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1.
Geomagnetic disturbance effects on the occurrence of range and frequency spread at Huancayo are studied for the period 1957–74. The occurrence of frequency spread is decreased on disturbed days forD-months andE-months while duringJ-months an increase is noted in the post-midnight period. The occurrence of range spread is decreased on disturbed days in the pre-midnight hours duringD andE-months. Post-midnight hours ofD andE-months and all hours of night duringJ-months show an increase of range spread on disturbed days, which is most prominent during low sunspot years. The mean occurrence of frequency spread (2300–0100 LT) and range spread (2000–2300 LT) decrease with increasing ΣKp. On the other hand, occurrence of range spread (0300–0500 LT) increases with increasing ΣKp, and is suggested as the consequence of the changes of the horizontal electric field in theF-region associated with the geomagnetic storms.  相似文献   

2.
Ionospheric scintillation observations of VHF radio signals from FLEETSAT satellite (73°E longitude) at Bhopal from January 1990 to December 1990 have been used to study the characteristic variations of scintillation activity. It is found that scintillation occurrence is essentially a night-time phenomenon and day-time scintillations are very rare. Annual average nocturnal variation of percentage occurrence of scintillations shows maximum at around 2100–2200 hours LT. Seasonally, scintillations are most prominent during equinoxes and least during summer. Geomagnetic disturbances tend to decrease the occurrence of scintillations in the pre-midnight period.  相似文献   

3.
Features of the equatorial electrojet are studied at Sao Luiz (2.6°S, 44.2°W, inclination −0.25°) in eastern Brazil and Sikasso (11.3°N, 5.7°W, inclination 0.1°) in the western African sector. The stations are situated on either side of the lowest magnetic field intensity in the region of rapid changes in the declination. The daily variations of ΔX at the two stations are almost similar with the peak around noon with maximum values during equinoxes and minimum values during J-solstices. Daily variations of ΔY differ with the maximum deviation of about −35 nT around noon at Sao Luiz and much smaller value of about −10 nT around 14 h LT for Sikasso. The direction of the H vector varies from 15°W of north at 08 h to more than 30°W of north at 17 h for Sao Luiz and from 14°E of north to 25°W of north at 18 h for Sikasso. The plot of the deviations in ΔX and ΔY at different hours for the two stations shows the points along narrow ellipses with major axis aligned along 22°W of north for Sao Luiz and along 3°W of north for Sikasso as compared to declination of 20°W for Sao Luiz and 6°W for Sikasso. The deviations in ΔX at the two stations are fairly well correlated.  相似文献   

4.
The Tianshan Carboniferous–Permian rift-related volcanism in northwestern China represents a newly recognized large igneous province extending over at least 1.5 × 106 km2. The volcanic successions comprise thick piles of basaltic lavas and subordinate intermediate and silicic lavas and pyroclastics, and are interpreted to result from a mantle plume head with component of εNd(t) ≈ +5, 87Sr/86Sr(t) ≈ 0.704 and La/Nb ≈ 0.9. On the basis of petrogeochemical data, the Carboniferous basic lavas can be generally incorporated into low-Ti/Y (LT, Ti/Y < 500) magma type that can be further divided into three subtypes: LT1, LT2 and LT3. The chemical evolution of the LT1, LT2 (in central Tianshan) and LT3 (in western Tianshan and Jungar) lavas is controlled by an olivine (ol) + clinopyroxene (cpx) fractionation, but gabbroic fractionation accounts for the chemical variation of the LT3 lavas from eastern Tianshan. Elemental and isotopic data suggest that the chemical variation of Tianshan Carboniferous basic lavas cannot be explained by crystallization from a common parental magma.The Sr–Nd isotopic variation of the crustally contaminated LT3 lavas is related to the nature of lithosphere through which the plume-derived melts have erupted. The involvement of an older (Precambrian) lithosphere led the LT3 lavas in western Tianshan to have lower to negative εNd(t) (−1.2 to +6.1) and variable 87Sr/86Sr(t) (0.7036–0.7061), whereas the LT3 lavas from eastern Tianshan and Jungar are characterized by high εNd(t) (+4.2 to +9.7) and low 87Sr/86Sr(t) (0.7035–0.7044), that are related to the contamination of upper crust containing early Paleozoic and Devonian arc-basin volcanic rocks and/or to a pre-Carboniferous subduction enrichment of the lithospheric mantle source region. The observed geochemical variations in the Tianshan data are consistent with an AFC process.The Tianshan Carboniferous rift-related volcanic rocks display a spatial petrogeochemical variation in which predominantly uncontaminated LT1 and less-contaminated LT2 tholeiitic lavas erupted in central Tianshan rift and predominantly the strongly contaminated LT3 tholeiites erupted in the circumjacent regions of the central Tianshan rift. The LT1 and LT2 lavas were generated by a higher degree (10–30%) of partial melting in the garnet stability field of the mantle plume compared to the LT3 lavas. The lower degree (<10%) of partial melting in the spinel–garnet transition zone of the mantle plume, as is characteristic of the LT3 lavas, may be the result of a relatively lower geotherm.  相似文献   

