Victor D. Kokourov

Dynamics of the quiet thermosphere (a review)

Abstract A survey of experimental data on thermospheric motions obtained by radiophysical methods is presented. Hydrodynamical motions in the height range 80–120 km have a clearly pronounced meteorological origin (viz. seasonal variations, longitudinal effects, local properties, etc.). A systematic statistical description of horizontal ionospheric drift measurements allows an empirical global model of motions at ionospheric levels to be obtained by the method of spaced receivers in a concerted effort of the world-wide network of stations. The prevailing zonal wind in the mid-latitude E -region is westward in the winter hemisphere and eastward in the summer hemisphere. The semi-diurnal tides at mid-latitudes and the diurnal tides at low latitudes are very intensive. At high latitudes, perhaps there is possibly an independent circulation cell that is defined by the properties of interactions of the high-latitude ionosphere with the magnetosphere and low atmosphere. In the F -region, the seasonal variations are delicately pronounced, while the manner of drift is determined mainly by the latitude. The prevailing drift during solstices is southwestward at high- and mid-latitudes and southeastward in the near equatorial zone. The transition zone coincides with the Sq -current system focus. Drift measurements in the F -region allow information about the space-time variations in ionospheric electric fields to be attained and the neutral wind evaluated at these altitudes. The results are in good agreement with incoherent scatter measurements.

The dynamical structure of the lower thermosphere during the CRISTA/MAHRSI campaign

Abstract As a part of co-operative “CRISTA / MAHRSI Campaign” the measurements of horizontal wind in the lower thermosphere were provided. The structures of lower thermosphere dynamics can be infered from these data. Some parameters of dynamical regime over East Siberia during campaign are presented.

The semidiurnal tides at the midlatitude lower thermosphere over East Siberia

Abstract Results from the study of semidiurnal tides in the horizontal wind field at 85–95 km over East Siberia are presented. The seasonal variation of tidal amplitudes and the effects of stratospheric warmings are discussed.

The dynamic regime of the ionospheric E-region as observed at Irkutsk

Presented are the results derived from analyzing the observations of horizontal ionospheric drifts at the ionspheric E-region heights. The observations were made during 1958-1982 using the method of spaced receivers at vertical-incidence radio soundings. The observational data were analyzed separately for the regular E-layer and for the sporadic Es-layer. The complex character of the dynamic regime in this region of space is emphasized. Inspection of the data suggests that the sporadic Es-features are fully involved in the drifts of regular layers. The difference of the horizontal drift parameters implies that for the regular E-layer the north-eastern directions occur by 0-15% more frequently when compared with the drifts of the sporadic Es-layer. It is pointed out that with increasing geomagnetic activity, there is an increase of the probability of the north-eastward transport. The mean value of the modulus of the drift velocity vectory in different directions is 100-110 m/s. The azimuth of the drift velocity vector changes clockwise on a diurnal scale. In this case the prevailing drift directions differ both in seasons and in the dayside and nightside ionosphere.

The dynamics of large-scale absorption variations (fmin) at Irkutsk

For the first time, using long-term fmin measurements acquired by the method of standard vertical-incidence radio incidence at st. Irkutsk (52° N, 105° E) for the time interval 1983-1991, we investigated the dynamics of large-scale variations of the minimum reflection frequency (fmin) of radio waves from the ionosphere. Sliding multiple correloperiodogram analysis was used. We detected regular seasonal (91 days) and 27-day oscillations with almost constant periods, the semi-annual wave characterized by a period fluctuating over a rather wide range (176-221 days), and the annual wave with the mean period of 365 days. A clearly pronounced annual trend of seasonal dfmin fluctuations was observed. Quasi-biennial variations may be classed as irregular fluctuations with a constantly varying period over a rather wide range (588-886 days). The large-scale variations of fmin under investigation which characterize its long-term variability, may serve as climatic characteristics for the description of this atmospheric region.

The fluctuations of 27-day periodicities of the total ozone content at middle latitudes

The time variations of daily values of the Total Ozone Content (TOC) during 1978-1992 at stations Irkutsk, Collm, Madrid and Saskatoon were investigated. These stations are located in the middle latitudes but have essentially different longitudes. The TOMS-database (satellite Nimbus-7) was used. The multi-correlation periodogramanalysis with sliding step 1 month was applied. The fluctuations with the periods lying in rater wide band including 27-day variations (21-39) days were revealed. Amplitudes of these fluctuations have a well-defined annual course with a maximum in the winter and a minimum in the summer. The variations of amplitudes of 27-day fluctuations contain an annual wave and quasibiennial oscillations with periods 20-28 months. At Saskatoon and Irkutsk, during years close to maxima of solar activity (1980 and 1990) the amplitudes of 27-day TOC fluctuations are the greatest. The significant correlation between amplitudes of fluctuations at considered middle-latitude stations practically without of phase lag is found out. Correlation functions are quasiperiodical with maxima of correlation coefficients each two years. It is possible to explain the results by joint influence of 27-day variations of solar activity connected with rotation of the sun and planetary atmospheric waves activity. The regular 27-day TOC fluctuations can be considered as climatic parameters of TOC variability in the Earth’s atmosphere.

The role of planetary waves in the atmosphere-ionosphere interactions

The main mechanism of interaction between lower neutral atmosphere and ionosphere are the upward propagating atmospheric planetary waves. In the Institute of Solar-Terrestrial Physics RAS the experimental investigations of the interaction between different atmospheric layers from the stratosphere up to ionospheric E- and F-regions were carried out. The long-term database of some midlatitude upper atmosphere parameters was used (the horizontal thermosphere/ionosphere wind, the minimal vertically reflected from the ionosphere radio-frequency fmin, total ozone content, stratospheric temperature). The seasonal variations and significant non-zonality (longitudinal dependence) of the quasi-periodical structure were revealed. The influence of solar and geomagnetic activity periodicities from above and stratospheric thermo-baric field from below on the upper atmosphere processes is evaluated. In spite of significant changeability of planetary waves in the atmosphere-ionosphere system (from day to day, seasonal, from year to year, with solar cycle etc.) there are some similarities for the different atmospheric layers. The empirical model for the lower thermosphere wind field quasi-periodical structure was constructed. It is shown that increasing of planetary waves (wave-numbers 1 and 2) activity leads to the decreasing of the prevailing zonal wind velocity. We consider the atmosphere-ionosphere planetary waves as indicators of the complex disturbances in the Sun-Earth system and important element of upper atmosphere climate.