E. M. Apostolov

Bulgarian Academy of Sciences, Ulpia Serdica, Sofia-Capital, Bulgaria

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Publications (20)23.02 Total impact

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    ABSTRACT: The aim of this work within the WP 3.1 of the COST 271 Action is the characterization of the variability introduced in the F-region ionosphere by -Planetary Wave Signatures- (PWS) and -Gravity Wave Signatures- (GWS). Typical patterns of percentage of time occurrence and time duration of PWS, their climatology and main drivers, as well as their vertical and longitudinal structure have been obtained. Despite the above characterization, the spectral distribution of event duration is too broad to allow for a reasonable prediction of PWS from ionospheric measurements themselves. GWS with a regular morning/evening wave bursts and specific GWS events whose arising can be predicted have been evaluated. As above, their typical pattern of occurrence and time duration, and their vertical structure have been obtained. The latter events remain in the ionospheric variability during disturbed days while additional wave enhancements of auroral origin occur. However, both types of disturbances can be distinguished. published
    01/2009;
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    E. M. Apostolov, D. Altadill, M. Todorova
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    ABSTRACT: Solar cycle variations of the amplitudes of the 27-day solar rotation period reflected in the geomagnetic activity index A<sub>p</sub> , solar radio flux F10.7cm and critical frequency fo F2 for mid-latitude ionosonde station Moscow from the maximum of sunspot cycle 18 to the maximum of cycle 23 are examined. The analysis shows that there are distinct enhancements of the 27-day amplitudes for fo F2 and A<sub>p</sub> in the late declining phase of each solar cycle while the amplitudes for F10.7cm decrease gradually, and the fo F2 and A<sub>p</sub> amplitude peaks are much larger for even-numbered solar cycles than for the odd ones. Additionally, we found the same even-high and odd-low pattern of fo F2 for other mid-latitude ionosonde stations in Northern and Southern Hemispheres. This property suggests that there exists a 22-year cycle in the F2-layer variability coupled with the 22-year cycle in the 27-day recurrence of geomagnetic activity. Key words. Ionosphere (mid-latitude ionosphere; ionosphere- magnetosphere interactions) ? Magnetospheric physics (solar wind-magnetosphere interactions)
    01/2004;
  • D. Altadill, E. M. Apostolov
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    ABSTRACT: The time and scale size of planetary wave signatures (PWS) in the mid latitude F region ionosphere of the Northern Hemisphere and the main pattern of their possible sources of origin are presented. The PWS involved in this study have periods of about 2–3, 5–6, 10, 13.5, and 16 days. The PWS in the ionosphere are large scale phenomena. PWS with periods of about 2–3 and 5–6 days have a typical longitudinal size of 80°, they are coherent some 6000 km apart, and they occur about 12% and 14% of the entire observational record respectively. The typical longitudinal size of PWS with periods of about 10 and 13 days is 100°, they are coherent some 7500 km apart, and they occur about 24% and 22% of the entire observational record respectively. PWS with periods of about 16 days seem to be global scale phenomena, and they occur about 30% of the entire observational record. The results estimate that geomagnetic activity variations play the most important role for driving PWS in the ionosphere. The geomagnetic activity variations can drive at least 20–30% of the PWS with periods of about 2–3, 5–6, 10 and 16 days, but even up to 65–70% for the PWS with periods of about 10 and 16 days, and they practically drive 100% of the PWS with periods of about 13.5 days. The planetary wave activity in the mesosphere/lower thermosphere (MLT) winds can drive about 20–30% of the PWS with periods of about 2–3, 5–6, 10 and 16 days. There is a significant percentage of existence of PWS in the F region apparently ‘independent’ from the geomagnetic activity variations and of the MLT winds. The latter is better expressed for PWS with shorter period. PWS with periods of about 13.5 days are an exception to that. A candidate mechanism for the ‘independent’ events may be the non linear interaction or the amplitude modulation between different PWS.
