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1.
Superposed epoch analysis (SEA) was used to examine ionospheric drift velocities measured by a digital ionosonde located at the mid-latitude station Bundoora (145.1°E, 37.7°S geographic), near Melbourne. The control times for the SEA were the times of cloud-to-ground (CG) lightning strokes measured from August 2003 to August 2004 by the World Wide Lightning Location Network (WWLLN). Statistically, regions of concentrated lightning activity migrated from west to east across Bundoora, and the stroke frequency was higher the day prior the activity reached the station, and lower on the day after it passed to the east. For the SEA, CG strokes were separated into four directional quadrants centred on north, south, east and west. No SEA results are shown for the south quadrant due to the relatively low detection frequency of strokes across the Southern Ocean (6% of all events). The strongest downward vertical perturbations in F-region drifts, ?4.5 m s?1, were found for lightning located towards the west during ?30 to ?16 h (i.e., the afternoon prior the activity passed near the station at t=0 h). The downward perturbation decreased in amplitude to ?1.5 m s?1 for lightning located towards the north during ?6–+6 h, and was weakest (?0.7 m s?1) for lightning located towards the east during +16–+28 h (i.e., the next afternoon). There were directionally consistent perturbations in the drift azimuths associated with the lightning located in their respective quadrants; lightning located to the west of the station caused eastward azimuth enhancements, northward lightning caused southward enhancements, and eastward lightning caused westward enhancements. Velocity magnitudes and fluctuations tended to increase during the passage of lightning. The observed responses were stronger when the SEA was performed with data selected using time windows of <2 min on either side of each lightning stroke. However, they persisted at longer time scales and were strong when thunderstorm onsets (instead of lightning times) were used as controls. Our results can be explained by thunderstorm-generated atmospheric gravity waves (AGWs) which subsequently gave rise to medium-scale travelling ionospheric disturbances (MSTIDs), with the lightning strokes acting merely as a proxy for this coupling. The prevailing thermospheric winds were flowing from east to west across the study region, and may have acted as a directional ‘filter’ for the MSTIDs, allowing waves generated in the west quadrant to reach the station and preventing those generated in other quadrants. Displacement of the MSTIDs in the direction anti-parallel to mean neutral wind flow has been observed by (Waldock, J.A., Jones, T.B., 1986. HF Doppler observations of medium-scale travelling ionospheric disturbances at mid-latitudes. Journal of atmospheric and terrestrial physics 48(3), 245–260).  相似文献   

2.
A large aerosol plume with optical depth exceeding 0.7 engulfs most parts of the Arabian Sea during the Asian summer monsoon season. Based on Micro Pulse Lidar observations during the June–September period of 2008 and 2009, the present study depicts, for the first time, the existence of an elevated dust layer occurring very frequently in the altitude band of 1–3.5 km over the west coast of peninsular India with relatively large values of linear depolarization ratio (δL). Large values of δL indicate the dominance of significantly non-spherical aerosols. The aerosol optical depth of this layer (0.2) is comparable to that of the entire atmospheric column during dust-free days. Back-trajectory analysis clearly shows the advection of airmass from the arid regions of Arabia and the west Arabian Sea, through the altitude region centered around 3 km. This is in contrast to the airmass below 1 km originating from the pristine Indian Ocean region which contains relatively spherical aerosols of marine origin with δL generally <0.05.  相似文献   

