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1.
We demonstrate that narrowband measurements can be used for rudimentary ranging of cloud-to-ground lightning flashes. The system at present responds to both intra-cloud and cloud-to-ground lightning; ranging is demonstrated for a subset of flashes known to be cloud-to-ground lightning. The system uses a ferrite-core antenna with a length of about 4 cm and diameter 4 mm, and operates on a narrow band at about 1 MHz, close to the HF band (3–30 MHz). It downmixes the signal to audio frequencies and operates in a manner which is very similar to an AM radio. The system triggers on all impulses which exceed a given adjustable threshold above the ambient noise level, and records 1 s of data. Such a system was used to collect lightning-caused electromagnetic disturbances during summer 2006 in Finland. The output is compared to two scientifically verified references: a flat-plate broadband antenna measuring the vertical electric field and a commercial lightning location network giving flash location. A key aim of the system is to reduce the information to as few parameters as possible. Peak intensity and full-flash energy were used as simple parameters. It is shown that accurate flash-by-flash ranging is not possible with this method; however, it is shown that the method can be used to track clusters of ground flashes within a range of about 50–100 km with an accuracy of about 10 km.  相似文献   

2.
During the summer of 2005, transient luminous events were optically imaged from the French Pyrénées as part of the EuroSprite campaign. Simultaneously, extremely low frequency (ELF: 3–3000 Hz) and broadband very low frequency (VLF: 3–30 kHz) data were recorded continuously at two separate receivers in Israel, located about 3300 km from the area of the parent lightning discharges responsible for the generation of sprites. Additionally, narrowband VLF data were collected in Crete, at about 2300 km away from the region of sprites.The motivation for the present study was to identify the signature of the sprite-producing lightning discharges in the ELF and VLF electromagnetic frequency bands, to qualify and compare their parameters, and to study the influence of the thunderstorm-activated region on its overlaying ionosphere. For the 15 sprites analyzed, their causative positive cloud-to-ground (+CG) discharges had peak current intensities between +8 and +130 kA whereas their charge moment changes (CMC) ranged from 500 to 3500 C km. Furthermore, the peak current reported by the Météorage lightning network are well correlated with the amplitudes of the VLF bursts, while showing poor correlation with the CMCs which were estimated using ELF methods.Additionally, more than one +CG was associated with six of the sprites, implying that lightning discharges that produce sprites can sometimes have multiple ground connections separated in time and space. Finally, for a significant number of events (33%) an ELF transient was not associated with sprite occurrence, suggesting that long continuing current of tens of ms may not always be a necessary condition for sprite production, a finding which influences the estimation of the global sprite rate based on Schumann resonance (SR) measurements.  相似文献   

3.
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   

4.
Two sprite-producing mesoscale convective systems above the South-Western part of France are studied. Three sprite events during the first night and seven during the second night were captured. Except for two events, the sprites could be associated with causative positive cloud-to-ground (+CG) discharges in the stratiform region of the storm. The analysis of lightning activity reveals that in both nights sprites occurred when lightning activity decreased rapidly and the ratio of +CGs to the total number of CGs decreased slightly. The average peak current of sprite-producing lightning was lower than 60 kA, in agreement with other observations. The delay times of sprites to their SPCGs varied from 57 to 140 m s and no correlation between events’ delay and shape was established.  相似文献   

5.
All-sky camera (ASC), Global Positioning System (GPS), and ionosonde measurements were used to investigate the upper atmospheric variations at mid-latitude during the strong geomagnetic storm on October 29–31, 2003. An arc-shaped 630.0 nm emission was observed in the northern sky on all-sky images taken at Mt. Bohyun (36.2°N, 128.9°E, GMLAT=29°N) in Korea during 17:48–8:58 UT in the main phase of the geomagnetic storm on October 29. The NmF2 and hmF2 from the ionosonde show strong disturbances at that time. The vertical profiles of electron densities, calculated by the ionospheric tomographic method using ground-based GPS slant total electron contents measurements, show the largest value at ∼440 km height at 18:30 UT on October 29 when the enhancements of OI 630.0 nm emission were observed. The arc-shaped red emission observed during the main phase of the magnetic storm is likely a low-latitude red aurora due to its short duration of ∼1 h. The result implies that the plasmapause was at L=1.4–1.6 during the geomagnetic storm. The fact that the arc did not follow a constant L-value appears to suggest that neutral precipitation and a traveling ionospheric disturbance could also be the cause of the arc.  相似文献   