5.
Radio beacon from ATS-6 at 140 MHz was used to measure the changes in the polarization angle (Faraday rotation) at Bombay, Rajkot, Ahmedabad, Udaipur and Patiala during October 1975 to July 1976. In this paper, results of diurnal, seasonal and latitudinal variations in total electron content (TEC) derived from these measurements are reported. The amplitude of diurnal peak is found to be higher at Rajkot, Ahmedabad and Udaipur as compared to that at Patiala or Bombay, indicating that the peak of Appleton anomaly in the latitudinal variation of TEC was close to the latitude of Ahmedabad. The diurnal maximum of TEC occurs around the same time during summer and winter months. The peak electron content shows a semiannual variation at all the stations with large values in equinoxes as compared to winter and summer. The TEC at Bombay shows a seasonal anamoly with high values in winter as compared to summer. The paper describes the development of latitudinal anomaly with the time of the day for different seasons. This anomaly is maximum during 1000 to 1800 LT and is located between 12° and 14° N (dip latitude) in summer and equinoxes and at about 10°N in winter.  相似文献   

6.
Short-period events such as bays and storm sudden commencements (SSCs) have been analysed to investigate the nature of induced magnetic variations at two Indian magnetic observatories: Shillong and Gulmarg. It seems that near Gulmarg there is obvious connection between the induced magnetic variations and the two large scale features; the main central thrust (MCT) and the main boundary fault (MBF) in the north-west direction. The Dauki fault, an approximately east-west conductor, seems to be responsible for the conductivity anomalies at SHL.  相似文献   

7.
The magnetic measurements of declination (D), horizontal (H) and vertical (Z) components of earth’s magnetic field, collected from ground surveys between 1962 and 1966, are used to develop an analytical model of geomagnetic field variations over Indian region for the epoch 1965. In order to reflect spatial features with wavelengths of approximately 1000 km, sixth degree polynomial as a function of differential latitude and longitude is calculated by the method of least squares. The root mean square fit of the model to the input data is better than that accounted by the International Geomagnetic Reference Field for 1965.0. Isomagnetic charts drawn forD, H, Z and total force (F) reflect more details than that shown on world magnetic charts. Further, the values of the field at common repeat stations recorded between 1962 and 1974, after eliminating the field values for the epoch 1965.0, are used to get the secular variation as well as its spatial dependence again by means of polynomial which now includes coefficients which are functions of time and of geographical locations. The accuracy of coefficients is tested against the behaviour of secular variation at permanent magnetic observatories. The merits and limitations of the model are discussed.  相似文献   

