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1.
Variations in the amplitude of the ordinary wave from a received signal on a partial reflection radar at a short-wave range on the Kola Peninsula during the appearance of noctilucent clouds on August 12, 2016, are examined. Noctilucent clouds are registered by the all-sky camera located 100 km southward of the partial reflection radar. They extended over the entire celestial hemisphere observed by the all-sky camera; all of them moved in the southern direction, and the clouds had a tenuous structure and showed gravity waves with spatial periods of 15–100 km. During the presence of noctilucent clouds over the partial reflection radar, polar mesospheric summer echoes (PMSEs) were recorded at heights of 83–86 km. It was found that the presence of only noctilucent clouds in diagram of the antenna pattern of partial frequency radar is not sufficient for the appearance of PMSEs; noctilucent clouds must also have irregularities of several kilometers. The PMSE heights decreased with a velocity of 0.5 and 1.3 m/s. The issue of aerosols that cause the appearance of PMSEs and noctilucent clouds is discussed.  相似文献   

2.
The solar wind velocity distribution in the heliosphere is best represented using a v-map, where velocity contours are plotted in heliographic latitude-longitude coordinates. It has already been established that low-speed regions of the solar wind on the source surface correspond to the maximum bright regions of the K-corona and the neutral line of the coronal magnetic field. In this analysis, v-maps on the source surface for Carrington rotations (CRs) 1787-1795, during 1987, have been prepared using the interplanetary scintillation measurements at Research Institute of Atmospherics (RIA), Nagoya Univ., Japan. These v-maps were then used to study the time evolution of the low-speed (\leq450 km s−1) belt of the solar wind and to deduce the distribution of solar wind velocity on the heliospheric current sheet. The low-speed belt of the solar wind on the source surface was found to change from one CR to the next, implying a time evolution. Instead of a slow and systematic evolution, the pattern of distribution of solar wind changed dramatically at one particular solar rotation (CR 1792) and the distributions for the succeeding rotations were similar to this pattern. The low-speed region, in most cases, was found to be close to the solar equator and almost parallel to it. However, during some solar rotations, they were found to be organised in certain longitudes, leaving regions with longitudinal width greater than 30○ free of low-speed solar wind, i.e. these regions were occupied by solar wind with velocities greater than 450 km s−1. It is also noted from this study that the low-speed belt, in general, followed the neutral line of the coronal magnetic field, except in certain cases. The solar wind velocity on the heliospheric current sheet (HCS) varied in the range 300–585 km s−1 during the period of study, and the pattern of velocity distribution varied from rotation to rotation.  相似文献   

3.
Bacterioplankton productivity, numbers, and cell specific activity were studied in nearshore waters of the southeastern U.S. continental shelf during seasons of maximum freshwater discharge. In April 1984, coastal waters were stratified from normal spring discharge and typical northeastward wind stress. In April 1985, shelf waters were vertically homogeneous due to below normal runoff and southwestward wind stress. In 1984, nearshore bacterial productivity ranged from 7.0 to 14.7 × 106 cells l−1 h−1 and midshelf rates were 40–50% less. In 1985, nearshore productivity ranged from 0.9 to 2.4 × 106 cells 1−1 h−1, and productivity was extremely patchy over the entire shelf. The cell-specific activity (thymidine incorporation per cell) suggests that although productivity was high in 1984, only a fraction of the bacterioplankton was actively growing or incorporating thymidine (0.9–2.9 × 10−21 mol cell−1 h−1). In 1985, a higher percentage of cells appeared to be active and incorporating thymidine (5–13 × 10−21mol cell−1h−1) even though productivity was low. Hydrographic conditions along the southeastern coastline may have had a significant impact on the overall community structure and carbon flow through the microbial food web. When coastal waters were stratified in 1984, bacterial biomass was a significant percentage (35–320%) of the phytoplankton biomass. During vertically homogeneous conditions of 1985, bacterial production and biomass were a small percentage (2–13%) of the phytoplankton production and biomass across the shelf. The interannual variation in the microbial food web was attributed to the interannual variability of the southeastern U.S. hydrology due to changes in freshwater discharge and wind direction and intensity. The ecological implications of these results extend to the potential impact of seasonal microbial food webs on nearshore allochothonous and autochothonous organics before removal from the southeastern U.S. coastline.  相似文献   