    Journal of Geophysical Research 01/2003; 108. · 3.17 Impact Factor
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    ABSTRACT: Analyses of time-spatial variations of critical plasma frequency f oF2 during the summer of 1998 reveal the existence of an oscillation activity with attributes of a 6-day westward propagating wave. This event manifests itself as a global scale wave in the f oF2 of the Northern Hemisphere, having a zonal wave number 2. This event coincides with a 6-day oscillation activity in the meridional neutral winds of the mesosphere/lower thermosphere (MLT). The oscillation in neutral winds seems to be linked to the 6?7-day global scale unstable mode westward propagating wave number 1 in the MLT. The forcing mechanisms of the 6-day wave event in the ionosphere from the wave activity in the MLT are discussed.
    Annales Geophysicae 01/2003; · 1.52 Impact Factor
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    ABSTRACT: This CD-ROM presents the Livingston Island Geomagnetic Observatory Bulletin, edited by Observatori de l'Ebre, containing the data obtained during the years 2000, 2001 and the first two months of the year 2002. For the first time this Bulletin is edited in digital format, being it the continuation of the paper-edited series as Misceláneas 41, 42 and 43 (ISSN 0211-4534). The structure of the CD-ROM consists of one file with the Bulletin contents in PDF and of a tree of directories and subdirectories with the data corresponding to the different years and months of the Bulletin. These data files and their names were built according to the IAGA-2000 data exchange format.
    01/2003;
  • D. Altadill, E.M. Apostolov, J.G. Solé, Ch. Jacobi
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    ABSTRACT: Starting from previous results of vertical propagating 6.5-day oscillations observed in the F region of the ionosphere during 1998, we investigated their possible forcing from both the planetary wave activity of the neutral winds in the mesosphere/lower thermosphere and the geomagnetic activity. For this purpose we used the electron density profiles and geomagnetic field components obtained at Observatori de l'Ebre station (40.8°N, 0.5°E), the mesopause region neutral wind data obtained at Collm Observatory (52°N, 15°E) and the 3-hourly geomagnetic index Ap. From the four events of 6.5-day oscillations observed in the ionosphere, we found that two of them are contemporary with two events of 6.5-day period in the neutral winds and the coherency between the neutral winds and ionosphere variations at 6.5-day periods is very large. The other ionospheric events are contemporary with the existence of two geomagnetic storms and the coherency between the ionospheric and geomagnetic activity variations is very large at periods at 6.5 days. The potential mechanisms linking the geomagnetic activity variations and the planetary wave activity of the neutral winds in the mesosphere/lower thermosphere with the wave events observed in the ionosphere are discussed in order to explain the possible origin of the ionospheric waves.
    Physics and Chemistry of the Earth Part C Solar Terrestrial & Planetary Science 01/2001;
  • David Altadill, Emil M Apostolov
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    ABSTRACT: We investigate the meteorology of wave-type oscillations in the period range of 2- and 6.5-days observed in the middle latitude ionosphere at the altitude range from 170 to 230 km during 1998, with an emphasis on their vertical propagation characteristics. The true height electron density profiles obtained at the Observatori de l'Ebre station (40.8°N, 0.5°E) have been used for this purpose. We found several events with occurrence of wave packets with periods near 2- and 6.5-days during the summer half-year. These events display a clear vertical progression in the ionospheric F region, with vertical wavelengths larger than 100 km. The events exist with upward or downward propagation and the different values between the phase and group velocities indicate the degree of dispersion that the ionosphere introduces on their propagation. The potential mechanisms linking the planetary wave activity in the mesosphere/lower thermosphere with the wave events observed in the ionosphere are discussed in order to evaluate the most likely mechanism.