6.
During the northern hemisphere winter of 2005–2006, transient luminous events (TLEs) known as ‘sprites’ and ‘elves’ were imaged over thunderstorm cells in the eastern Mediterranean. Simultaneously, extremely low frequency (ELF) data (ELF: 3–3000 Hz) were recorded at two observation stations in Israel and Hungary in order to qualify and quantify parameters of the parent lightning discharge associated with the transient optical emissions in the upper atmosphere. In this study, we found that for 87% (Israel) and 77% (Hungary) of optically observed TLEs an intense ELF transient event was recorded. These stations are located some 500 and 2100 km, respectively, from the region of the TLEs. All ELF transients that were associated with TLEs were caused by lightning discharges with positive polarity. Calculation of the charge moment change showed values between 600 and 2800 C km with a peak around 1000 C km. Additionally, the time delay between the +CG and ensuing sprite was 76±34 ms and it was displaced up to 50 km from its parent CG.One of our objectives in the present study was to characterize, based on the ELF radiation from lightning, the electromagnetic (EM) waveforms of the lightning discharges which generate TLEs in the time and frequency domains, and to compare them with other lightning discharges occurring in the same thunderstorm cell at approximately the same time, but which did not produce TLEs. The survey for a typical EM waveform showed no unique ELF signature for lightning discharges associated with either sprites or elves.  相似文献   

7.
《Continental Shelf Research》2006,26(17-18):2260-2280
On October 3, 2002 Hurricane Lili made landfall on a previously studied region of the inner Louisiana shelf as a Category 2 storm with winds over 160 km/h. A week after the hurricane, major impacts of the storm were not evident in the water column except for the lower than expected inshore salinities (∼12 psu) for this time of year, which was characterized by low river discharge. Turbidity profiles were typical of those measured during previous investigations with suspended sediment concentrations >75 mg/L at inshore stations and <50 mg/L in surface waters and offshore. The implication is that the sediments resuspended during the hurricane settled soon after the storm passage. Water column particulate organic carbon (POC) concentrations ranged from 0.1 to over 2.0 mg/L, with the highest concentrations measured near the seabed and in the inshore portions of the study area. Suspended particles were characterized by low organic matter content (%POC of 0.5–2 wt%), low chlorophyll:POC ratios (Chl:POC<4 mg/g) and moderately elevated POC:particulate nitrogen ratios (POC:PN of 10–14 mol/mol), all suggesting their source was locally resuspended seabed sediment rather than from algal biomass or land-derived vascular plant detritus.Post hurricane sediment deposition throughout the study area resulted in a storm layer that ranged from <0.5 to 20 cm in thickness. In most locations sediment accumulation ranged from 3 to 10 cm. The storm deposits were generally composed of silty clays with a coarser, somewhat sandy 1–2 cm basal layer. Surface sediments from the storm layer were characterized by relatively high mineral surface areas (SA of 30–50 m2/g) and elevated OC contents (%OC of 1.0–2.0%). The dispersal of fine sediments following the hurricane resulted in marked changes in the SA and %OC values of surface sediments from offshore locations, which prior to the storm contained coarser, organic-poor particles (SA of 5–15 m2/g and %OC of 0.2–0.6%). The OC:SA and OC:N ratios of storm layer sediments ranged from 0.4 to 0.6 mg OC/m2 and from 10 to 12 mol/mol, respectively, and were comparable to those measured in surface sediments prior to the hurricane. Such similarities in the composition of the organic matter reinforce the idea that the source of the storm deposits was the finer fraction of resuspended seabed sediments, with little evidence for inputs from local land-derived sources or autochthonous algal production. Overall, the magnitude of sediment and organic matter deposition on the seabed after the storm greatly exceeded the annual inputs from the Atchafalaya River and coastal primary production. The combined effects of hurricane-driven erosion and post-storm deposition represent a major perturbation to the benthic community of the region, which is already subject to these types of disturbances due to the combined effects of peaks in river discharge and the passage of storm fronts.  相似文献   