4.
A mesoscale model has been applied to calculate climatological means of the surface wind. A reliable average requires more than 40 model runs, which are differentiated by the direction and speed of the geostrophic wind under the assumption of neutral stratification. The frequency distributions of the geostrophic wind have been taken from observations of the 850-hPa winds at the radiosonde station in Prague for a 10-year period. The simulation results have been averaged over all sectors and speed classes of the geostrophic wind according to their frequencies. A comparison of the calculated mean wind speeds with observed ones shows deviations of about 0.4 ms−1 outside the mountains. The representation of steep topography and isolated mountains on the basis of a 3-km horizontal resolution of the simulations needs special treatment in order to reduce the gap of up to 4 ms−1 between observed and simulated mean wind speeds over mountains. Therefore, an empiric speed-up formula has been applied to the isolated mountains that otherwise would fall through the 3-km meshes. The corresponding deviations have been reduced to 1.5 ms−1.  相似文献   

5.
Mesospheric wind profiles with an altitude resolution of 25 m have been obtained by means of radar tracking of foil chaff clouds. Such experiments were performed during winter 1990 at Biscarrosse, France (44°N, 1°W). On one flight, a wind shear as high as 330 m s−1 km−1 at 87.4 km and a region of dynamical instability between 86 and 88 km was measured. This wind shear is believed to be the largest value ever measured in the mesosphere. The region of dynamical instability results from a superposition of two wave motions, and is found to link well with enhanced turbulence and small-scale wave activity.  相似文献   

6.
A sizable total-pressure (magnetic pressure plus kinetic pressure) enhancement was found within the high-speed wind stream observed by Helios 2 in 1976 near 0.3 AU. The proton density and temperature and the magnetic magnitude simultaneously increased for about 6 h. This pressure rise was associated with a comparatively large southward now velocity component (with Vz – 100 km · s–1) and magnetic-field rotation. The pressure enhancement was associated with unusual features in the electron distribution function. It shows a wide angular distribution of electron counting rates in the low-energy (57.8 eV) channel, while previous to the enhancement it exhibits a wide angular distribution of electron count rate in the high-energy (112, 221 and 309 eV) channels, perhaps indicating the mirroring of electrons in the converging field lines of the background magnetic field. These fluid and kinetic phenomena may be explained as resulting from an interplanetary magnetic flux rope which is not fully convected by the flow but moves against the background wind towards the Sun.  相似文献   

7.
An RH-560 rocket flight was conducted from Sriharikota rocket range (SHAR) (14°N, 80°E, dip latitude 5.5°N) to study electron density and electric field irregularities during spread F. The rocket was launched at 2130 local time (LT) and it attained an apogee of 348 km. Results of electron density fluctuations are presented here. Two extremely sharp layers of very high electron density were observed at 105 and 130 km. The electron density increase in these layers was by a factor of 50 in a vertical extent of 10 km. Large depletions in electron density were observed around 175 and 238 km. Both sharp layers as well as depletions were observed also during the descent. The presence of sharp layers and depletions during the ascent and the descent of the rocket as well as an order of magnitude less electron density, in 150/300 km region during the descent, indicate the presence of strong large-scale horizontal gradients in the electron density. Some of the valley region irregularities (165/178 km), in the intermediate scale size range, observed during this flight, show spectral peaks at 2 km and can be interpreted in terms of the image striation theory suggested by Vickrey et al. The irregularities at 176 km do not exhibit any peak at kilometer scales and appear to be of new type. The growth rate of intermediate scale size irregularities, produced through generalized Rayleigh Taylor instability, was calculated for the 200/330 km altitude, using observed values of electron density gradients and an assumed vertically downward wind of 20 ms–1. These growth rate calculations suggest that the observed irregularities could be produced by the gradient drift instability.  相似文献   

8.
Common programme observations by the EISCAT UHF radar revealed an extended interval, post geomagnetic local noon on 03 April 1992, during which the F-region ion velocity orthogonal to the geomagnetic field was significantly enhanced, to values exceeding 2 km s−1 corresponding to a perpendicular electric field of some 100 mV m−1. Observations from this interval are used to illustrate a method by which estimates of the E-region ion-neutral collision frequency may be derived in the presence of enhanced electric field. From both the rotation of the ion velocity vector and the reduction in the ion velocity magnitude relative to that in the F-region, independent estimates of the normalised ion-neutral collision frequency are made at the UHF E-region tristatic altitudes; the derived values are, in general, lower than model predictions. Although initial calculations assume a stationary neutral atmosphere, first-order estimates of the E-region neutral wind are subsequently employed to calculate revised estimates of the normalised ion-neutral collision frequency; these neutral winds are derived by attributing the difference between predicted and observed enhancements in field-parallel ion temperature to thermospheric motion. The inclusion of neutral winds, which are themselves not inconsiderable, appears to have only a limited effect on the normalised collision frequencies derived.  相似文献   

