind and a decrease of the southward wind. Moreover the SD of both components of the prevailing wind decreases in winter and spring and increases in summer and autumn. For the samples considered, the observed reaction of the seasonal mean components of the prevailing wind caused by the variations of the geomagnetic activity level is statistically significant in 38% cases and the SD variations are significant in 63% cases.
At the second stage, the influence of geomagnetic activity
on the
value and direction of the prevailing wind was studied as a
function of height and season. Figures 1 and 2 show the results. It
follows from these figures that the influence of geomagnetic
activity on the zonal component of the prevailing wind is most
considerable in summer, autumn, and winter. In summer, a typical
height inversion of the zonal prevailing wind direction not related
to the magnetic field state is observed in the height range 80-95 km
(Figure 1).
Moreover in periods of magnetic disturbances the
zonal component of the prevailing wind demonstrates a change of the
wind velocity vertical gradient: the wind velocity gradient
decreases at heights of 80-95 km and increases at heights of
95-100 km. Averaged over a season, the amplitude of the prevailing
wind zonal component decreases during magnetic disturbances in the
entire height region considered. The variations of the seasonal
mean values of the zonal component for disturbed magnetic
conditions reaches 50% at some altitudes. A strong influence of
geomagnetic activity on the meridional component of the prevailing
wind up to a reversal of its direction is observed in all seasons.
In winter and in autumn, an anti-clockwise turn of the prevailing wind vector (in the coordinate system shown in Figure 2) under disturbed geomagnetic conditions as compared with quiet conditions is observed in the entire height region considered. The turn is especially pronounced in winter at altitudes above 104 km and in autumn above 89 km. In summer the clockwise turn of the prevailing wind vector with a decrease of altitude typical for this height region is observed for quiet geomagnetic conditions. Under disturbed geomagnetic conditions, some advance and some delay of the resulting wind vector as compared with quiet geomagnetic conditions are observed at altitudes of 89-110 km and 80-89 km, respectively. Therein in winter and summer under increased levels of geomagnetic disturbances a decrease of the prevailing wind vector value is observed in the entire height region considered.
The influence of geomagnetic activity on the diurnal and semi-diurnal tides of the neutral wind was studied. Tables 3, 4, 5, and 6 show the influence of geomagnetic activity on the parameters of the tides considered for four seasons on the average in the height range 80-110 km. The following symbols are used: A1,2 and F1,2 are the amplitude and phase of the diurnal (1) and semi-diurnal (2) tides, respectively. In this case the tidal phase corresponds to the local time of the maximum of the corresponding component (zonal or meridional) of the tide. The statistical significance of the observed changes caused by the variations of the magnetic activity level was checked by the method described above.
The zonal component of the diurnal tide demonstrates an increase of the seasonal mean amplitude for disturbed magnetic conditions in winter, spring, and summer and a decrease of this amplitude in autumn. For the zonal component of the diurnal tide a phase advance for disturbed magnetic conditions as compared with quiet conditions in winter and spring and a delay in summer and autumn are observed. In spring the above advance observed for disturbed magnetic conditions as compared with quiet conditions at heights of 100-107 km reaches 2.5 hours. The SD of the amplitude of the diurnal tide zonal component decreases in winter and autumn and increases in spring and summer. On the contrary, the SD of the phase of the diurnal tide zonal component increases in winter and autumn and decreases in spring and summer.
The meridional component of the diurnal tide demonstrates an increase of the seasonal mean amplitude during magnetic disturbances in all seasons except spring when a decrease is observed. The meridional component of the diurnal tide averaged over season demonstrates a phase advance for disturbed magnetic conditions as compared with quiet magnetic conditions in autumn and a phase delay in other seasons. The SD of the amplitude of the diurnal tide meridional component decreases in winter, spring, and autumn and increases in summer. The SD of the phase of the diurnal tide meridional component decreases in all seasons.
For the samples considered the observed reaction of the seasonal mean amplitudes of the diurnal tide components caused by variations of the geomagnetic activity level and the changes of SD are statistically significant in 38% and 75% cases, respectively. The changes of the seasonal mean values of the diurnal tide phase caused by an increase of the geomagnetic activity level and the changes of SD are statistically significant in 25% and 38% cases, respectively.
The amplitude of the zonal and meridional components of the semi-diurnal tide demonstrates an increase of the seasonal mean amplitude during magnetic disturbances in all seasons except winter, when the meridional component decreases. For both components of the semi-diurnal tide a phase advance under disturbed magnetic conditions in comparison with quiet conditions is observed for all seasons. The zonal component in winter and meridional component in spring when a delay is observed present an exception. The changes observed of the seasonal mean parameters of the semi-diurnal tide and the SD changes are statistically significant in 44% and 50% cases, respectively, though the sign of the changes is not constant and vary depending on season.
The increase of tide amplitude detected in most cases under an increase of geomagnetic activity related to an increase of solar activity corresponds to current hypothesis on the source of tidal motions.
