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Mid latitude spread-F has extensively studied recently as  compared to the earlier periods. In the course of these  investigations two concepts have appeared in the literature concerning the origin of the mid-latitude spread-F. The first concept based on radar measurements attributes the development of spread-F to plasma instabilities (Fukao et al.\ 1988), while according to the other concept based on the vertical incidence sounding of the ionosphere the occurrence of mid-latitude spread-F is related to travelling ionospheric disturbances (Bowman 1981). Therefore, it seemed worthy to study mid-latitude spread-F from the point of view of its origin. This study has been carried out by analysing the connection between  the occurrence of spread-F and the occurrence of sporadic E layers, keeping in mind that the development of sporadic E layers at mid-latitude is due to atmospheric gravity waves, which initiate plasma instabilities. On the other hand, the relation of the occurrence of spread-F to enhancements of the auroral electrojet index (AE) has been studied indicating the occurrence of large scale travelling ionospheric disturbances.

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Ground based observations of the “Széchenyi István” Geophysical Observatory of the Hungarian Academy of Sciences (Nagycenk) offer a unique opportunity for investigation of the complex processes of Earth’s environment and of its effect on radio wave propagation. High temporal resolution recording of variations of the geomagnetic field, vertical incidence sounding of the ionosphere and observation of whistlers enable determination of plasma density profiles in the inner magnetosphere (plasmasphere) and their changes. Study of ionospheric processes and their effects in the upper atmosphere (thermosphere) is supported by a permanent GPS station. Both DC and AC fields of lightning discharges are also recorded in the Observatory. Measurement of the DC field is related to the atmospheric electric circuit, where the global thunderstorm activity is the generator and fine weather areas play the role of loading. Loading of the atmospheric electric field depends on rate of ionization, ionization being caused by galactic cosmic rays. However, galactic cosmic rays are modulated by solar activity. Similarly amplitude of Schumann resonances depend on thunderstorm activity and its distance from the recording site. Resonance frequencies occurring at wave-lengths approximating dimensions of the Earth-ionosphere waveguide are affected by conductivity variations of the upper boundary of the cavity resonator and thus, by changes of thickness of the cavity resonator.

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Using data of Es parameters obtained at a high mid-latitude station in Germany (Juliusruh; geogr. lat.: 54º 38' daily and seasonal variations were derived and certain characteristics of these variations have been analysed during a period of changing solar activity. It has been confirmed that diurnal maxima appearing both in f oEs and f bEs are generally higher around solar activity maximum than around the minimum. Seasonal maxima appearing in summer are also higher around solar activity maximum than towards the minimum. The daily variation of h'Es showed two maxima: one in the morning and another in the afternoon. The seasonal maximum of h'Es generally appeared in April or May while the minimum in October or November. All these results confirm earlier findings derived for a station in Hungary (Békéscsaba). The present study particularly intends to show changes in the diurnal variation of Es parameters associated with geomagnetic disturbances. The difference between f oEs and f bEs might  be used as indicator of the transparency of the Es layer, and this quantity seems to increase on days with strong geomagnetic storms. Analysing the transparency based on the maximum values appearing in the daily variations of f oEs and f bEs, a rather high quantity has been derived for stormy days in the year 1971. Actually, in this year, the daily variation of h'Es was also characterised by an extremely high afternoon maximum. The wind-shear theory is generally suitable for explaining the generation of sporadic E layer at mid-latitude. The present results are not inconsistent with the points of this theory.

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In addition to the original equipment, a new measuring set has been constructed for observing the atmospheric electric potential gradient (PG) at Nagycenk station. The study describes this new set and reports on PG variations derived from data obtained by the two simultaneous measurements. Both runs of the mean diurnal variations determined on the basis of the two data sets are quite similar, as well as they resemble the appropriate diurnal variation derived from earlier Nagycenk data, or fairly the `Carnegie Curve'. Nevertheless, the measured PG level is somewhat higher in the case of the new equipment (PG 2) than that measured with the original set (PG 1). This is also true in the case of single days diurnal variations, however, the daily runs derived from the individual measurements are hardly different. Whichever of the two data sets is applied, the PG's seasonal (annual) variation shows a distinct maximum in winter, even if this maximum is again higher for PG 2 than that for PG 1. Certain ideas on this behaviour are also suggested by the study