8.
This paper describes a field test to verify a newly discovered phenomenon of microwave emission due to rock fracture in a volcano. The field test was carried out on Miyake Island, 150 km south of Tokyo. The main objective of the test was to investigate the applicability of the phenomenon to the study of geophysics, volcanology, and seismology by extending observations of this phenomenological occurrence from the laboratory to the natural field.We installed measuring systems for 300 MHz, 2 GHz, and 18 GHz-bands on the mountain top and mountain foot in order to discriminate local events from regional and global events. The systems include deliberate data subsystems that store slowly sampled data in the long term, and fast sampled data when triggered. We successfully obtained data from January to February 2008. During this period, characteristic microwave pulses were intermittently detected at 300 MHz. Two photographs taken before and after this period revealed that a considerably large-scale collapse occurred on the crater cliff. Moreover, seismograms obtained by nearby observatories strongly suggest that the crater subsidence occurred simultaneously with microwave signals on the same day during the observation period.For confirmation of the microwave emission caused by rock fracture, these microwave signals must be clearly discriminated from noise, interferences, and other disturbances. We carefully discriminated the microwave data taken at the mountaintop and foot, checked the lightning strike data around the island, and consequently concluded that these microwave signals could not be attributed to lightning. Artificial interferences were discriminated by the nature of their waveforms. Thus, we inferred that the signals detected at 300 MHz were due to rock fractures during cliff collapses. This result may provide a useful new tool for geoscientists and for the mitigation of natural hazards.  相似文献   

9.
Mid-latitude Digisonde Doppler velocities, auroral electrojet (AE) indices and cloud-to-ground (CG) lightning strokes during August 2003–2004 were used to study the perturbations in the F-region vertical drift associated with terrestrial thunderstorms. A superposed epoch analysis (SEA) showed that the F-region vertical drifts Vz had a net descent of ～0.6 m s?1 peaking ～3 h after lightning. Stronger downward perturbations of up to ～0.9 m s?1 were observed in the afternoon on the day prior to lightning days. The perturbations were less significant on the day after and insignificant during the remaining intervals up to 144 h on either side of the lightning. The stronger responses on the day before are consistent with causality because the lightning times were merely proxies for the physical mechanisms involved. The actual causes are unclear, but we discuss the possible roles of lightning-induced ionisation enhancements, intense electric fields penetrating upward from electrified clouds, and atmospheric gravity waves (AGWs) radiated from thunderstorms or from the accompanying tropospheric fronts. There is no doubt that the behaviour of the mid-latitude F-region is controlled by the thermospheric winds and the solar wind-magnetosphere electrical generators, but our results suggest that electrified clouds also account for a significant, albeit relatively small component of the ionospheric variability.  相似文献   

10.
We present observations of radar volume reflectivities under conditions of polar mesosphere summer echoes (PMSE) at three frequencies, i.e., 53.5, 224, and 930 MHz corresponding to Bragg wavelengths of 2.8, 0.67, and 0.16 m. These measurements were made with the ALWIN radar in Andenes and the EISCAT VHF and UHF radars in Tromsø. Contributions to the signal at 930 MHz by incoherent scatter are used to estimate electron number densities and their gradient at PMSE altitudes, and spectral width measurements of Doppler spectra recorded at 224 MHz are used to estimate the turbulent energy dissipation rate. We further derive a theoretical expression for the radar volume reflectivity for the case of turbulent scatter aided by a large Schmidt number (i.e., the current standard theory of PMSE) and show that our observations quantitatively agree with this theory if Schmidt numbers between 2500 and 5000 are assumed. We then show that these Schmidt numbers correspond to ice particles with radii in the range 20–30 nm which should frequently occur in the polar summer mesopause region. In addition, we show that for the short period when PMSE was observed at UHF frequencies the volume reflectivity is proportional to a factor determined by the turbulent energy dissipation rate, electron number density, and the electron number density gradient in agreement with theory. We consider our findings as strong support that PMSE at all considered frequencies is indeed created by turbulent scatter in the presence of a large Schmidt number. We finally highlight that ultimate proof of this concept will require the direct measurement of ice particle sizes in a PMSE environment probed by radars covering frequencies between 50 MHz and 1 GHz.  相似文献   