3.
At longer periods, scattered ScS waves sometimes dominate over coda waves at large lapse times. Examining recordings of seismic envelopes at 9 IRIS seismic stations of regional earthquakes with focal depths deeper than 150 km in periods from 1 to 20 s for a wide lapse time range up to 2000 s, we found significant frequency dependence. The coda decay gradient at short periods is steeper than that at longer periods; however, the change of coda gradient associated with the ScS arrival becomes distinct as the period becomes longer. In particular, a clear offset of coda amplitude appears in central Asia for 10 and 15 s period bands. The multiple isotropic scattering process of S-waves in the heterogeneous mantle can be simply simulated by using the Monte Carlo simulation method based on the radiative transfer theory in scattering media. Assuming a two-plane-layer attenuation structure and smoothed velocity model of the PREM, we estimated the average total scattering coefficients of S-waves such as 7.52 × 10− 4∼1.32 × 10− 3 km− 1 and 2.08 × 10− 4∼6.23 × 10− 4 km− 1 at 4 s, and 4.51 × 10− 4∼7.37 × 10− 4 km− 1 and 2.80 × 10− 5∼2.71 × 10− 4 km− 1 at 10 s, for the lithosphere and the upper mantle and for the lower mantle, respectively. Our results indicate that scattering occurs mostly in the lithosphere and the upper mantle and support that medium heterogeneity spreads over the whole mantle though its scattering power is small. Strong scattering occurs beneath central Asia and Papua New Guinea, whereas the scattering beneath Italy and regions of east Russia is much weaker. The numerical calculation enables us to confirm that much stronger scattering than intrinsic attenuation causes the offset behavior with coda decay gradient change after the ScS arrival for 4 and 10 s period bands in some regions.  相似文献   

4.
Increasing evidence from fission track studies in Sweden indicate that large parts of the Fennoscandian Shield have been affected by a large-scale thermotectonic event in the Palaeozoic. In this study the results of 17 apatite fission track analyses from central Sweden are presented collected along three NW–SE transects trending from the Bothnian Sea to the Caledonides. On the Bothnian coast samples have been collected directly from the Sub-Cambrian Peneplain. The sedimentary cover protecting this surface until recently is responsible for the thermal increase detected through apatite fission track (FT) thermochronology.The apatite FT ages range between 516 ± 46 Ma (±1σ) on the Bothnian coast around sea level to 191 ± 11 Ma in the Caledonides (～500–1500 m.a.s.l.). The mean track lengths vary from 11.3 ± 2.2 μm (±1σ) in the east to 14.2 ± 2.8 μm in the west, indicating a longer stay in the PAZ in the east, versus a continuous cooling pattern in the west. This pattern in combination with other geological constraints indicates that the crystalline basement rocks near the Caledonian deformation front in the west experienced higher temperatures after the formation of the Sub-Cambrian Peneplain followed by denudation, compared with the basement rocks in the east near the Bothnian coast.The apatite FT data near the Caledonian deformation front indicates prevailing temperatures of more than 110 ± 10 °C prior to the Mid Palaeozoic, causing a resetting of the apatite fission track clock. The temperatures were progressively lower away from the deformation front. Apatite fission track analysis of samples collected from the Sub-Cambrian Peneplain along the Bothnian coast indicate maximum temperatures of 90 ± 15 °C during Late Silurian–Early Devonian time. This heating event is argued to be the result of burial beneath a developing foreland basin in front of the Caledonian orogeny. Assuming a geothermal gradient of 20 °C/km, this temperature increase can be converted to a total burial of the samples. The resulting geometry of this basin can be described as an asymmetrical basin at least 3.5 km deep in the vicinity of the Caledonian deformation front decreasing to about 2.5 km on the Bothnian coast, continuing further onto Finland. The width of this basin was in thus in the order of 600 km. Whether this was formed completely synorogenic or partly synorogenic, broadening after cessation of the orogeny, could not be revealed.The Late Palaeozoic and Mesozoic thermal evolution of this area is related to the extensional tectonics in the North Atlantic Domain.  相似文献   

5.
Narrow bipolar events (NBEs) are a distinct class of intra-cloud lightning discharge. In this paper we present observations of 10 negative and 67 positive such events in East China. Positive NBEs occurred at 7–12 km altitude above mean sea level (MSL) with a mean altitude of 9.5 km, and negative NBEs occurred at 14–16 km altitude. Electrical/channel characteristics of these events were derived from NBE pulse waveforms based on the transmission-line model. On average, the peak current moment and the charge moment change of a NBE event is 15 kA km, and 0.12 C km, respectively. The mean time for the propagation of current front along the channel is 2.2 μs. The upper limit on channel length for NBEs in this study is 510–1060 m, the lower limit on discharge current amplitude is 12.5–43.2 kA, and the minimum charge transfer is 0.1–0.3 C.  相似文献   