    Journal of Atmospheric and Solar-Terrestrial Physics 01/2001; · 1.42 Impact Factor
  • D. Altadill, J. G. Solé, E. M. Apostolov
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    ABSTRACT: The dependence in altitude and time of the ionospheric electron density and its altitude variations during the solar eclipse of August 11, 1999, have been studied. The true height electron density profiles recorded at the Observatori de l'Ebre station (40.8°N, 0.5°E) during a rapid sequence vertical ionospheric sounding campaign have been used. A gravity wave like oscillation in the ionosphere has been characterized after the maximum of solar occultation by the Moon. The event is clearly seen in both the plasma frequency and in the altitude variations of the F region in the altitude range from 150 to 250 km, and it has a dominant oscillating period T=57.5min. The oscillation activity linked with the gravity wave event is found to exist in the variations of the ionospheric parameters, being coherent with the oscillations characterized in the plasma frequency variations. The source-origin of the event was located around the transition region between F1 and F2 layers and from that altitude the wave propagates vertically drawing energy upwards and downwards simultaneously. The decreasing/increasing solar ionizing radiation and the cooling/heating processes linked with the solar eclipse are discussed as a possible mechanism to explain the gravity wave event observed in the electron density and dynamics of the ionosphere.
    Journal of Geophysical Research 01/2001; 106:21419-21428. · 3.17 Impact Factor
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    ABSTRACT: The authors give details of the magnetic observations recorded at the Livingston Island Geomagnetic Observatory during 1998, including the 1998 - 1999 Austral summer survey. The data presented are: 1. Computer-produced K indices by means of the FMI method. 2. Table with a summary of the monthly and annual means of the magnetic elements. 3. Month-at-a-glance daily magnetograms of declination (D), horizontal intensity (H), and vertical intensity (Z). 4. Month-at-a-glance daily magnetograms of total intensity (F).
    01/1999;
  • D. Altadill, E. M. Apostolov
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    ABSTRACT: The altitude development of the 2-day wave in the ionospheric F region (from 170 to 230 km) has been investigated from the true height electron density profiles obtained at Observatori de l'Ebre station (40.8°N, 0.5°E). We found five time intervals of enhanced activity of the 2-day wave, bursts type, during 1995. The altitude variations of the 2-day wave period, amplitude, and phase are presented. The phase and group velocities of the wave are evaluated for each case. The observed changes in the electron density indicate three types of dominant vertical propagation of the 2-day wave in the F region: (1) upward propagation of the wave, (2) simultaneous 2-day oscillation of the electron density at all altitude range under study, and (3) downward propagation of the wave. The potential mechanisms proposed for explaining the vertical propagation of the 2-day wave in the ionospheric F region are discussed in order to evaluate the most likely mechanism.
    Journal of Geophysical Research 01/1998; 103:29199-29206. · 3.17 Impact Factor
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    E. M. Apostolov, D. Altadill, R. Hanbaba
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    ABSTRACT: The relative contributions of quasi-periodic oscillations from 2 to 35 days to the variability of foF2 at middle northern latitudes between 42°N and 60°N are investigated. The foF2 hourly data for the whole solar cycle 21 (1976–1986) for four European ionospheric stations Rome (41.9°N, 12.5°E), Poitiers (46.5°N, 0.3°E), Kaliningrad (54.7°N, 20.6°E) and Uppsala (59.8°N, 17.6°E) are used for analysis. The relative contributions of different periodic bands due to planetary wave activity and solar flux variations are evaluated by integrated percent contributions of spectral energy for these bands. The observations suggest that a clearly expressed seasonal variation of percent contributions exists with maximum at summer solstice and minimum at winter solstice for all periodic bands. The contributions for summer increase when the latitude increases. The contributions are modulated by the solar cycle and simultaneously influenced by the long-term geomagnetic activity variations. The greater percentage of spectral energy between 2 to 35 days is contributed by the periodic bands related to the middle atmosphere planetary wave activity.