8.
Analysis of over 1400 earthquakes in the North Island of New Zealand from 1955 to 1969, comprising all shocks with ml ? 4.3 for shallow, and ML ? 4.0 for deep events, reveals several empirical relationships between the depth and the equivalent radius of the area occupied by shocks, the number and density of the shocks, and the coefficient b and the maximum magnitude. The coefficient b increases linearly with depth from 1.0 for shallow earthquakes to 1.4 for those at a depth of 120 km, and then decreases to 0.75 at 300—350 km. The variation with depth shows clear inverse correlation with the distribution of maximum stress along the downgoing slab, calculated for several slab models by Smith and Toksöz. Similarly, the maximum magnitude at different depths correlates distinctly with the distribution of the principal stress. Time variations of the coefficient b and the rate of earthquake occurrence, for both shallow and deep earthquakes, have an oscillatory character, with a period of 7–8 years. These variations also imply that shallow and deep seismicity are mutually dependent.  相似文献   

9.
Using stationary zooplankton nets that fished the tidal current we measured the daily abundance ofCancer crab megalopae near the mouth of Coos Bay, Oregon, during the 1997 spring settlement season. During the spring of 1997, the coastal waters were dominated by a significant El Niño event. Sea surface temperatures (SST) were higher than normal, upwelling indices were an order of magnitude smaller than during the two previous springs, and upwelling favorable winds were weak. Daily catches ofCancer magister megalopae ranged from 0 to 78 with 61% of the total catch occurring during four pulses. Peak catches tended to occur every 13.6 d close to 13.8 d average period between spring tides. Significant cross correlations were found between the maximum daily tidal range and the catch ofC. magister megalopae; large catches tended to occur 4 to 7 d after the spring tide. Daily catches ofCancer oregonensis andCancer productus ranged from 0 to 307 with catch significantly positively cross correlated to the maximum daily tidal range at a lag of ?5 days suggesting that the largest catches tended to occur after the spring tides. We hypothesize that a tidally-generated phenomenon internal waves, transportedCancer megalopae shoreward and caused the observed variation in their abundance in Coos Bay.  相似文献   

10.
SOHO-MDI daily magnetic field synoptic data (a 14-year series of daily maps of the solar magnetic field intensity B available at the site ) have been used to analyze the dynamics of the photospheric magnetic field in the vicinity of the solar equator. The standard deviation s B of the field B calculated over areas of tens of square degrees on the solar disk was taken as a basic index. An 11-year variation similar to that observed at higher latitudes is observed in the vicinity of the equator, and is similar for weak and strong fields; i.e., the solar cycle exists in the sunspot-free zone. New qualitative data support the idea that the weak background magnetic field increases toward the solar limb. This angular dependence suggests the existence of a transverse component of the background field. The magnetic fields in the vicinity of the equator were significantly different in the initial phases of Cycles 23 and 24. Annual variations of s B were observed near the center of the solar disk. These variations are due to two factors: the annual variation of the distance from the equator to the disk center and the increase of s B with with distance from the equator. Reliable detection of these variations is an evidence of high accuracy of the s B estimates.  相似文献   

11.
Synoptic maps of the intensity of the λ530.5 nm FeXIV green coronal line and maps of computed coronal magnetic fields for the period 1977–2001 are compared. For quantitative comparisons, the correlation coefficients r for the correlation between these two parameters at corresponding points of the synoptic maps are calculated. This coefficient exhibits cyclic variations in the spot-formation zone, ±30° and the zone above 30° and is in antiphase in these two zones. In the low-latitude zone, the correlation coefficient is always positive, reaches its maximum at activity minimum, and strongly decreases by activity maximum. Above 30°, r reaches maximum positive values at activity maximum and then gradually decreases, passing through zero near the beginning of the phase of activity minimum and becoming negative during this phase. A Fourier analysis of r as a function of time reveals a wavelike variation with a period close to 1.3 yr (known also from helioseismological data for the tachoclinic region of magnetic-field generation), as well as a pronounced wave with a period of about 5 yr. The latitude dependence of r seems to be related to variations in the contributions from local, large-scale, and global fields. Our analysis suggests an approach to studying the complex problem of mechanisms for coronal heating.  相似文献   