6.
Observations of lake ice at the shore, complete ice cover, ice duration, ice thickness and other measures for 18 Polish lakes were collected for the 50 year period (1961–2010). Average ice dates in early winter became later: first appearance of ice along shore 2.3 days decade−1 and complete ice cover 1.2 days decade−1 while complete ice cover disappeared earlier (5.6 days decade−1) as did last ice at the shore (4.3 days decade−1). The duration of ice cover decreased by 5.6 days decade−1 and average ice thickness declined by 6.1 cm decade−1. The magnitude of these values for individual lakes decreased from eastern to western Poland. This geographic gradient is likely related to regional atmospheric circulation because in winter this part of Europe is strongly affected by continental air, an influence that is greater in the east. A multivariate redundancy analysis (RDA), used in order to examine the dependence of ice measures on lake physical properties and location, indicated longitude and altitude as key factors explaining lake ice dynamics such as the disappearance of ice and ice cover, ice cover duration and thickness. Lake volume and average depth influenced mostly the appearance of ice and ice cover.  相似文献   

7.
Kutch (northwest India) experienced lithospheric thinning due to rifting and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event. Alkalic lavas, containing mantle xenoliths, form plug-like bodies that are aligned along broadly east–west rift faults. The mantle xenoliths are dominantly spinel wehrlite with fewer spinel lherzolite. Wehrlites are inferred to have formed by reaction between transient carbonatite melts and lherzolite forming the lithosphere. The alkalic lavas are primitive (Mg# = 64–72) relative to the tholeiites (Mg# = 38–54), and are enriched in incompatible trace elements. Isotope and trace element compositions of the tholeiites are similar to what are believed to be the crustally contaminated Deccan tholeiites from elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all except two alkalic basalts plot in a tight cluster that largely overlap the Indian Ridge basalts and only slightly overlap the field of Reunion lavas. This suggests that the alkalic magmas came largely from the asthenosphere mixed with Reunion-like source that welled up beneath the rifted lithosphere. The two alkalic outliers have an affinity toward Group I kimberlites and may have come from an old enriched (metasomatized) asthenosphere. We present a new model for the metasomatism and rifting of the Kutch lithosphere, and magma generation from a CO2-rich lherzolite mantle. In this model the earliest melts are carbonatite, which locally metasomatized the lithosphere. Further partial melting of CO2-rich lherzolite at about 2–2.5 GPa from a mixed source of asthenosphere and Reunion-like plume material produced the alkalic melts. Such melts ascended along deep lithospheric rift faults, while devolatilizing and exploding their way up through the lithosphere. Tholeiites may have been generated from the main plume head further south of Kutch.  相似文献   

8.
The optical properties and light climate in the deep and extremely acid Lake Caviahue have been studied in order to better understand its characteristics and the possible influence upon the phytoplankton community. The absorption coefficients for the dissolved fraction were maximal in the ultraviolet (UV) region and the water absorption spectra showed a shoulder around 300 nm, which was attributed to the concentration of Fe(III). No radiation was detected in the water column below 360 nm. The depth of the 1% incident radiation was dependent of wavelength, showing its maximum of 13.3 m at 565 nm, compared to 1.7 m and 4.8 m at 400 nm and 700 nm, respectively. Phytoplankton biomass was low and showed an almost constant profile with depth despite the relative darkness of the water column. Optical climate of Lake Caviahue is not typical of high elevation lakes but is more similar to low elevation shallow lakes of the Andean region. The chemical composition of the water, mainly Fe oxidation state and concentration, is the responsible for the high attenuation of the UV radiation (UVR). Living organisms are protected of UVR because Lake Caviahue waters are a shield against UV-B.  相似文献   