    Annales Geophysicae 01/1998; 16(2):168-175. · 1.52 Impact Factor
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    D. Altadill, J. G. Solé, E. M. Apostolov
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    ABSTRACT: The existence and development of the quasi-2-day oscillations in the plasma frequency variations of the F region at northern middle latitudes are investigated. A new approach to study the quasi-2-day oscillations is presented, using a methodology that allows us to do such a study at fixed heights. The hourly values of plasma frequency at fixed heights, from 170 km to 220 km at 10 km step, obtained at the Observatori de l'Ebre station (40.8°N, 0.5°E) during 1995 are used for analysis. It is found that quasi-2-day oscillations exist and persisted in the ionospheric plasma frequency variations over the entire year 1995 for all altitudes investigated. The dominant period of oscillation ranges from 42 to 56 h. The amplitude of oscillation is from 0.1 MHz to 1 MHz. The activity of the quasi-2-day oscillation is better expressed during the summer half year when several enhancements, about 15?30 days in duration, were observed. The largest enhancements of the oscillation occurred during early June, July and early August; i. e., near and after the summer solstice when the 2-day wave in the middle neutral atmosphere typically displays its largest activity in the Northern Hemisphere. The results obtained may help us understand better the possible influencing mechanisms between the 2-day wave in the middle neutral atmosphere and the ionospheric quasi-2-day oscillations.
    Annales Geophysicae 01/1998; · 1.52 Impact Factor
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    ABSTRACT: The authors give details of the magnetic observations recorded at the Livingston Island Geomagnetic Observatory during 1997, including the 1997 - 1998 Austral summer survey. The data presented are: 1. Computer-produced K indices by means of the FMI method. 2. Table with a summary of the monthly and annual means of the magnetic elements. 3. Daily magnetograms of declination, horizontal intensity and vertical intensity. 4. Daily magnetograms of total intensity.
    01/1998;
  • D. Altadill, E. M. Apostolov, L. Alberca
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    ABSTRACT: The annual variations of the amplitude, period, and probability of existence of foF2 quasi-2-day oscillations at middle latitudes in both northern and southern hemispheres are investigated. The foF2 hourly data for the period 1977-1982 for stations Kiev (50.5°N, 30.5°E) and Wakkanai (45.39°N, 141.69°E) for the northern hemisphere and Kerguelen (49.35°S, 70.24°E) and Campbell Island (52.6°S, 169.1°E) for the southern hemisphere are used for analysis. It is found that there is a strong tendency for hemispheric similarities in the seasonal variations of oscillation amplitude, period, and probability of existence. The annual variations of the oscillation amplitude are modulated by the 12-month hemispheric and semiannual geomagnetic waves; the dominant period of oscillation is maximum in summer (50-51 hours) and minimum in winter (47-49 hours), and the probability of existence has a maximum in the summer half year and minimum in the winter half year. The seasonal similarities in the annual variations of the oscillation period and the probability of existence show the possible influence of the planetary 2-day wave in the middle neutral atmosphere on the electron density variations in the F region.
    Journal of Geophysical Research 01/1997; 102:9737-9740. · 3.17 Impact Factor
  • E. M. Apostolov, D. Altadill, L. Alberca
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    ABSTRACT: The temporal variations of the characteristics of f(sub 0)F2 quasi-2-day oscillations at middle latitudes in the northern hemisphere are investigated. The hourly data for the period 1976-1986 for four European ionospheric stations, Kiev (50.5 deg N, 30.5 deg E), Kaliningrad (54.7 deg N, 20.62 deg E), Lannion (48.75 deg N, 3.45 deg W) and Slough (51.48 deg N, 0.57 deg W), are used for analysis. Periodogram and complex demodulation analysis are used to evaluate the period and amplitude time variations of this oscillation. It is found that quasi-2-day oscillations in f(sub 0)F2 are present almost continuously during the year. The amplitude is larger in the equinoxes and lower in the solstices. The amplitude variations are modulated by the long-term geomagnetic variations during the solar cycle. There is a clear expressed seasonal variation of the oscillation period with minimum in the winter, November-February (42-47 hours), and maximum near summer solstice (48-55 hours). The observations suggest that there are three types of f(sub 0)F2 quasi-2-day oscillations with behavior of (1) westward traveling planetary wave with zonal number one, predominantly during summer half year, with greatest occurrence in August, (2) stationary planetary wave with a maximum occurrence in the summer solstice with unknown wave number, and (3) independent oscillations in separate locations on relatively long distances. The vertical plasma drift variations can explain the annual and long-term modulation effects of f(sub 0)F2 quasi-2-day amplitude variations but encounter significant difficulties in explaining the discrepancy between zonal wave numbers in the mesosphere (three) and upper ionosphere (one). Another possibility is that the arising of quasi-2-day oscillation in the upper ionosphere may be induced by the forcing of such oscillation in the mesosphere with further independent development in the F region.