9.
Collapsed calderas are the structural surface expression of the largest volcanic eruptions on Earth and may reach diameters of tens of kilometres while erupting volumes larger than 1000 km3. Remnants of collapse calderas can be found along the South American volcanic arc and are thought to be inactive. However, this study shows that systems of such dimension may become active in a relatively short period of time without attracting much attention. Using satellite-based InSAR data, a 45 km wide elongated area of ground deformation was observed in the Lazufre volcanic region (Chile), where no deformation was detected 10 years ago. The deformation signal shows an uplift of up to ~ 3 cm yr− 1 during 2003–2006, affecting an area of about 1100 km2, comparable in size to super-volcanoes such as Yellowstone or Long Valley. This deformation signal can be explained by an inflating magma body at about 10 km depth, expanding and propagating laterally at a velocity of up to 4 km per year. Although it is not clear whether this intrusion will lead to an eruption, its dimensions and the rapid deformation rate insinuate that a potentially large volcanic system is forming.  相似文献   

10.
Simultaneous observations of polar mesosphere summer echoes (PMSE) have been carried out during summer 1994 in northern Norway using three radars on different frequencies: the ALOMAR SOUSY radar at Andenes on 53.5 MHz, the EISCAT VHP radar at Tromsø on 224 MHz and the MF radar at Tromsø on 2.78 MHz. During the common measuring period in July/August 1994, PMSE could be detected at 224 and 53.5 MHz, and there are strong hints that PMSE also occur at 2.78 MHz. Reliable correlations between hourly backscattered power values indicate that the PMSE structures have zonal extensions of more than 130 km and can be detected at very different scales (half wavelength) between 0.67 (EISCAT VHP radar) and 54 m (MF radar). Using the wind values derived by the MF radar it can be shown that the mesospheric wind field influences the structure of PMSE. The diurnal variation of PMSE is strongly connected with tidal-wind components, whereas spatial differences of PMSE can partly be explained by the mean wind field.  相似文献   

11.
A study of the formation and movement of sequential Sporadic-E layers observed during the night-time hours at two Indian low-latitude stations, SHAR(dip 10°N) and Waltair (dip 20°N) shows that the layer are formed around 19:00 h. IST at altitudes of ≈180 km. They descend to the normal E-region altitude of about 100 km in three to four hours and becomes blanketing type of Es before they disappear. However, the absence of these descending layers at an equatorial station, Trivandrum (dip 2°N) gives the experimental evidence for wind shear theory. The meridional neutral wind derived from the height variation of the F-layer showed significant poleward wind during the descent of these layers. Hence it is inferred that these layers are formed as a consequence of the convergence of plasma by the poleward wind and the equatorward propagating gravity waves (inferred from the height fluctuations of F-layer).  相似文献   

12.
An improved resonant fluorescence instrument for measuring atomic oxygen concentration was developed to avoid the Doppler effect and the aerodynamic shock effect due to the supersonic motion of a rocket. The shock effect is reduced by adopting a sharp wedge-shaped housing and by scanning of the detector field of view to change the distance between the scattering volume and the surface of the housing. The scanning enables us to determine absolute values of atomic oxygen concentration from relative variation of the scattered light signal due to the self-absorption. The instrument was calibrated in the laboratory, and the numerical simulation reproduced the calibration result. Using the instrument, the altitude profile of atomic oxygen concentration was observed by a rocket experiment at Uchinoura (31°N) on 28 January 1992. The data obtained from the rocket experiment were not perfectly free from the shock effect, but errors due to the effect were reduced by the data analysis procedure. The observed maximum concentration was 3.8× 1011 cm−3 at altitudes around 94 km. The systematic error is estimated to be less than ±0.7×1011 cm−3 and the relative random error is less than±0.07× 1011 cm−3at the same altitudes. The altitude profile of the OI 557.7-nm airglow was also observed in the same rocket experiment. The maximum volume emission rate was found to be 150 photons cm−3 s−1 at 94 km. The observed altitude profiles are compared with the MSIS model and other in situ observations.  相似文献   

