Ionospheric Precursors of Geomagnetic Storms. 1. A Review of the Problem

Geomagnetism and Aeronomy - The problem of the occurrence of disturbances of the ionospheric F2-layer parameters prior to the beginning of a geomagnetic storm is discussed. It is shown that...     

Long-term changes in the Delta <Emphasis Type="Italic">foF</Emphasis>2 parameter based on the data of two European ionospheric stations

Analysis of changes in the critical frequency foF2 in recent decades has been performed by determination of “Delta foF2” parameter. These values determine the mean change of foF2 values from the “etalon period” (1958–1980) to later periods. The results are compared with the determination of foF2 trends, which was performed in a series of previous publications of the authors. The data of two most reliable ionospheric stations of the European region, Slough and Juliusruh, are analyzed. The results confirm all principal conclusions obtained earlier, which were based on analysis of the trends. A systematic decrease of foF2 with time occurs (which corresponds to a negative trend), and the character of changes in the Delta values with season and local time on the whole agrees with the character of changes in the trend. It is shown that the results based on the data of both considered stations show good agreement.     

Diurnal and Seasonal Variations in Trends in the <Emphasis Type="Italic">E</Emphasis>-Layer Critical Frequency

Long-term changes in the E-layer critical frequency foE at three stations in the European region (Juliusruh, Slough, and Rome) are analyzed by the method described in detail in the previous paper by the authors. It is found that two former stations demonstrate a well-pronounced change in foE (a trend) during the two previous decades. At the same time, the same features of the behavior of the aforementioned trend k(foE) are obtained for both stations. The trend is positive and negative in the morning and evening hours, respectively. It is minimal near the local noon. That explains the small value of k(foE) obtained in the previous paper for 1200 LT. A well-pronounced seasonal behavior of k(foE) is detected: the trend is minimal and maximal in the summer period and at the end of fall—beginning of winter, respectively. The trend maximal amplitude in the morning hours reaches +0.04 MHz per year, whereas the minimal amplitude in evening hours is–0.06 MHz per year. No systematic changes in foE exceeding 0.01 MHz in magnitude per year are found for Rome station.     

Rb-Sr and U-Pb systematics of metasedimentary carbonate rocks: The Paleoproterozoic Kuetsjarvi Formation of the Pechenga Greenstone Belt, Kola Peninsula

The Rb-Sr and U-Pb systematics have been studied in the metasedimentary carbonate rocks from the Paleoproterozoic Kuetsjarvi Formation. Samples were taken from the borehole drilled in the northern zone of the Pechenga Greenstone Belt in the northwestern Kola Peninsula. The carbonate section of the formation is made up of three units (from the bottom to top): (I) dolomite (68 m), (II) calcareous-dolomite (9 m), and (III) clayey calcareous (1 m) ones. Dolomites (Mg/Ca = 0.55–0.61) from the lowermost unit I contain 70.3–111 ppm Sr. Initial ⁸⁷Sr/⁸⁶Sr ratio in them varies within 0.70560–0.70623 and characterizes the primary continental-lacustrine carbonate sediments. Calcareous dolomites (Mg/Ca = 0.39–0.59) and dolomitic limestones of units II and III (Mg/Ca = 0.02–0.36) are enriched in Sr (285–745 and 550–1750 ppm, respectively). Initial ⁸⁷Sr/⁸⁶Sr ratios in these rocks (0.70406–0.70486 and 0.70407–0.70431, respectively) fall within the range typical of the Jatulian seawater, which indicates that the carbonate sediments of two upper units were formed in an open marine basin. Study of dolomites from unit I showed that the Svecofennian metamorphism more significantly affected the U-Pb systems of carbonate rocks as compared to their Rb-Sr systems. In the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagram, most data points corresponding to the carbonate constituent of dolomites define isochron with an age of 1900 ± 25 Ma (MSWD = 0.5). The same samples define a positive correlation in the ²⁰⁸Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb plot. Since sedimentary carbonates usually do not contain Th, this correlation points to secondary enrichment of the studied dolomites in Th or thorogenic ²⁰⁸Pb. Hence, the obtained Pb-Pb dating can be regarded as the age of the Svecofennian metamorphic event. Three samples from dolomites of unit I lack any disturbance of the initial U-Th-Pb systematics, but their trend in the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagram deviates from the 1900 Ma isochron. Based on these samples, the model U-Pb premetamorphic age of the Kuetsjarvi carbonate sediments is 2075–2100 Ma. This interval is consistent with the age range of the Lomagundi-Jatulian event, which was responsible for the formation of carbonate sediments with high positive δ¹³C values.     

