Dynasonde observations of electron concentration gradients above Tromsø

The Dynasonde is a digital ionosonde which measures the amplitude and phase of radio echoes reflected from the ionosphere. In addition to the frequency and virtual range of each echo, one parameter of particular interest is the direction of arrival which allows horizontal structures in the ionosphere to be studied. Under suitable ionospheric conditions, namely where structures or strong gradients are present in the electron concentration, echoes may be observed out to horizontal distances of several hundred kilometres. We describe an automated method for latitudinally mapping these gradients in F-region peak electron concentration using the Dynasonde. Good agreement is found when the results are compared with a co-located incoherent scatter radar lending credibility to observations of electron concentration gradients by a Dynasonde alone. This is significant for sites where no supporting instruments exist. Although Dynasondes account for only a small fraction of digital ionosondes in use today, the techniques described in this paper could be extended to other digital ionosondes with similar directional capabilities. The combination of routine soundings, and the worldwide distribution of such instruments, would provide a powerful means to monitor ionospheric structures on a regular basis.     

Comparison of model electron densities and temperatures with Millstone Hill observations during undisturbed periods and the geomagnetic storms of 16–23 March and 6–12 April 1990

Measurements of F-region electron density and temperature at Millstone Hill are compared with results from the IZMIRAN time-dependent mathematical model of the Earths ionosphere and plasmasphere during the periods 16–23 March and 6–12 April 1990. Each of these two periods included geomagnetically quiet intervals followed by major storms. Satisfactory agreement between the model and the data is obtained during the quiet intervals, provided that the recombination rate of O⁺(⁴S) ions was decreased by a factor of 1.5 at all altitudes during the nighttime periods 17–18 March, 19–20 March, 6–8 April and 8–9 April in order to increase the NmF2 at night better to match observations. Good model/data agreement is also obtained during the storm periods when vibrationally excited N₂ brings about factor-of-2-4 reductions in daytime NmF2. Model calculations are carried out using different expressions for the O⁺ – O collision frequency for momentum transfer, and the best agreement between the electron-density measurements and the model results is obtained when the CEDAR interim standard formula for the O⁺ – O collision frequency is used. Deviations from the Boltzmann distribution for the first five vibrational levels of NI were calculated. The calculated distribution is highly non-Boltzmann at vibrational levels j > 2, and the Boltzmann distribution assumption results in the increase of 10–30% in calculated NmF2 during the storm-time periods. During the March storm at solar maximum the model results obtained using the EUVAC solar flux model agree a little better with the observations in comparison with the EUV94 solar flux model. For the April storm period of moderate solar activity the EUV94X model results agree better with the observations in comparison to the EUVAC model.     

Geomagnetic activity as a possible cause of changes in the middle atmosphere and troposphere; observations made in Prague in the years 1932–1987

Summary A possible relation between the 10-day, 1-month, and 2-month geomagnetic as well as solar activity, characterized by Kp and Wolf's number R, and temperatures measured at the Prague-Klementinum meteorological station in January–February of the years 1932–1987 was sought. It was found that:1) Kp correlates with the temperature in Prague better than Wolf's number R.2) If the QBO phase is taken into account, the correlations improve.3) Even if the correlations improves with increasing length of the subintervals into which the individual winter periods (January-February) were derived, their statistical significance declines.     

A potential experiment for in-situ measurement of atmospheric temperature and atomic oxygen density in the 90–150 km altitude range by a Raman Lidar

The Raman backscatter cross sections for a 355 nm light source for the three fine-structure components are calculated. The signal-to-noise considerations show that the determination of the densities of the three fine-structure components separately is a feasible experiment. Since these fine-structure components are calculated to be in local thermodynamic equilibrium up to at least 350 km altitude, this experiment also gives atmospheric temperature. It is pointed out that this experiment does not suffer from the drawbacks of the previous efforts to determine atomic oxygen density and should yield reliable results for this density as well as temperature.     

Ionospheric effects of Poincaré waves in the observations of the total electron content

Disturbances of the daily variations in the total electron content (TEC) of the ionosphere during and after geomagnetic storms, obtained from the observations of the GPS satellite signals, are considered. The specific features of these disturbances consist in the intensification of the variations with periods of 4–6 h, the amplitude of which increase at midlatitudes, and in a weak dependence of local maximums on latitude and their considerable longitudinal variability. The possibility of explaining observed disturbances of the considered daily variations by the generation of standing planetary Poincaré waves is discussed. The estimated periods of Poincaré waves, latitudinal structure of these waves, and their ionospheric effects make it possible to qualitatively explain the observed specific variations in TEC.     