12.
Observations of the total magnetic field in the active region NOAA 6757 have been used to study the turbulence regime from 2.5 h before the onset of a 2B/X1.5 flare until two minutes after its maximum. The curvature of the exponent ζ(q) for the structure functions of the B z field increases monotonically before the flare (i.e., the multifractal character of the B z field becomes more complex) but straightens at the flare maximum and coincides with a linear Kolmogorov dependence (implying a monofractal structure for the B z field). The observed deviations of ζ(q) from a Kolmogorov line can be used for short-term forecasting of strong flares. Analysis of the power spectra of the B z field and the dissipation of magnetic-energy fluctuations shows that the beginning of the flare is associated with the onset of a new turbulence regime, which is closer to a classical Kolmogorov regime. The scaling parameter (cancellation index) of the current helicity of the magnetic field, k h , remains at a high level right up until the last recording of the field just before the flare but decreases considerably at the flare maximum. The variations detected in the statistical characteristics of the turbulence can be explained by the formation and amplification of small-scale flux tubes with strong fields before the flare. The dissipation of magnetic energy before the flare is primarily due to reconnection at tangential discontinuities of the field, while the dissipation after the flare maximum is due to the anomalous plasma resistance. Thus, the flare represents an avalanche dissipation of tangential discontinuities.  相似文献   

13.
The spatial and temporal variations in aerosols and precursor gases over oceanic regions have special importance in the estimation of radiative forcing parameters and thereby in the refinement of general circulation models. Extensive observations of the columnar aerosol optical depth (AOD), total column ozone (TCO) and precipitable water content (PWC) have been carried out using the on-line, multi-band solar radiometers onboard ORV Sagar Kanya (Cruise # SK 147B) over Bay of Bengal during 11th–28th August 1999. Aerosol optical and physical properties (optical depth and angstrom parameter) have been estimated at six wavelengths covering from UV to NIR (380–1020 nm) while TCO and PWC have been determined using the UV band around 300 nm and NIR band around 940 nm, respectively. Added, concurrent meteorological and satellite observations during this field phase of BOBMEX-99 have been utilized to investigate spectral-temporal variations of AOD, TCO and PWC in marine environment. The results indicate lower AODs (around 0.4 at characteristic wavelength of 500 nm) and size distributions with abundance of coarse-mode particles as compared to those aerosols of typical land origin. An interesting result that is found in the present study is the significant reduction in AOD at all wavelengths from initial to later part of observation period due to cloud-scavenging and rain-washout effects as well as signature of coastal aerosol loading. The clear-sky daytime diurnal variation of TCO shows gradual increase during post-sunrise hours, broad maximum during afternoon hours and gradual decrease during pre-sunset hours, which is considered to be due to photochemical reactions. The diurnal variation curve of PWC showed maximum (~ 4 cm) during morning hours and gradual decrease (~ 3.5 cm) towards evening hours, which are found to be greater as compared to typical values over land. Another interesting feature observed is that although the PWC values are very high, there was no proportionate or appreciable enhancement in AOD—a feature that can be utilized to infer composition of aerosols over the study region.  相似文献   

14.
The spatio-temporal variations of soil gas in the seismic fault zone produced by the 12 May 2008 Wenchuan Ms 8.0 earthquake were investigated based on the field measurements of soil gas concentrations after the main shock. Concentrations of He, H2, CO2, CH4, O2, N2, Rn, and Hg in soil gas were measured in the field at eight short profiles across the seismic rupture zone in June and December 2008 and July 2009. Soil-gas concentrations of more than 800 sampling sites were obtained. The data showed that the magnitudes of the He and H2 anomalies of three surveys declined significantly with decreasing strength of the aftershocks with time. The maximum concentrations of He and H2 (40 and 279.4 ppm, respectively) were found in three replicates at the south part of the rupture zone close to the epicenter. The spatio-temporal variations of CO2, Rn, and Hg concentrations differed obviously between the north and south parts of the fault zone. The maximum He and H2 concentrations in Jun 2008 occurred near the parts of the rupture zone where vertical displacements were larger. The anomalies of He, H2, CO2, Rn, and Hg concentrations could be related to the variation in the regional stress field and the aftershock activity.  相似文献   