13.
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.  相似文献   

14.
The landslide and cataclysmic eruption of Mount St. Helens on May 18, 1980 triggered a sequence of explosive eruptions over the following five months. The volume of explosive products from each of these eruptions decreased uniformly over this period, and the character for each eruption progressed from a fairly continuous eruption lasting more than eight hours on May 18 to a series of short bursts, some of which were spaced 12 hours apart, on October 16–18. The transition in the character of these eruption sequences can be explained by a difference between the magma supply rate and the magma discharge rate from a shallow reservoir.The magma supply rate (MSR) is the rate with which magma is supplied to the level where disruption due to vesiculation occurs. It is determined by dividing the dense-rock-equivalent volume of eruptive products by the total duration of each eruption sequence. The magma discharge rate (MDR) is the rate with which the disrupted magma is discharged through the vent. It is determined by dividing the volume of erupted products by the duration of each explosive burst. The relative magnitude of these two quantities controls the temporal evolution of an explosive event. When MDRMSR the explosive phase of the eruption lasts for several hours as a single continuous event. When MDR>MSR, an eruption is characterized by a series of short explosive bursts at intervals of several minutes to several days. The MSR of the eruptions of 1980 decreased with time from 5500 m' s−1 on May 18 to 7 m3 s−2 on October 16–18 and approximately fits an exponential decay. The MDR for the same events remained approximately constant at 2000 m3 s−1. Each explosive event has been followed by an aftershock-like series of earthquakes located beneath the volcano at depth mostly between 7 and 14 km. The seismic energy released during each of these series is proportional to the corresponding volume of erupted magma. Deformation data between June and November, 1980 indicate a subsidence of the volcanic structure which can be modeled by a volume collapse of 0.25 km3 located at 9 km depth.We propose a model in which magma is supplied from depths of 7–14 km through a narrow conduit during each eruption. It erupts to the surface at a uniform rate during each eruption. The deep seismic activity following each eruption is related to a readjustment and volume decrease in the deep feeding system. The decrease of the MSR over time is explained by an increase in the viscosity of a progressively water-depleted magma. The amount of water necessary to explain the observed decrease of the MSR is of the order of 4.6%.  相似文献   

15.
The descent and ejection of matter in the solar atmosphere observed in the CaII 8498-Å line have been studied. In the NOAA active region no. 10 792 on July 30, 2005 before the flare, the dense cold gas cloud descended with a ray velocity of ～8 km/s and then ascended in the impulsive phase. The plasma ascended with an acceleration reaching 0.4 km/s2 in the flare maximum. The acceleration of the matter likely continued after the flare maximum, because an acceleration of higher than 0.5 km/s2 was required for the appearance of the ejection at the edge of the occulting disk of the LASCO C2 coronagraph at 0557 UT. The descent of the matter resulting in the local heating of the chromosphere was also observed in the NOAA active region no. 10656 on August 9, 2004 before the flare. The maximum descent velocity was no more than 24.7 km/s.  相似文献   

16.
The polar wind is an ambipolar outflow of thermal plasma from the high-latitude ionosphere to the magnetosphere, and it primarily consists of H+, He+ and O+ ions and electrons. Statistical and episodic studies based primarily on ion composition observations on the ISIS-2, DE-1, Akebono and Polar satellites over the past four decades have confirmed the existence of the polar wind. These observations spanned the altitude range from 1000 to ∼50,500 km, and revealed several important features in the polar wind that are unexpected from “classical” polar wind theories. These include the day–night asymmetry in polar wind velocity, which is 1.5–2.0 times larger on the dayside; appreciable O+ flow at high altitudes, where the velocity at 5000–10,000 km is of 1–4 km/s; and significant electron temperature anisotropy in the sunlit polar wind, in which the upward-to-downward electron temperature ratio is 1.5–2. These features are attributable to a number of “non-classical” polar wind ion acceleration mechanisms resulting from strong ionospheric convection, enhanced electron and ion temperatures, and escaping atmospheric photoelectrons. The observed polar wind has an averaged ion temperature of ∼0.2–0.3 eV, and a rate of ion velocity increase with altitude that correlates strongly with electron temperature and is greatest at low altitudes (<4000 km for H+). The rate of velocity increase below 4000 km is larger at solar minimum than at solar maximum. Above 4000 km, the reverse is the case. This suggests that the dominant polar wind ion acceleration process may be different at low and high altitudes, respectively. At a given altitude, the polar wind velocity is highly variable, and is on average largest for H+ and smallest for O+. Near solar maximum, H+, He+, and O+ ions typically reach a velocity of 1 km/s near 2000, 3000, and 6000 km, respectively, and velocities of 12, 7, and 4 km/s, respectively, at 10,000 km altitude. Near solar minimum, the velocity of all three species is smaller at high altitudes. Observationally it is not always possible to unambiguously separate an energized “non-polar-wind” ion such as a low-energy “cleft ion fountain” ion that has convected into a polar wind flux tube from an energized “polar-wind” ion that is accelerated locally by “non-classical” polar-wind ion acceleration mechanisms. Significant questions remain on the relative contribution between the cleft ion fountain, auroral bulk upflow, and the topside polar-cap ionosphere to the O+ polar wind population at high altitudes, the effect of positive spacecraft charging on the lowest-energy component of the H+ polar wind population, and the relative importance of the various classical and non-classical ion acceleration mechanisms. These questions pose several challenges in future polar wind observations: These include measurement of the lowest-energy component in the presence of positive spacecraft potential, definitive determination and if possible active control of the spacecraft potential, definitive discrimination between polar wind and other inter-mixed thermal ion populations, measurement of the three-dimensional ion drift velocity vector and the parallel and perpendicular ion temperatures or the detailed three-dimensional velocity distribution function, and resolution of He+ and other minor ion species in the polar wind population.  相似文献   