Distribution of the low velocity bulk in the middle-to-lower crust of southern Tibet: implications for formation of the north–south trending rift zones

We conducted the ambient noise tomography to image the shallow crustal structure of southern Tibet. The 2D maps of phase velocity anomalies at the periods of 10–16 s show that the low velocities are mainly confined along or near some of the rift zones. While the maps at the periods of 18–25 s show that the coherent patterns that the low velocities expand outside of the rift zones. It means that the low velocities are prevailing in the middle crust of southern Tibet. According to the previous study of surface wave tomography with teleseismic data, we find that the low velocities from the lower crust to the lithospheric mantle are also restricted to the same rift zones. Thus, the integrated knowledge of the distribution of the low velocities in southern Tibet provides some new insight on the formation of the north–south trending rift zones. Compiling the multidiscipline evidences, we conclude that the rifting was an integrated process of the entire lithosphere in the early stage (~26–10 Ma), but mainly occurred within the upper crust due to the weakening a decoupling in the low velocity middle crust in the late stage (later than ~8 Ma).     

Inter-hemispheric asymmetry in polar mesosphere summer echoes and temperature at 69° latitude

An inter-hemispheric asymmetry is found in the characteristics of polar mesosphere summer echoes (PMSE) and upper mesosphere temperatures at conjugate latitudes (～69°) above Antarctica and the Arctic. The second complete mesosphere–stratosphere–troposphere (MST) radar summer observation season at Davis (68.6°S) revealed that PMSE occur less frequently, with lower strength and on average 1 km higher compared with their northern counterparts at Andenes (69.3°N). We consider the thermodynamic state of the mesosphere for conjoining hemispheric summers based on satellite and ground-based radar measurements, and show the mesopause region near ～80–87 km of the Southern Hemisphere (SH) to be up to 7.5 K warmer than its Northern Hemisphere (NH) counterpart. We show that this is consistent with our observation of asymmetries in the characteristics of PMSE and demonstrate how the mesosphere meridional wind field influences the existence and strength of the echoes in both hemispheres.     

High resolution dating of loess profile from Biały Kościół, south–west Poland

Results of high resolution optically stimulated luminescence (OSL) dating of one of the most complex loess/palaeosol complex in Poland located in the vicinity of the village Biały Kościół (south–west Poland) are presented. Twenty one samples were collected from the 9-m loess profile and dated using the single-aliquot regenerative method applied to coarse grained quartz fraction (90–125 μm). The obtained single-aliquot results were analysed using the central age model. OSL ages are in good agreement with the chronology inferred from the marine oxygen isotope (MOIS) stratigraphy and ages reported for similar lithostratigraphic units for other loess profiles in Europe. However, for the deepest part of the profile underestimation of OSL is observed.     

Regional characteristics of stress field in the southern part of the north－south seismic belt in China and its relation with plate movement

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Groundwater–surface water interactions in a river estuary and the importance of geomorphology: Insights from hydraulic,thermal and geophysical observations

This study presents the groundwater flow and salinity dynamics along a river estuary, the Werribee River in Victoria, Australia, at local and regional scales. Along a single reach, salinity across a transverse section of the channel (~80 m long) with a point bar was monitored using time-lapse electrical resistivity (ER) through a tidal cycle. Groundwater fluxes were concurrently estimated by monitoring groundwater levels and temperature profiles. Regional porewater salinity distribution was mapped using 6-km long longitudinal ER surveys during summer and winter. The time-lapse ER across the channel revealed a static electrically resistive zone on the side of the channel with a pronounced cut bank. Upward groundwater flux and steep vertical temperature gradients with colder temperatures deeper within the sediment suggested a stable zone of fresh groundwater discharge along this cut bank area. Generally, less resistive zones were observed at the shallow portion of the inner meander bank and at the channel center. Subsurface temperatures close to surface water values, vertical head gradients indicating both upward and downward groundwater flux, and higher porewater salinity closer to that of estuary water suggest strong hyporheic circulation in these zones. The longitudinal surveys revealed higher ER values along deep and sinuous segments and low ER values in shallow and straighter reaches in both summer and winter; these patterns are consistent with the local channel-scale observations. This study highlights the interacting effects of channel morphology, broad groundwater–surface water interaction, and hyporheic exchange on porewater salinity dynamics underneath and adjacent to a river estuary.     

Spatial and temporal variation of precipitation in Sudan and their possible causes during 1948–2005

Temporal and spatial patterns of precipitation are essential to the understanding of soil moisture status which is vital for vegetation regeneration in the arid ecosystems. The purposes of this study are (1) to understand the temporal and spatial variations of precipitation in Sudan during 1948–2005 by using high quality global precipitation data known as Precipitation REConstruction (PREC), which has been constructed at the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center, and (2) to discuss the relationship between precipitation variability and moisture flux based on the NCEP/NCAR reanalysis data in order to ascertain the potential causes of the spatial and temporal variations of precipitation in the region. Results showed that (1) annual and monthly precipitation in Sudan had great spatial variability, and mean annual precipitation varied from almost nil in the North to about 1500 mm in the extreme Southwest; (2) precipitation of the main rain season, i.e., July, August and September, and annual total precipitation in the central part of Sudan decreased significantly during 1948–2005; (3) abrupt change points were found in the annual, July, August and September in the late 1960s, when precipitation decreased more rapidly than in other periods; and (4) the decreasing precipitation was associated with the weakening African summer monsoon. The summer moisture flux over Sudan tended to be decreasing after the late 1960s which decreased the northward propagation of moisture flux in North Africa. This study provides a complementary view to the previous studies that attempted to explain the Sahel persistent drought and possible causes.     