15.
Quasi-biennial oscillations (QBOs) can clearly be distinguished in uniform series of data on the solar magnetic-field polarity derived from Hα observations in 1915–1999. These have been proven to represent oscillations of the global magnetic field of the Sun. This is verified by spectral analyses executed using various methods: the QBOs are clearly visible in low harmonics (l=1–3), but abruptly disappear for l=4 and higher. First and foremost, the QBOs are displayed in variations of the sector structure of the large-scale magnetic field, demonstrating that they correspond to variations of the horizontal multipoles.  相似文献   

16.
Xihu desert wetland is an important and unusual environment in China or even in the world. However, until now, little research has been focused on the microclimate and CO2 flux characteristics in this area. This paper reports the characteristics of daily variations of microclimate and CO2 flux in the Dunhuang Xihu desert wetland, based on data observed in the desert wetland during a period of continuous fine weather in summer 2012. Results indicate that the characteristics of the micrometeorology were significantly affected by the land–lake breeze during the study period, and updrafts were prevalent in this region. The friction wind speed and the vertical velocity were much greater than those in the Maqu grasslands. The energy budget was strongly imbalanced: the latent heat flux was significantly higher than the sensible flux. The daily mean values of total solar radiation and net radiation were larger than those in Maqu grasslands and Jinta oasis. There was a temperature inversion and inverse humidity gradient in the atmospheric surface layer at night. The desert wetland ecosystem was a carbon sink during the whole of the observation period, and the maximum rate of carbon absorption usually occurred at about 11:00 hr each day in this region.  相似文献   

17.
With this study, I suggest that changes in biogeochemical processes in northern Norwegian fjords during the last glacial/interglacial transition (14.3 to 6.3 ka B.P.) are predominantly climate induced. Variable strength in Atlantic water inflow, intrusion of Arctic waters and sea-ice coverage, various nutrient supply, as well as several re-advances of continental ice-sheets are recorded by both bulk organic and selected biomarkers in sediments of the Andfjord. Stable isotope (δ13Corg, δ15N) and Rock-Eval pyrolysis data indicate a strong supply of terrestrial organic matter during stadials (e.g., Younger Dryas, Older Dryas) whereas enhanced input of marine organic matter prevailed during the interstadials (Bølling/Allerød). Profiles of selected biomarkers of marine origin show a characteristic climate-induced variation during the whole time interval, where relatively low concentrations of dinosterols and C37 alkenones may indicate—similar to today—a minor contribution of their respective plankton groups (coccolithophorids, dinoflagellates). The prevalence of Arctic waters and seasonal sea-ice coverage during the Younger Dryas is indicated by a single maximum of 24-methylenecholesterol probably indicating the dominance of sea-ice diatoms and/or the prevalence of diatoms of Thalassiosiraceae in surface waters. The sediments of the Allerød, in contrast, show a pronounced maximum in dinosterols, indicating a better adaptation of dinoflagellates to an oceanic regime that is characterized by incipient intrusion of Atlantic water, highly variable sea surface temperatures, and well stratified water masses. The concentration of higher-molecular-weight n-alkanes, however, follows the deglacial-early Holocene trend of climate amelioration from the last deglaciation to the middle Holocene, with maxima during ice advances (stadials) and minima during the middle Holocene warm period.These new results significantly improve the understanding of climate-induced response on organic matter supply in fjords and provide a better knowledge of past variations of biogeochemical processes in high-latitude coastal environments.  相似文献   