17.
Inorganic particulate material uptake and release over an oyster reef during a tidal cycle was measured every 11.8 days for one year. There was a net uptake on the flooding tides and a net release on ebbing tides. Particulate uptake was highest in late summer reaching a maximum of 230 g m−2 h−1. Particulate release was also highest in late summer reaching a maximum of 94 g m−2 h−1. On an annual basis, 86.7 kg m−2 y−1 of inorganic particulate material was taken up on flooding tides and 56.5 kg m−2 y−1 was released on ebbing tides. The pattern of uptake on flooding tides and release on ebbing tides was hypothesized to be the result of higher water velocities on ebbing tides.  相似文献   

18.
The time evolution of negative buoyancy of a subducting slab is modelled from the beginning of subduction under various kinematic conditions (dip angle and subduction velocity). The calculations take into account the thermal and density effects of the variations of the thermophysical parameters with temperature and pressure, and of phase transitions. The magnitude of the negative buoyancy increases during subduction of oceanic lithosphere, up to values in the (2–4) × 1013 N m−1 range when the tip of the slab reaches a depth of 600–700 km. If continental material arrives at the trench and is subducted, the downward buoyancy decreases by an amount proportional to the volume of the subducted continental crust. Assuming that subduction stops when the buoyancy becomes zero, and that delamination of the continental crust or slab breakoff do not occur, the maximum downdip length of the subductable continental crust is estimated as a function of the dip angle, subduction velocity and geometry of the margin. In most cases, subduction of continental material down to depths of 100–250 km is possible, and continental subduction can continue for times up to 10–15 Ma if the velocity is low. These estimates are not significantly affected by the hypothetical occurrence of a metastable olivine wedge within the slab, and could be lower bounds if the lower continental crust is mafic and transforms to eclogite.  相似文献   

19.
Profiles of velocity turbulence in Monterey Canyon, made with a recently developed expendable probe, show the existence of a very turbulent bottom boundary layer. The turbulent flow is up to 170 m thick and has peak microscale shears of 1 m s−1 per meter. The rate of dissipation of kinetic energy, based on the observed shear variance, averaged over the depth of the turbulent boundary layer ranged from 70 to 500 × 10−6W m−3. Temperature measurements indicate that the flow was up canyon at a time of low tide. The upper bound for the vertical eddy viscosity is estimated to be17 × 10−4m2s−1 and for the vertical eddy diffusivity is estimated to be 15 × 10−4m2s−1. The large vertical scale and the intensity of the observed boundary layer suggest that the flow in Monterey Canyon may be important for the renewal and circulation of water over the continental shelf in the bay area.  相似文献   

20.
Permanent magnetism and conventional dynamo theory are possible but problematic explanations for the magnitude of the Mercurian magnetic field. A new model is proposed in which thermoelectric currents driven by temperature differences at a bumpy core-mantle boundary are responsible for the (unobserved) toroidal field, and the helicity of convective motions in a thin outer core (thickness 102 km) induces the observed poloidal field from the toroidal field. The observed field of 3 × 10−7 T can be reproduced provided the electrical conductivity of Mercury's semiconducting mantle approaches 103 Ω−1 m−1. This model may be testable by future missions to Mercury because it predicts a more complicated field geometry than conventional dynamo theories. However, it is argued that polar wander may cause the core-mantle topography to migrate so that some aspects of the rotational symmetry may be reflected in the observed field.  相似文献   

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