* G. Petrunov, N. Miloshev, L. Draganov, G. Miloshev* – Introducing regular climatic investigations of the solar ultraviolet radiation in Sofia

**Abstract.**The beginning of regular instrumental investigations and measurements of the solar ultraviolet radiation in Sofia are presented in this paper. The constructed instrumentation (apparatuses) and the used methods are described in details. The first results for the year 1997 are graphically presented. The future scientific and practical applications of the obtained results are pointed out in the conclusion.

**Key words:**solar ultraviolet radiation, UV-A radiation, UV-B radiation, UV measurements, UV forecasting.

* L. Simeonova, D. Yordanov, M. Kolarova*– Diffusion of admixtures from a limited linear source under windless conditions

**Abstract:**In the paper analytical expressions concerning the diffusion from a limited linear source under windless conditions are found. First, simple analytical solutions for the ground level concentration of pollutants from continuously acting limited linear source at large and small diffusion times are proposed. Second, analytical expressions for the ground level concentration of pollutants from an instantaneous limited linear source at large and small diffusion times are found. Using the laboratory experimental data of Willis and Deardorff for the dispersion parameters linear and quadratic asymptotes are found at large and small diffusion times and the corresponding expressions about the ground level concentrations of pollutants from a limited linear source under calm conditions are obtained. These expressions are used for estimations of the road transport pollution at small and large distances from the line source.

**Key words:**diffusion, linear source, windless conditions, analytical solution

* E.Georgieva*– Flow modifications by groups of buildings – some results from a diagnostic model

**Abstract**. A diagnostic mass-consistent flow model is used to simulate the main flow patterns in two groups of buildings with the same individual geometry but with different alignment (regular rows and staggered). Possible flow regimes are discussed and parameterization schemes based on wind tunnel data and on model results are introduced. The influence of building heights on flow modification for both arrays is studied and qualitatively compared to wind tunnel results for simple building arrangements.

**Key words**: Street canion flows, mass consistence, wind regimes.

* R. Dimitrova* -Numerical simulation of flow data over two-dimensional hills and valleys and comparison with wind tunnel experiment data

**Abstract.**The present study demonstrates some comparisons of simulated turbulent wind fields over a complex terrain with wind tunnel experiment data. A quasi-hydrostatic model of the mesoscale dynamics, based on the Businesque approximation was used. The calculations were made for three isolated hills and valleys with different slopes. A large number of numerical experiments were carried out, by which the model sensitivity to input parameters was proved and partially studied. Reasonable quantitative agreement with wind tunnel experiment data was obtained. The important qualitative effect of the vortex formation can also be seen in the model results.

**Key words**: comparison, flow over complex terrain.

* D. Altadill, J. G. Sole, E. M. Apostolov*– Gravity wave type disturbances observed in the ionosphere during the solar eclipse of 11 August 1999

**Abstract**. We use the electron density profiles from Observatori de l’Ebre (40.8°N, 0.5°E) of the ionospheric sounding campaign during the solar eclipse of 11 August 1999 in order to investigate the possible ionospheric disturbances generated by the eclipse. We find a gravity wave type disturbance in the ionosphere with a dominant oscillating period of 57 minutes that occurs after the maximum of solar occultation by the Moon. The wave event arises near the altitude of the F1 ionisation maximum and draws energy upwards and downwards simultaneously.

**Key words:**solar eclipse, ionospheric gravity waves, phase velocity, group velocity.

* B. Andonov, P. Muhtarov*– Ellipticity and orientation polarization analysis of geomagnetic pulsations as deduced from ground-based magnetometric measurements

**Abstract**. A method for deriving the characteristics of the geomagnetic pulsations: period, amplitude, phase, ellipticity and polarisation is presented. The magnetometric data are considered as consisting of separate sinusoidal pulses possessing random initial moments, amplitudes periods, phases and duration. For the purpose, a least square trigonometric approximation is applied to obtain amplitudes, periods and phases. A confidence criterion is introduced which allows an amplitude with a certain period to be revealed, when it exceeds by a given number the amplitudes of other pulsation periods and the signal to (signal+noise) ratio.