Further the influence of geomagnetic activity on the vertical
profiles of the amplitude of the diurnal tide zonal and meridional
components was analyzed. Figure 3 shows the results of this
analysis. It shows that the influence of geomagnetic activity on
the amplitude of the zonal and meridional components of the diurnal
tide is most significant in winter and spring when at some
altitudes the variations of the seasonal mean amplitudes of the
diurnal tide components reaches 40% for the geomagnetic activity
levels considered. However for these parameters the presence of
height regions where the influence of geomagnetic activity is not
seen or is poorly pronounced is also typical. For the diurnal tide
zonal component the influence of geomagnetic activity is small at
heights of 89-92 km in summer, autumn, and winter, at heights
101-104 km in spring and winter, and at heights of 104-107 km in
summer and autumn. The influence of magnetic activity on the
meridional component is the weakest in the height range 92-95 km
during the entire year and at heights of 86-89 km in winter,
spring, and summer. Moreover the influence of geomagnetic activity
on the vertical profiles of the phase of the diurnal tide zonal and
meridional components was analyzed. It was found that the phase
reaction on magnetic disturbances also depends on altitude. A
minimum of the phase change is observed in all seasons within the
height range 95-101 km. The seasonal mean change (advance or
delay) of the phase of the diurnal tide components under influence
of geomagnetic activity for some height ranges reaches 3 hours.
Figure 4 shows the influence of geomagnetic activity on the
vertical profiles of the amplitude of the semi-diurnal tide zonal
and meridional component. It follows from Figure 4 that the
influence of geomagnetic activity on the amplitude of the
semi-diurnal tide components is most significant in winter and
summer when at some heights the variations of the seasonal mean
amplitudes of the semi-diurnal tide components reach 15% for the
geomagnetic activity levels considered. In spring and winter the
influence is much less pronounced. The amplitude of the
semi-diurnal tide components also have height regions where the
geomagnetic activity influence is not manifested or is weakly
pronounced. For example, for the both components this height region
is 92-95 km in winter and summer and 89-92 km in spring and
autumn. The geomagnetic activity influence on the vertical profiles
of the phase of the semi-diurnal tide zonal and meridional
components was also studied. The reaction of the semi-diurnal phase
on magnetic disturbance depends on altitude (as is the case for the
diurnal tide), but this dependence varies from season to season. In
autumn a change of the vertical gradient of the seasonal mean
values of the phase of the semi-diurnal tide zonal component is
observed for various geomagnetic conditions. The phase of the
semi-diurnal tide meridional component demonstrates a seasonal
dependence of the vertical gradient of the seasonal mean values: in
winter the gradient is maximum and in summer it is minimum.
It is discovered that the reaction of the diurnal tide phase to magnetic disturbance is better pronounced than the reaction of the semi-diurnal tide phase, this fact probably being one of the causes of the phase instability of the diurnal tide.
Besides the influence of geomagnetic factors, the seasonal mean vertical profiles of the tide amplitude demonstrate also a difference in vertical profiles for the zonal and meridional components in quiet magnetic conditions caused evidently by the influence of physical factors which are not related to geomagnetic activity and in a different way influence the zonal and meridional components of the tidal wind. The observed effect has the order of magnitude compared with the influence of magnetic activity and is better pronounced at heights above 95 km in winter and summer.
We considered the influence of geomagnetic activity on irregular structure of the neutral wind in the region of mesoscale disturbances on the example of the parameter B. Table 7 shows the analysis results of this influence on the average for the height range 80-110 km. The analysis shows that the influence of geomagnetic activity on the irregular structure of the neutral wind in the mesoscale disturbance region is less pronounced than the influence on larger-scale components. The observed changes of the mean values and SD for the sample of the parameter B considered has no permanent sign.
The influence of geomagnetic activity on vertical profiles of the neutral wind irregular structure in the mesoscale disturbance region are weakly pronounced except the height range 104-110 km. Large increases of B (up to 60%) related to increases of geomagnetic activity are observed here in summer.
The analysis performed demonstrates the following main features of the geomagnetic activity influence on the dynamic of the upper mesosphere and lower thermosphere:
1) The geomagnetic activity influence on the neutral wind in the region of the upper mesosphere and lower thermosphere is most pronounced for large-scale components of the wind (prevailing wind, tides) and decreases with a decrease of the spatial and temporal scales of the dynamical processes.
2) Under disturbed geomagnetic conditions in winter and autumn in the entire height region considered and in summer at heights of 80-89 km the anti-clockwise turn of the prevailing wind vector is observed and in summer at heights of 89-110 km the clockwise turn is observed as compared with quiet geomagnetic conditions, the decrease of the prevailing wind vector being observed in winter and summer.
3) The maximum influence of geomagnetic activity on the diurnal and semi-diurnal tides is observed in spring-autumn and summer, respectively, an increase of the tide amplitudes being mainly observed. Under disturbed magnetic conditions, a phase advance is observed for the diurnal tide zonal component in winter and spring and for the meridional component in autumn. For the rest of the seasons a phase delay is found for the meridional component as compared with quiet geomagnetic conditions. It is found that the reaction of the diurnal tide phase to magnetic disturbances is better pronounced than that of the semi-diurnal tide.
4) The geomagnetic activity influence on the neutral wind irregular structure of in mesoscale disturbance region is less manifested than that for larger-scale components such as the prevailing wind and tides. The considerable (up to 60%) increases (related to increases of geomagnetic activity) of the parameter B which characterizes the neutral wind irregular structure in the mesoscale disturbance region are observed in summer at heights of 104-110 km.
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