9.
Space-based tectonic studies on the western part of the North Anatolian Fault Zone (NAFZ) have been conducted over two decades. After the August 17, 1999, Izmit earthquake (Mw = 7.4), this region attracted greater scientific interest, and the collected data became more valuable. The Geodesy Department of the Kandilli Observatory and Earthquake Research Institute (KOERI) at Bogazici University established three micro-geodetic networks to the east of Akyazi, east of Iznik, and west of Lake Sapanca in the eastern part of the Marmara region; GPS data have been continually collected at these locations since 1994. The NAFZ branches out in the western part of the Marmara region and extends up to the Aegean Sea. Segments of the fault passing through the Marmara Sea are considered active, and this has increased concern regarding imminent earthquakes. Conventional geodetic measurements made between 1990 and 1994 are not sufficient for monitoring small movements. However, GPS has played a very important role in detecting such deformations in the area after 1994. The Iznik network, with 10 points, is bilaterally located on the Iznik-Mekece fault. Six years of GPS data for 2004–2010 collected for the monitoring of crustal deformation showed that the Iznik-Mekece fault segment moves westward at about 22 ± 1 mm/yr with respect to the Eurasia fixed reference frame. The GPS observations show that there is no strain accumulation in the area.  相似文献   

10.
The distribution and abundance of dinoflagellate cysts in recent sediments from Visakhapatnam harbour, east coast of India was investigated and compared with sediment characteristics and physico-chemical variables of the overlying water column. The cyst abundance varied from 11 to 1218 cysts g–1 dry sediment. Changes in the cyst assemblages from phototrophic to heterotrophic forms were observed from inner to outer harbour stations, and related to changes in environmental characteristics. Enhanced cyst production of potentially harmful dinoflagellate Protoceratium reticulatum was recorded in the inner harbour stations with higher nutrient concentrations. Protoperidinium cysts were the most diversified group, and were dominant in the outer harbour stations having improved water conditions and circulation. This study points out the potential use of dinoflagellate cyst populations in providing information on environmental conditions.  相似文献   

11.
Reflection seismic data were acquired along a c. 23 km long profile over the Pärvie Fault system with a nominal receiver and source spacing of 20 m. An hydraulic breaking hammer was used as a source, generating signals with a penetration depth of about 5–6 km. Steeply dipping reflections from the end-glacial faults are observed, as well as sub-horizontal reflections. The location and orientation of the reflections from the faults agree well with surface geological observations of fault geometries. Reflections from a potential fourth end-glacial fault is observed further to the east along the profile. The more sub-horizontal reflections may originate from gabbroic bodies within the granitic basement or from deeper lying greenstones. Our results indicate that the end-glacial faults dip at moderate to steep dips down to at least 2–3 km depth, and possibly continue at this dip to depths of 6 km. This result has significant implications for determining the state of stress required to activate the faults in the past and in the future.  相似文献   

12.
The central part of Europe north of the alpine orogenic belt is generally seen as a relatively stable area of the western tip of the Eurasian plate. Indeed, up to now, no geodetically significant motions have been detected although an active rift system running roughly in SSE–NNW direction along the Rhine valley could have some effect on the stability of this region. Presently, the increasing accuracy of geodetic point motions should allow the study of small motions at levels down to nearly 0.1 mm/yr. We start our investigation with a closer look at the ‘true’ accuracy and significance of GPS derived point velocities of permanent stations. We compare and discuss the different levels of formal errors obtained by the three analysis centers considered in this study (EPN, JPL and SOPAC) and present additional ways of assessing the accuracy using the redundancy offered by different independent analyses and multiple systems operating at one site. On the average, all results indicate that a one-sigma level of ±0.3 mm/yr can be seen as a conservative estimate for the horizontal accuracy of point motions in central Europe. On the basis of this assumption we find that at present, the actual velocity field as determined by different analysis groups and centers does not show any significant east–west extensional deformation. We do however see a prominent north–south compressional velocity gradient of about 1 mm/yr/1000 km (1 nanostrain/yr) which could be associated with the Alpine thrust in conjunction with a south-directed horizontal component of the Fennoscandian Glacial Isostatic Adjustment.  相似文献   