11.
A 100-year climatology of tropical storms and hurricanes within a 200-km buffer was developed to study their impacts on coral reefs of the Flower Garden Banks (FGB) and neighboring banks of the northwestern Gulf of Mexico. The FGB are most commonly affected by tropical storms from May through November, peaking in August–September. Storms approach from all directions; however, the majority of them approach from the southeast and southwest, which suggests a correlation with storm origin in the Atlantic and Gulf of Mexico. A storm activity cycle lasting 30–40 years was identified similar to that known in the Atlantic basin, and is similar to the recovery time for impacted reefs. On average there is 52% chance of a storm approaching within 200 km of the FGB every year, but only 17% chance of a direct hit every year. Storm-generated waves 5–25 m in height and periods of 11–15 s induce particle speeds of 1–4 m s?1 near these reefs. The wave–current flow is capable of transporting large (～3 cm) sediment particles, uplifting the near-bottom nepheloid layer to the banks tops, but not enough to break coral skeletons. The resulting storm-driven turbulence induces cooling by heat extraction, mixing, and upwelling, which reduces coral bleaching potential and deepens the mixed layer by about 20 m. Tropical storms also aid larvae dispersal from and onto the FGB. Low storm activity in 1994–2004 contributed to an 18% coral cover increase, but Hurricane Rita in 2005 reduced it by 11% and brought coral cover to nearly pre-1994 levels. These results suggest that the FGB reefs and neighboring reef banks act as coral refugia because of their offshore location and deep position in the water column, which shields them from deleterious effects of all but the strongest hurricanes.  相似文献   

12.
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   

13.
Relationships between the polar cap magnetic activity index PC and the magnetic storm Dst index have been studied for the magnetic storms with duration more 12 h and peak value Dst<?30 nT and, observed in 1998–2002 and 2004–2005. Along with PC index the geoeffective interplanetary electric field Em was also examined. It has been found that all examined storms, lying in range from ?30 to ?373 nT, started when the PC index and, correspondingly, the Em field firmly exceeded the threshold >2 mV/m. In particular, the “anomalous” magnetic storm on January 21–22, 2005 occurring under conditions of northward IMF BZ (Du et al., 2008) is usual phenomena fitted well with the threshold restriction owing to the large IMF By component input. The maximal storm depression (the peak value of Dst) is linearly related to the quantities Em and PC, averaged for the time interval from the storm beginning to the storm maximum. The correlation between Dst and PC is more steady and larger than correlation between Dst and Em, the latter being dependent on Em value (effect of “Dst saturation”). The moment of the firm descent of the Em and PC quantities below the threshold level ～2 mV/m is indicative of the depression damping and transition to the recovery phase. The results are consistent with the similar peculiarities revealed for substorms development (Troshichev and Janzhura, 2009) and support the conclusion that the PC index is a reliable proxy characterizing the solar wind energy having been entered into the magnetosphere.  相似文献   

14.
We describe attempts to create ball lightning by directing lightning, triggered from natural thunderclouds using the rocket-and-wire technique, through a variety of materials. Some of the observed phenomena have features in common with natural ball lightning or with laboratory attempts to create it: flame-like luminosity for up to 0.5 s above salt water; constant-luminosity silicon fragments falling for about 1 s under the influence of gravity; a 0.7 m region of stationary luminosity whose bottom was 0.3 m above a stainless steel surface to which arcing had occurred; and a glow for about 0.5 s above pine tree sections.  相似文献   