**Key words**: geomagnetic pulsations, ellipticity, polarisation, trigonometric approximation, sliding time segment.

* N. Isaev, O. Serebriakova, D. Teodosiev* – Electromagnetic and plasma effects of the seismological activity in the earth ionosphere

**Abstract**. Data from the low-orbiting satellite COSMOS-1809 were used to search for a correlation between seismic activity and increases in ELF/VLF emissions and plasma density fluctuations for different regions. The data of ~ 100 revolution of satellite were chosen for the analysis of ionospheric effects of this seismic activity. The analysis enables to draw the following conclusions. The electromagnetic emissions in the frequency range of 140-450 Hz are regularly observed within the L-shells with the root in the seismoactive zone. Near a zone of seismic activity (Dl=±6) at the altitude ~ 950 km bursts of ELF radiation are observed at the frequencies f=140 and 450 Hz in the magnetic and electric components. The radiation excess above the background level can reach more than an order of magnitude. The radiation bursts are observed minutes to hours beforehand an earthquake and during several hours after it. The sizes of the disturbed region are usually ~400-600 km, and greater in the period of aftershock activity than during the earthquake development. The analysis of seismogenic ELE/VLF radiation had shown that the main signal characteristics (frequency, amplitude etc.) were sufficiently different for different seismoactive regions of the Earth. This fact is evidence of the seismoelectromagnetic parameters dependence on the physical-chemical properties of the Earth’s crust and lithospheric processes.

**Key words:**seismic activity, ELF/VLF emissions, plasma density fluctuations, satellite data.

* P. A. Bradley, P. Muhtarov, I. Kutiev* – Probability statistics of the relative deviations of foF2 from a reference level over the European region

**Abstract**. foF2 measurements selected from the COST251 data base for 15 European stations over a period of 11 years (1981-1991) are used to reveal some statistical properties of the relative daily deviations of foF2 (denoted as F) from their corresponding sliding medians over different days for the same hours. Emphasis is placed on composite statistics rather than a break-down by hours and months. The standard deviations (SD’s) of F (relative square-root deviations of foF2 from the medians), calculated for each station for the whole period, are used to generate a map over the area (40°, 60°) N and (-10°, 45°)E. This map shows the overall variability of foF2 in the European region. The form of the probability-density distribution of F averaged over all stations is compared with those of the theoretical Laplacian and Gaussian or normal distributions. The storm-time F is further analysed in terms of the probability distribution of the time length of positive and negative disturbances. The time and spatial autocorrelation functions are calculated to reveal the characteristic time and space scales of the main ionospheric disturbances. The spatial correlation distance is determined separately as a function of latitude and longitude to yield an isotropy ratio which is contrasted with measured values determined by other groups. For the purposes of short-term forecasting, a median based prediction is compared with predictions that follow persistence criteria.

**Key words**: ionosphere, probability statistics, time and space autocorrelation, ionospheric disturbances

* S. Stankov* – The oxygen-helium ion transition level during low solar activity: I. Comparison of AE-C satellite data with IRI model calculations

**Abstract**. The latitudinal, longitudinal, diurnal and seasonal variations of the O+ – He+ ion transition level during low solar activity are analysed using in-situ ion density measurements from the Atmosphere Explorer – C satellite. The results are compared with the International Reference Ionosphere model calculations.

**Key words:**transition level, satellite measurements, empirical modelling

* G. Mardirossian, L. Petrov, L. Dimitrova, B. Tzvetkov* – Investigating the influence of the microseismic noise on the precision of measuring the horizontal component of the geomagnetic field

**Abstract**. In the paper, the experiments and the obtained results are described from the study of the eventual impact of microseismic noise on the accuracy of measurement of the swinging period of a permanent magnet, and from there, on the accuracy of calculation of the horizontal component of the geomagnetic field. The experiments were conducted in the spring and summer of 1999; they were related mainly with the registration of microseismic noise at the time of geomagnetic measurements. Based on the obtained results, it may be ascertained that the variations in the 3rd to 4th digit after the decimal point of the measured values of the swinging period of the reference permanent magnet are not due to seismic noise.

**Key words:**microseismic noise, swinging period, permanent magnet, horizontal component of the geomagnetic field.

* Tz. Darakchiev* – Nontidal variations of the gravity direction and their determi-nation by permanent astronomical observations with a zenith telescope.