13.
This study seeks to establish a new system characteristic describing dayside convective flows in the coupled magnetosphere–ionosphere: the low-pass filter function through which interplanetary magnetic field (IMF) fluctuations are processed as they are communicated from the magnetopause to the high-latitude ionosphere near local noon. In doing so, this study confirms that variations in the ionospheric flows at high-latitudes near local noon are well correlated with variations in the IMF orientation and magnitude on short timescales. We construct the filter function by comparing time series of the ionospheric equivalent flows at a fixed location at magnetic local noon and 80° latitude with time series of the IMF. The coherence spectra of these two parameters—averaged over 330 h of comparison—indicate that there is a low-pass cutoff in the ionospheric response to IMF driving at a periods shorter than 20 min (frequencies higher than 0.8 mHz). When there is sufficient power in the IMF fluctuations, this cutoff is relatively sharp—the coherence drops by roughly a factor of three between the periods 32 and 21 min (0.5 and 0.8 mHz). The results also show that on average the coherence between the east–west component of the equivalent flows and IMF By tends to be less than the coherence between the north–south component of the equivalent flows and IMF Bz.  相似文献   

14.
The present study focuses on the chronological relationship between alternating dune sand and silty water-lain sediments in the central part of the Khongoryn Els dune field in the Gobi Desert, Southern Mongolia. The 23 m high section evolved from the construction of a natural dam by west–east moving sand dunes and fluvial inundation by a river system from the mountain ranges in the south. To resolve the chronology of events, optically stimulated luminescence (OSL) dating was applied and from sedimentological and geochemical analysis the depositional processes could be characterised. Quartz OSL dating of these sediments is hindered by feldspar contamination. Thus, dating of coarse-grain K-feldspars is applied to provide a more reliable chronostratigraphy. As this fraction might be influenced by signal loss over geological time scales, the extent of fading is measured and corrected for. The resulting ages fit well in a supraregional reconstruction of Central Asian palaeoclimate. The data imply that the basal aeolian sediments were deposited 27 ka ago, while the major part of the profile was accumulated in a rather short period of time around ～15 ka. The temporal differentiation implies that the stratification of aeolian and fluvial sediments is not caused by long term climatic variations. It rather represents arid conditions with episodical fluvial activity. Samples between 20 and 15 m depth could not be taken, but it is assumed that this part of the section represents an extremely arid time period with an intensive remobilisation of sand around LGM. A dune overlying the section was deposited during the Late Holocene and represents the ongoing aridity in this region.  相似文献   

15.
Mercury (Hg) can reach the environment through natural and human-related sources, threatening ecosystems all over the planet due to its well known deleterious effects. Therefore, Antarctic trophic webs, despite being relatively isolated, are not exempt of its influence. To evaluate Hg concentrations in an Antarctic ecosystem, different tissues from 2 species of invertebrates, 2 of fish, 8 of birds, 4 of pinnipeds and at least 5 of vegetation were investigated (n = 176). For animals, values ranged from 0.018 to 48.7 μg g? 1 dw (whole Antarctic krill and Antarctic Fur Seal liver). They were generally correlated to trophic position (assessed by δ15N and δ13C) but also to cephalopods and myctophids consumption. For vegetation, values ranged from 0.014 to 0.227 μg g? 1 dw (Colobanthus quitensis and an unidentified lichen), with lichens presenting significantly higher values than mosses, likely due to year-round exposure and absorption of animal derived organic matter, as hypothesized by literature.  相似文献   