A comparative flood frequency study of Ganga and Brahmaputra river systems of north India — a brief appraisal

An attempt has been made to study the occurrence of floods in the two important river systems of north India, viz. Brahmaputra and Ganga. Both these river systems are located north of Lat. 22°N and lie in the longitudinal belt of north India from Long. 73°E to 97°E. Both these river systems are affected by floods during monsoon months of June–September. It has been seen after examining rainfall and floods of the period 1986–1999 that although variation of monsoon rainfall magnitudes received by these two basins differ considerably in each monsoon season, but by and large, the frequency of floods at their terminal gauge/discharge (G/D) sites at Dhubri and Farakka do not differ very much from each other.     

Core–mantle boundary topography as a possible constraint on lower mantle chemistry and dynamics

The origin of large low shear-wave velocity provinces (LLSVPs) in the lowermost mantle beneath the central Pacific and Africa is not well constrained. We explore numerical convection calculations for two proposed hypotheses for these anomalies, namely, thermal upwellings (e.g., plume clusters) and large intrinsically dense piles of mantle material (e.g., thermochemical piles), each of which uniquely affects the topography on Earth's core–mantle boundary (CMB). The thermochemical pile models predict a relatively flat but elevated CMB beneath piles (presumed LLSVPs), with strong upwarping along LLSVP margins. The plume cluster models predict CMB upwarping beneath upwellings that are less geographically organized. Both models display CMB depressions beneath subduction related downwelling. While each of the two models produces a unique, characteristic style of CMB topography, we find that seismic models will require shorter length scales than are currently being employed in order to distinguish between the end-member dynamic models presented here.     

Evaluation of temperature and freeze–thaw effects on excess pore pressure generation of fine-grained soils

The November 3, 2002 Denali-Alaska earthquake (M_w=7.9) caused significant liquefaction associated damage to various infrastructure built on fine-grained soils. The seismic response, liquefaction potential, and excess pore pressure generation of soils in cold regions, especially those of fine-grained nature, have not been studied thoroughly and therefore are not well-understood. This paper presents results from an extensive laboratory study on the characteristics of excess pore pressure generation and liquefaction potential of fine-grained soils. Laboratory-constituted soils specimens were tested in four categories: (1) tests on specimens subjected to no thermal conditioning or freeze–thaw cycles; (2) tests on specimens conditioned at 24, 5, 1, 0.5, and −0.2 °C; (3) tests on specimens subjected to 1–4 freeze–thaw cycles; and (4) tests on specimens conditioned at near-freezing temperatures of 0.5 and −0.2 °C through different freeze–thaw paths. Strain-controlled, undrained, cyclic triaxial tests were performed at shear strain levels of 0.005–0.8%. Specimens conditioned at different temperatures were found to generate significantly different pore pressures with cyclic loading. The excess pore pressure generation at near or slightly below freezing was found to change dramatically. A transitional change in the dynamic soil behavior, attributed to unfrozen- or frozen-dominant pore water, was discovered. The threshold shear strain was also found to be influenced by the temperature. Subjecting the soil specimens to 1, 2 and 4 freeze–thaw cycles caused a reduction in excess pore pressure generation and slight change in the threshold shear strain. The temperature conditioning path to reach the target temperature was found to be important on the development of excess pore pressure at near and slightly below-freezing temperatures.     

Geochemistry of the High Dam Lake sediments,south Egypt： implications for environmental significance

The goal of the present work is to perform a geochemical assessment of High Dam Lake bottom sediments for determining the fate, dispersion and levels of trace elements causing environmental pollution, and provided an access to their probable sources. The sediment samples were analyzed using ICP-MS for 20 elements; Ag, Ba, Cr, Cu, Ga, Hf, Mn, Pb, Rb, Sc, Se, Sn, Sr, Ta Th Ti, T1, U, V and Zr, and their obtained data were treated using statistical, graphical and mapping techniques. The results showed the data set of all analyzed elements affected by outliers and extreme values that caused deviation away from normality. Kruskal-Wallis test revealed that median of some trace element levels in Lake sediments, was not significantly different and other elements rejected the Null hypothesis. Most analyzed elements had high values of median and mean in sediments of Lake Nubia rather than Lake Nasser and their normalization gave the same results of calculated environment factors. Subsequently, Lake Nubia sediments possessed high combined EF levels ofTh, Sc, Sn, Ag, Zr, Hf, Ta, Sr, U, Ti, V, Cr, and Mn, causing significant contamination, which was great potentially related to industrial, agricultural, urbanization and mining activities. Whereas, combined EF of Se, Cu, Ga, Pb, Ba, Rb, and Tl, which are highly elevated in southern Lake Nasser sediments owing to their source are great possibly derived from Lake Nubia and geogenic activities. With decreasing distance towards the High Dam body, the contaminant elements were diminished due to reduction in the environmental factors and Sudanese pollution sources leading to the northern Lake Nasser considered to be less contaminated. Overall, the present study is an environmental alert for contaminated sediments that carried contaminants and considered the secondary source of pollution impact on ecosystem, and subsequently, their environmental risk on Human health.