Structural damage patterns in the villages of Korf and Tilichiki due to the <Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">S</Emphasis></Subscript> 7.7 April 20(21), 2006 Olyutorskii earthquake

The area of the Koryak Autonomous Okrug was hit by an M _S 7.7 earthquake on April 20(21), 2006, the largest to have occurred in the area during the period of historical and instrumental observation. This event is now referred to as the Olyutorskii earthquake. We present results from a study of the associated macroseismic effects as observed in the villages of Korf and Tilichiki. The intensity was IX at Korf and VIII at Tilichiki on the MSK-64 scale.     

Isotopic geochronology and biostratigraphy of Riphean deposits of the Anabar Massif,North Siberia

The structure of Riphean deposits developed on the western slope of the Anabar Massif is described with analysis of their depositional environments, distribution of stromatolite assemblages and organic-walled and silicified microfossils through sections, and evolution of views on stratigraphic significance of some of these assemblages. The investigation included complex mineralogical, geochemical, structural, and isotopic?geochronological study of globular phyllosilicates (GPS) of the glauconite?illite series from paleontologically well substantiated Riphean sequences (Ust’-Il’ya and Yusmastakh formations of the Billyakh Group) of the Anabar Massif in the Kotuikan River basin. Isotopic dating of monomineral size and density fractions of GPS from the Billyakh Group was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mössbauer spectrometry data. The applied approach is based on an assumption that the formation and transformation of Rb?Sr and K?Ar systems in GPS are synchronous with stages in their structural evolution, which are determined by the geological and geochemical processes during depositional history. Such an approach combined with the mineralogical and structural analysis contributes to correct interpretation of stratigraphic significance of isotopic data. The results obtained provide grounds for the conclusion that isotopic dates of GPS from the Ust’-Il’ya (Rb?Sr, 1485 ± 13 Ma; K?Ar, 1459 ± 20 Ma) and Yusmastakh (Rb?Sr, 1401 ± 10 Ma; K?Ar, 1417 ± 44 Ma) formations mark the stage of early diagenesis of sediments and are suitable for estimating the age of formations in question.     

Behavior of critical frequency <Emphasis Type="Italic">foF2</Emphasis> at various moments of the day: 1. Dependence on solar activity

Variation with solar activity level of the ratios of the critical frequencies of the F2 layer, foF2, is considered for various pairs of local time moments T1 and T2 for two seasons (winter and summer). It is found that, as a rule, the foF2(T1)/foF2(T2) behavior with the solar activity index F10.7 is different in winter and summer. The variations of foF2(T1)/foF2(T2) with F10.7 obtained from experimental data are considered from the viewpoint of the current theory of F2-layer formation. It is shown that the majority of observed characteristics of these variations are explained using the above-indicated theory. However, special cases of foF2(T1)/foF2(T2) behavior with solar activity are found for which there is still no physical explanation.     

Behavior of parameters of the ionospheric F2 layer at the turn of the Centuries: 1. critical frequency

The available massifs of experimental data on the critical frequency of the ionospheric F2 layer, foF2, covering the first decade of the new century, are considered. On the basis of studying these massifs, a conclusion is drawn that the scatter of foF2 values (measured by the standard deviation (SD)) relative to the dependence on solar activity has grown substantially over recent decades as compared to the period 1958–1979. The possible causes of the SD increase are considered. It is shown that the foF2 values for the period 1998–2010 decreased as compared to the period 1958–1979 by an average of 0.6 MHz which gives an estimate of the foF2 trend of ～-0.03 MHz per year. Linear trends in foF2 for some ionospheric stations are analyzed. It is obtained that, in spite of the scatter in the data, it is possible to obtain statistically significant trends for each considered situation (day and postsunset period in summer and winter). At the same time, the winter negative trends (～-0.052 MHz per year) are approximately a factor of 2 higher than the summer ones (～-0.024 MHz per year). Comparisons with the trends obtained for earlier periods show that the negative trend in foF2 increased substantially towards the first decade of our century.