16.
《Marine pollution bulletin》2012,64(5-12):255-261
Mussels were maintained for 4 weeks under different combinations of dissolved oxygen concentration (1.5, 3.0 and 6.0 mg O2 l−1) and salinity (15, 20, 25 and 30) in a 3 × 4 factorial design experiment. Clearance rate (CR), absorption efficiency (AE), respiration rate (RR) and scope for growth (SFG) decreased with decreasing salinity and dissolved oxygen concentration (DO), while excretion rate (ER) increased with decreasing salinity and increasing DO. The O:N ratio was <10 at salinities of 15 and 20, irrespective of DO levels. SFG was negative in most of the treatments, except for those under 6.0 mg O2 l−1 or at a salinity of 30 when DO was lower. The results may help explain the distribution pattern of Perna viridis in Hong Kong waters and provide guidelines for mussel culture site selection.  相似文献   

17.
Continuous magnetotelluric (MT) measurements were conducted from May 2008 to July 2009 at Sakurajima, one of the most active volcanoes in Japan. Two observation sites were established at locations 3.3 km east and 3 km west–northwest of the summit crater. At both observation sites, the high-quality component of the impedance tensor (Zyx) showed variations in apparent resistivity of approximately ± 20% and phase change of ± 2°, which continued for 20–180 days in the frequency range between 320 and 4 Hz. The start of the period of changes in apparent resistivity approximately coincided with the start of uplift in the direction of the summit crater, as observed by a tiltmeter, which is one of the most reliable pieces of equipment with which to detect magma intrusion beneath a volcano. A 2D inversion of MT impedance suggests that the resistivity change occurred at a depth around sea level. One of the possible implications of the present finding is that the degassed volatiles migrated not only vertically through the conduit but also laterally through a fracture network, mixing with shallow groundwater beneath sea level and thereby causing the observed resistivity change.  相似文献   

18.
Spatial correlations between total column ozone observed by TOMS and equatorial zonal winds from 1979 to 2003 have been assessed. Four months and three different altitude levels have been analyzed: January and July (solstice months), April and October (equinoctial months), and 10, 30 and 50 hPa. The results are different for the months and altitudes considered. The highest correlations values appear in tropical zone at 30 hPa. The Brewer–Dobson circulation plays a key role in regulating the abundance of ozone, influenced by the quasi-biennial oscillation (QBO) circulation. Since the Brewer–Dobson is a slow circulation, correlations considering lags between one and 12 months were estimated. In this case, the highest correlations values are moving to subtropical latitudes at winter hemisphere, with different behaviors for three altitude levels considered.  相似文献   

19.
Association between semitransparent cirrus (STC) and tropospheric dynamics during the two contrasting seasons (Summer-monsoon and Winter) is studied using lidar and MST radar observations at Gadanki (13.5°N, 79.2°E) along with data from Geo-stationary satellite (KALPANA-1). The tropical tropopause layer (TTL) is found to be very conducive for the formation of STC. For those thicker STCs forming within TTL, the cloud top remains mostly steady with the top of the TTL, while the cloud base varies significantly in accordance with the altitude extent of tropospheric convective outflow. Based on depolarization characteristics of STCs (from lidar) along with regional distribution of tropospheric clouds derived from satellite data, altitude profile of horizontal wind, wind field at 150 hPa and air mass back trajectories, it can be reasonably inferred that, while the multi-layered and highly structured STCs during the monsoon period originate mostly from the deep convections, the sub-visual/ultra thin STCs during winter could mostly be of in situ origin.  相似文献   

20.
We have considered spatial distributions of positive lightning discharges in the east of Siberia for the summer seasons of 2003–2007 and properties of their electromagnetic signals with the ELF “slow tail”, which, as is known, can be accompanied by sprites. There are two main regions of positive discharges located in the south and west of Yakutsk. Two other “centers” (the northeastern and the eastern) are located in high-mountainous regions. In these regions the positive discharges intensity can exceed the negative discharge intensity.The electromagnetic signals in the ELF range (usually in the form of two half-cycles) were observed after the VLF atmospherics were recorded in the high-latitude regions. The delay of ELF pulses relative to the corresponding atmospherics was 0–7 ms. The long (up to 350 ms) events of quasi-periodic ELF oscillations with the period of about 7 ms (which corresponds to the quasi-period of ELF pulses) were revealed.  相似文献   

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