    Journal of Geophysical Research 01/1995; 100:12163-12171. · 3.17 Impact Factor
  • E. M. Apostolov, L. F. Alberca, D. Pancheva
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    ABSTRACT: In this paper we intend to enlarge the fof2 long-term prediction accuracy taking into account the systematic variations of the hysteresis and secular-time phenomena.
    Advances in Space Research 01/1994; -1:178. · 1.18 Impact Factor
  • D. V. Pancheva, E. M. Apostolov
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    ABSTRACT: Different statistical methods are used to study the long-period (2.5-20 d) oscillations exhibited on four radiopaths for ionospheric absorption (747, 1412, 3985, and 5055 kHz) in the winter of 1984/1985. Special attention is given to the time changes of the basic wave characteristics (period, amplitude, and phase), as well as to their connection with the stratospheric warming in that period. Planetary wave amplification was observed before the major stratospheric warming at the end of December 1984, as well as considerable phase changes during the stratospheric warming.
    01/1991;
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    ABSTRACT: [EN] This paper is a résumé of the development of the ionospheric research at Ebro Observatory. The work is divided in two main parts. The first one highlights the historical achievements, and, again, it is divided in two parts: one explains the contribution to the study of the total electron content, and the other the contribution to the study of the ionospheric layers. In this last part, a distinction is made between D and Es layers, and te so called «regular layers»: E1 and higher ones. The second main part is a description of the most recent lines of research in the Observatory, with special emphasis on the most important achievement on the dynamic coupling in the Mesosphere-Thermosphere-Ionosphere system. [ES] Este trabajo quiere ser un recorrido a través de las diferentes etapas de la investigación ionosférica en el Observatorio del Ebro. Dividiremos el trabajo en dos partes principales. En la primera daremos una reseña histórica en que separaremos la contribución al estudio del contenido total de electrones de la Ionosfera, del estudio de las capas ionosféricas, en el que, a su vez distinguiremos la dedicación a las capas D, Es y las que hemos denominado «capas regulares»: E1 y superiores. En la segunda haremos un esbozo de la línea de investigación más reciente en la que se desglosarán los resultados más importantes sobre el acoplamiento dinámico en el sistema Mesosfera-Termosfera-Ionosfera. Peer reviewed
  • [Show abstract] [Hide abstract]
    ABSTRACT: This CD-ROM presents the Livingston Island Geomagnetic Observatory Bulletin, edited by Observatori de l'Ebre, containing the data obtained during the years 2003 including the 2003-2004 Austral summer survey. edited in digital format, The structure of the CD-ROM consists of one file with the Bulletin contents in PDF and of a tree of directories and subdirectories with the data corresponding to the different years and months of the Bulletin. These data files and their names were built according to the IAGA-2002 data exchange format.
    Boletín del Observatorio del Ebro, CD-ROM (ISSN 1579-4032), 121 pp. (2004). -1.

Publication Stats

273 Citations
23.02 Total Impact Points

Institutions

  • 1998–2001
    • Bulgarian Academy of Sciences
      Ulpia Serdica, Sofia-Capital, Bulgaria
  • 1994
    • Ebro Observatory
      Tortoza, Catalonia, Spain