**Abstract**. The notion “nontidal variations of the gravity direction” (NVGD) is defined and the problems of their determination are discussed. A classification of the astronomic geographic latitude variations in a given point of the Earth surface in the time is given. The importance for the geosciences and for Bulgaria of studying the NVGD is pointed out.

**Key words:**Zenith telescope, variations of the astronomical geographic latitude, nontidal variations of the gravity direction (vertical).

* R. Raditchev, St. Dimovski, M. Tokmakchieva* – Modeling of gravity and magnetic anomalies for the conditions of the Panagjurishte ore region

**Abstract**. The performed investigations are aimed towards receiving a summarized idea of the density and magnetic characteristics of the main rock types and the porphyry-copper ore bodies in the Panagjurishte ore region, and towards illustrating the modeling approach and the analysis of the possible gravity and magnetic anomalies. The rocks that are composing the Panagjurishte ore region are well differentiated according to their density and magnetic permeability. The performed gravity and magnetic modeling is aimed towards two basic aspects : estimating the possible distribution of the gravity and the magnetic fields on the territory of the Panagjurishte ore region after taking into account the differentiation of the rocks according to their density and magnetic permeability, illustrated for the basaltic andesites ; modeling of the copper deposits, illustrated for the porphyry-copper ore bodies. The distribution of the gravity field for the density model of the basaltic andesites varies in a large interval and depends mainly on the anomaly causing residual density and on the depth from the surface. A well-expressed zoning of the ore mineralization and the hydrothermal changes for the porphyry-copper deposits is observed, with gradual changes starting from the interior zone, passing through the intermediate, and ending in the exterior zone. The performed modeling is illustrating the wide range of possibilities for the formation of positive or negative gravity anomalies, as well as for the gravity field compensation, dependent on the zoning peculiarities and the composition of the porphyry-copper ore bodies. The basaltic andesites are characterized by an increased magnetic permeability. The distribution of the magnetic field for the summarized model shows, that for the normal for Bulgaria angle, two extremes are well exposed – a maximum and a minimum. The presented metasomatic zoning has no practical influence over the magnetic permeability distribution and for this reason the ore deposits can be presented by a summarized magnetic anomalous body. The anomalous magnetic field is characterized by a low intensity.

**Key words**: modeling, gravity, magnetic anomalies, Panagjurishte ore region.

* L. Christoskov* – Review on the mathematical models for conventional seismic sources. Part I: Introductory considerations, concentrated force, linear dipoles and some combinations

**Abstract**. The present review has two parts and it is meant to offer a convenient variant for unified presentation of the known formulas for the basic conventional source models used for determining the focal mechanism of seismic events. In the practice, still arise some problems, as the formulas for the different source models are originally derived in non uniform coordinate systems. Furthermore, the notations used by different authors vary considerably, a fact, which introduces some supplementary obstacles. A limited number of papers are describing the subsequent “evolution” of source models, for example, starting from a simple concentrated force to derive the more compound models of couples with a moment and without a moment. The predominant number of the published solutions do not consider the general case of forces acting in arbitrary directions. Taking into account the above mentioned circumstances, in the present paper a standard right hand coordinating system (rectilinear Ox1x2x3 or spherical ORJj) is considered. In addition, within the system Ox1x2x3 an arbitrary oriented coordinate system Oabg is introduced. The later is used to present the source models in the general case when the acting forces are arbitrary oriented in the space. The systems Ox1x2x3 or ORJj should also be treated as “geographic” when axis x1 is oriented to N, axis x2 to E and x3 along the vertical Z towards the Earth’s center (nadir). Thereby oriented coordinate systems Ox1x2x3єONEZ exactly correspond to the standard ones used in practice for obtaining the fault plane solutions. Considering this, the formulas for displacement components of models on the axis a, b or g, namely for simple concentrated force and linear couple (part I) and dipole with a moment and double couple without a moment (part II) are evaluated. From these formulas it is easy to obtain the transformations in spherical coordinates, as well as to separate the displacement components of the longitudinal (P) and the shear (S) waves.