15.
Experimental measurements of fracture-induced seismic waves velocity variations at frequencies ~ 1 kHz, ~ 40 kHz and ~ 1 MHz were performed directly in the field at the rocky outcrop and in the laboratory on specific rock samples collected from the outcrops. The peridotite–lherzolite outcrop appeared macroscopically uniform and contained three systems of visible parallel sub-vertical fractures. This rock has substantial bulk density and higher than average value of seismic wave velocity. The presence of fracture systems gives rise to its velocity anisotropy. The seismic waves passing through the rock fractures are subject to velocity dispersion and frequency dependent attenuation. Our data, obtained from field and laboratory measurements, were compared with theoretical model predictions. In this model we successfully used displacement discontinuity approach. For the velocity dispersion evaluation we used multi-frequency measurements. The a priori observation of orientations and densities of fracture sets allowed evaluation of their stiffness. Our approach revealed that the first arrivals of seismic waves can be used for evaluation of P-wave group velocities, the specific case, in which we expect anomalous velocity dispersion. Our observations contribute to the issue of up-scaling of well-log derived velocities in fractured rock to the scale of standard seismic exploration frequencies.  相似文献   

16.
Shallow carbonates are of utmost importance as potential sources of groundwater in karstified semi-arid terrains. Ground-Penetrating Radar (GPR) is being increasingly used as a prominent mapping tool in such environments. However, its potential in exploring and identifying shallow water-saturated zones (WSZs) in carbonates is constrained by the geoelectrical properties of carbonate soils as a function of moisture content. We report results of a case study that includes laboratory geoelectrical characterization and their comparison to in situ GPR attenuation measurements performed on Cretaceous Edwards Formation rudist mounds in central Texas, which we hypothesize as analogs for water-bearing formations in semi-arid karstified carbonate terrains. Dielectric measurements on field-collected rock samples carried out in the laboratory under controlled conditions of moisture saturation suggest that real and imaginary parts of dielectric constants of rocks with higher porosity and/or permeability have steeper dependence on pore moisture content; they produce better dielectric contrasts but allow shallower penetration. Our analyses suggest that within carbonates, dielectric contrasts improve with decrease in sounding frequency and/or increase in moisture content; and the relationship between dielectric permittivity and moisture content may be represented by 3rd order polynomial equations. GPR surveys using a wide-band 400 MHz antenna reveal subsurface mound morphologies with heights of ～ 1–2 m and basal diameters of ～ 8–10 m resembling outcrop analogs. Each mound appears to be composed of smaller amalgamated lithounits that seem geoelectrically similar. Amplitudes decays of the backscattered radar signal correlate to moisture distribution. Measuring the differences in signal attenuation allows differentiation between saturated and non-saturated zones. Velocity analyses of GPR profiles enable estimation of moisture distribution in the vicinity of the mounds. Optimal delineation and production of high-resolution GPR data up to a depth of ～ 10 m were observed for a sounding frequency of ～ 250 MHz with moisture content of ～ 5% by weight. Below this moisture level, the dielectric contrast is insufficient to uniquely identify water-saturated zones from the surrounding geoelectrical context, and above it, the radar signal is substantially attenuated leading to a total inefficiency of the method.  相似文献   

17.
Lightning flashes are usually preceded by preliminary breakdown processes (PBPs) before a stepped leader is initiated. These breakdown processes are not well understood. An early model, the so-called BIL model, has been called into question in later studies. However, we have found that the BIL model is quite successful in describing initial processes at least in high-latitude Scandinavian lightning. We present results from one summer of measurements in Finland, during which the vertical electric field was measured with a standard broadband plate antenna system. Lightning flash locations were provided by a lightning detection network and magnetic fields were measured with an experimental narrowband detection system. The relationship between the preliminary breakdown and the first return stroke (RS) is studied for 193 flashes at distances of 5–70 km. We can identify a preliminary breakdown in at least 90% of the flashes. The peak electric field of the RS is on average four times as intensive as the highest peak of the PBP. However, in 25% of the cases the PBP peak is more intensive. On the other hand, we show that this method of comparing intensities is physically arbitrary, since the PBP is continuous and the RS is impulsive. The narrowband measurement allows a physically consistent definition for intensities as the root-mean-square (RMS) sum of the most intense parts of signals. The PBP and RS are shown to have almost equal intensities at small distances. At larger distances, the PBP weakens more rapidly. This is suggested to be due to different propagation regimes, with the PBP signal changing from space-wave to ground-wave propagation with increasing distance, while the RS is predominantly ground wave at all distances. The result may have practical applications in narrowband detection of lightning. The BIL model suggests a characteristic signal in the narrowband signal, which could be used to identify the start of a lightning flash. The change in the RS–PBP ratio as a function of distance is statistically significant, but is too weak to significantly improve ranging methods.  相似文献   