18.
Variability of the ocean surface boundary layer characteristics on daily time-scale is studied utilizing the 3-hourly hydrographic data collected at a stationary location (20°N, 89°E) in the Bay of Bengal during August (18th–31st) and September (9th–19th), 1990 under MONTBLEX-90 field programme. The daily variations of temperature, salinity, σ0, mixed layer thickness, stability, heat content and rate of change of heat content in the upper 100 m are discussed in relation to prevailing weather (depressions) and hydrographic conditions (influx of fresh water, presence of eddies). The mixed layer thickness is examined through temperatureand σ0-based criteria considering also the surface salinity in the latter. TheT-based mixed layer thickness is always higher than that of σ0-based thickness. The rate of change of heat content is also computed up to the depth of 20°C and 14°C isotherms which takes into account the vertical motion and hence divergence. With the development of a low into a deep depression close to the study area, intense upwelling of subsurface cold waters is noticed from 100 m to the bottom of the surface mixed layer (20m) from 18th to 20th August. The upwelling is weakened by 21st August when the depression moved away from the study location. This variation of upwelling is supported by the variation of surface mixed layer thickness, static stability at 30 m depth, heat content in the upper 100 m and the heat content up to the depth of 20°C isotherm from 18th to 21st August. The rate of change of heat content in the upper 100 m and up to the depths of 20°C and 14°C isotherms leads to net heat storage during August and to net heat depletion during September. This together with the net surface heat gain lead to an import (197Wm?2) and export (233 Wm?2) of heat during August and September respectively through horizontal advective processes. These advective processes are attributed to the presence and movement of a warm core eddy through the study location.  相似文献   

19.
This study provides a compilation of the paleointensity records for the Cretaceous period derived from sediments of the Russian Plate and adjacent areas. The paleoinetensity values were calibrated using the laboratory redeposition experiments. Remarkable differences in the relative paleointensity variations were detected at the Berriasian–Early Barremian, Late Barremian–Santonian and Early–Late Maastrichtian boundaries. In the Berriasian–Early Barremian interval, the paleointensity varied stochastically, with the amplitude of about 1Ho and the mean value of 0.63Ho (Ho is the present-day geomagnetic field intensity assumed to be 40 μT). The records for the Barremian–Santonian give a picture of the geomagnetic field with alternating high- and low-amplitude features. The mean paleointensities remain constantly high (being on average 0.87Ho), and intervals of low-amplitude variation alternate with the pronounced bursts (3.5Ho). The Late Maastrichtian interval is characterized by high-amplitude paleointensity variations (4Ho) and a sharp drop towards the end of the interval. All records show remarkable similarities near the boundaries between geological time intervals, which are an increase in the amplitude and mean values of intensity at the end of intervals followed by a decrease towards the beginning of the subsequent interval.  相似文献   

20.
Monthly and inter-annual variation in tropospheric nitrogen dioxide (NO2) have been examined over metropolitan cities (New Delhi, Kolkata, Mumbai and Chennai) and hill stations (Mount Abu, Nainital, Srinagar, Kodaikanal, Dalhousie, Gulmarg, Shimla and Munnar) of India during the period 2004?C2010 using satellite-based SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY). It is observed that the monthly variation in NO2 over the metropolitan cities is higher during winter (November?CDecember?CJanuary?CFebruary) months and lower during summer monsoon (June?CJuly?CAugust?CSeptember) months. Lower NO2 in summer monsoon leads to the presence of deep convection and higher in winter leads to calm winds and more residential time of gases. Moreover, rapid industrialization and traffic growth are also responsible for the higher NO2. Mean values of NO2 over New Delhi and Mumbai as well as hill stations, such as Mount Abu, Nainital and Shimla, have exhibited more pollution. Similarly, maximum NO2 occurred over the hill stations during pre-monsoon months (April?CMay) and early part of summer monsoon (June). Higher NO2 values are observed in November?CDecember months. All the hill stations also show increasing trend of NO2 during the period 2004?C2010. Increasing pollution of NO2 over the hill stations might also be due to forest fires, biomass burning and long-range transport. Back trajectory analysis shows that the observed peaks in NO2 are a resultant of the long-range transported component amplified by the local environment. In the northern hill stations, pollution seems transported from west Asian and European countries while in the southern hill stations, pollution is originated from southern Indian Ocean and East Asian countries.  相似文献   

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