**Key words:**seismic source, earthquake mechanism, mathematical models.

* L. Christoskov* – Review on the mathematical models for conventional seismic sources. Part II: Dipoles with a moment, double couples without a moment and some combinations

**Abstract**. The present review has two parts and it is meant to offer a convenient variant for unified presentation of the known formulas for the basic conventional source models used for determining the focal mechanism of seismic events. In the practice, still arise some problems, as the formulas for the different source models are originally derived in non uniform coordinate systems. Furthermore, the notations used by different authors vary considerably, a fact, which introduces some supplementary obstacles. A limited number of papers are describing the subsequent “evolution” of source models, for example, starting from a simple concentrated force to derive the more compound models of couples with a moment and without a moment. The predominant number of the published solutions do not consider the general case of forces acting in arbitrary directions. Taking into account the above mentioned circumstances, in the present paper a standard right hand coordinating system (rectilinear Ox1x2x3 or spherical ORJj) is considered. In addition, within the system Ox1x2x3 an arbitrary oriented coordinate system Oabg is introduced. The later is used to present the source models in the general case when the acting forces are arbitrary oriented in the space. The systems Ox1x2x3 or ORJj should also be treated as “geographic” when axis x1 is oriented to N, axis x2 to E and x3 along the vertical Z towards the Earth’s center (nadir). Thereby oriented coordinate systems Ox1x2x3єONEZ exactly correspond to the standard ones used in practice for obtaining the fault plane solutions. Considering this, the formulas for displacement components of models on the axis a, b or g, namely for simple concentrated force and linear couple (part I) and dipole with a moment and double couple without a moment (part II) are evaluated. From these formulas it is easy to obtain the transformations in spherical coordinates, as well as to separate the displacement components of the longitudinal (P) and the shear (S) waves.

**Key words**: seismic source, earthquake mechanism, mathematical models.

* T. Delipetrov, D. Karakasev, J. Zivanovic* – Structural composition of the Strumica depression

**Abstract**. The paper presents geophysical investigations carried out in the Strumica depression with a brief summary of the location and geological composition. Calculations carried out on gravimetric data indicated the maximum depth of the depression. Analyses of total vector anomalies of magnetic field gave data about the complex block composition and magnetic causes lying deep in the depression. The geodynamic model of the depression was established based on the geological and geophysical data obtained.

D. Stefanov – Ultimate capacity of reinforced concrete shear wall under seismic excitation

Abstract. The seismic response of a reinforced concrete shear wall is investigated, taking into account the nonlinear behaviour of the materials – steel and concrete. Parametric and comparative investigations are done on the influence of the main mechanical characteristics of the materials on the structure capacity. The analytical and experimental results are compared in order to evaluate different factors, affecting the precision of the numerical solutions. A procedure for evaluation of ultimate capacity of shear walls under seismic excitations is proposed. The new procedure gives the possibility to get a good evaluation for failure mechanism of the investigated structure.

**Key words**: nonlinear seismic behavior, cracked concrete, seismic response.

* E.Botev, B.Babachkova, B.Dimitrov, S.Velichkova, I.Tzoncheva, K.Donkova* – Preliminary data on the seismic events recorded by NOTSSI in July – December 1998

**Abstract**. A map of the epicentres of 835 earthquakes that occurred in te part of the Balkan Peninsula outlined by longitude l=190-290E and latitude j =370- 470N is presented. A more detailed analysis of the seismicity of the territory of Bulgaria and its adjacent lands is made.

**Key words**: seismicity

* E.Botev, B.Babachkova, B.Dimitrov, S.Velichkova, I.Tzoncheva, K.Donkova* – Preliminary data on the seismic events recorded by NOTSSI in January – June 1999

**Abstract**. A map of the epicentres of 458 earthquakes that occurred in the part of the Balkan Peninsula outlined by longitude l=190-290E and latitude j =370- 470N is presented. A more detailed analysis of the seismicity of the territory of Bulgaria and its adjacent lands is made.

**Key words:**seismicity

### News in brief

* L. Christoskov, A. Velev* – Applied geophysics in Bulgaria and 45 years scientific work of Prof. Ch. Dachev

* C. Christova* – Second National Geophysical Conference with international participation Sofia’98