18.
《Marine pollution bulletin》2012,64(5-12):500-507
Using the dogwhelk Nucella lapillus as a biomonitor, we evaluated the organotin contamination along the Icelandic coast over nearly two decades. In 2008, adult dogwhelks collected from 30 locations were examined for imposex status. In 2009, tissue concentrations of six organotin species in dogwhelks from 16 of the 30 locations were analysed using gas chromatography–mass spectrometry. Tributyltin was the most predominant residue ranging from 2.07 to 70.38 μg kg−1 dw, while triphenyltin was also detected at concentrations up to 22.79 μg kg−1 dw. Higher total organotin concentrations and imposex indices were observed at locations near large harbours (Reykjavík and Hafnarfjörður). By comparing the current imposex results with those of previous surveys (1992–2003), we found that recovery was slow near the large harbours, but more apparent near the smaller harbours. We also observed a notable increase in imposex at several northwest sites implying incessant input of organotins.  相似文献   

19.
The Pacific margin of Canada has been subjected to tectonism, dramatic sea level change and vigorous storm and tidal energy since glacial times resulting in a complex seafloor. Extensive multibeam mapping of this shelf has provided an opportunity to understand how these processes have impacted sedimentology and morphology. Bathymetric restriction of the tidally dominated flow between the inland seas and the open Pacific has resulted in the development of very large subaqueous dune fields and terrace moats. For example, in the southern Strait of Georgia nearly symmetrical dunes with wavelengths between 100 and 300 m, dune heights up to 28 m, cover the seafloor in 170–210 m water depth. In northern Hecate Strait a 72 km2 area of large 2D dunes occurs at the transition with Dixon Entrance which opens to the Pacific Ocean and steep (>10°) wave-cut terraces and drowned spits, a result of sea level changes during the Holocene, are now being undercut to generate moats 7 m deep, in a narrowing shelf trough. Currents, with velocities ranging between 0.2 and 2.2 m s?1, are dominated by semi-diurnal tidal streams that are continually modified by wind and estuarine circulation. There appears to be a clear association of grain size, water depth and flow velocity controlling the size of the subaqueous dunes.  相似文献   

20.
Measurements of the radial gradient of the phase space density (PSD) at constant first and second invariants provide a test of whether storm time electron acceleration processes are dominated by inward radial transport of electrons or whether other processes must be considered. We used the detailed energetic electron angular and spectral data from the SCATHA satellite to determine the evolution of electron PSD radial profiles through a moderate magnetic storm (2 May 1986; Dst=−95 nT). We compared the changes that occur in the profiles in the storm recovery period to the pre-storm profiles for first invariant values M=200–2500 MeV/G and for K values of 0.055–0.65 Re√G over the L* range of ∼5.2–7.3 Re. The PSD radial profiles showed a range of features from peaked in L* at small K and M during the pre-storm period to those in the late storm recovery phase, that were flat, had negative slopes for small K and had peaks in the L*=5.2–6.5 range for intermediate to large K. There were significant differences in the radial profiles for small and intermediate K values at constant M. For example, during the recovery period the PSD profiles were flat or decreasing for K=0.06 and M>1200 MeV/G, while they were peaked near L*=5.75 for K>0.2. These results imply that radial diffusion is a reasonable explanation of the near-equatorial post-storm PSD enhancements for L*>5.2 for this storm but that either significant electron pitch angle transport, losses, and/or acceleration of off-equatorial mirroring electrons by waves play an important role in the evolution off-equator PSD profiles during the storm recovery.  相似文献   

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