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Acta Geodaetica et Geophysica Hungarica
Authors:
P. Bencze
,
B. Heilig
,
B. Zieger
,
J. Szendrői
,
J. Verő
,
H. Lühr
,
K. Yumoto
,
Y. Tanaka
, and
J. Střeštík

The total solar eclipse of August 11, 1999 offered a unique opportunity for the study of its geomagnetic effect — more specifically, of its effect on geomagnetic (Pc3) pulsations, as it swept through Europes many observatories, and additional temporary stations have also been established by Japanese, German and Hungarian groups. The present paper starts with an analysis of the ionospheric-interplanetary background. In the interplanetary medium, no indication was found which could result in any extraordinary event in pulsation activity. The both horizontally and vertically widespread ionospheric effect (electron density decrease) explains a change of the polarisation angle by about ten degrees in the local field line resonance (FLR) band. However, the most significant solar eclipse effect was identified as dramatic clockwise rotation (up to 70 degrees) of the polarisation ellipse of Pc3, Pc4 and Pc5 pulsations. Pulsation data exhibit a strong amplitude decrease (roughly by a factor of two) in and around the totality spot of the eclipse. The decrease is most significant at the local field line resonance (FLR) period. In the actual case, the FLR decrease swept over Europe with a speed being similar to the speed of the dark spot. We suppose that the FLR mechanism was disturbed by the change of particle distribution along the field lines ending in the dark zone due to upward propagation of the electron density decrease caused by the lack of ionising solar radiation in the E-layer of the ionosphere. Thus, the FLR mechanism can be disturbed both from outside, by a sudden change of the interplanetary magnetic field (Verő et al. 1998) and from inside, by a change of the particle density/distribution along the actual field line.

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Acta Geodaetica et Geophysica Hungarica
Authors:
A. Ádám
,
P. Bencze
,
F. Märcz
,
D. Martini
,
G. Sátori
,
L. Szarka
,
J. Verő
,
V. Wesztergom
, and
B. Zieger

This is a summary of the activities of the Geophysics Department of the Geodetic and Geophysical Institute in the framework of the Scientific School Earth Electromagnetism. Ten reports cover the most important topics and emphasize interconnections between the different phenomena. The topics include interplanetary space, magnetosphere and ionosphere, electromagnetic waves in a wide spectrum, and electromagnetic induction, also within a wide range of frequencies and applications. Geomagnetism, especially the Széchenyi István Geophysical Observatory Nagycenk with a lot of different recordings of electromagnetic parameters play important roles in most topics.

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Acta Geodaetica et Geophysica Hungarica
Authors:
A. Ádám
,
P. Bencze
,
J. Bór
,
B. Heilig
,
Á. Kis
,
A. Koppán
,
K. Kovács
,
I. Lemperger
,
F. Märcz
,
D. Martini
,
A. Novák
,
G. Sátori
,
S. Szalai
,
L. Szarka
,
J. Verő
,
V. Wesztergom
, and
B. Zieger

The paper describes research in geomagnetism and aeronomy carried out in the framework of a project organized by the Geodetic and Geophysical Research Institute of the Hungarian Academy of Sciences. It includes the development of the instrumentation of the Nagycenk Geophysical Observatory (geomagnetic measuring systems, ionosonde), moreover other instrumental and methodological developments, too. Observatory data are available in a database. Based on results of the Nagycenk and Tihany observatories and on data of permanent and temporal networks, long-term trends of different electromagnetic parameters were investigated. Thus geomagnetic activity was found secularly increasing, a decrease of the atmospheric electric potential gradient and a 11-year modulation of the winter/December attenuation of the geomagnetic pulsation activity were confirmed. Several possibilities (pulsations, whistlers, modelling) were used to improve knowledge about structure and parameters of the magnetosphere. Electromagnetic precursors of earthquakes were looked for. A significant increase of understanding was obtained in connection with Schumann resonances and electromagnetic transients caused by lightning. It was shown that see-coasts influence characteristically changes in ionospheric trends ( h m F2). When looking for the effect of the global climate changes in the subsurface electric resistivity, an example was discovered for the decrease of the resistivity due to infiltrating water from precipitation. Electromagnetic exploration of tectonically conditioned weak zones was continued, too.

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