Showing papers in "Annales Geophysicae in 1994"
TL;DR: The Hydrologic Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) as discussed by the authors was carried out in Niger, West Africa, during 1991 - 1992, with an intensive observation period (IOP) in August - October 1992, aiming at improving the parameterization of land surface atmosphere interactions at the Global Circulation Model (GCM) gridbox scale.
Abstract: The Hydrologic Atmospheric Pilot EXperiment in the Sahel (HAPEX-Sahel) was carried out in Niger, West Africa, during 1991 - 1992, with an intensive observation period (IOP) in August - October 1992. It aims at improving the parameterization of land surface atmosphere interactions at the Global Circulation Model (GCM) gridbox scale. The experiment combines remote sensing and ground based measurements with hydrological and meteorological modelling to develop aggregation techniques for use in large scale estimates of the hydrological and meteorological behaviour of large areas in the Sahel. The experimental strategy consisted of a period of intensive measurements during the transition period of the rainy to the dry season, backed up by a series of long term measurements in a 1^ by 1^ square in Niger. Three “supersites” were instrumented with a variety of hydrological and (micro) meteorological equipment to provide detailed information on the surface energy exchange at the local scale. Boundary layer measurements and aircraft measurements were used to provide information at scales of 100 - 500 km2. All relevant remote sensing images were obtained for this period. This programme of measurements is now being analyzed and an extensive modelling programme is under way to aggregate the information at all scales up to the GCM grid box scale. The experimental strategy and some preliminary results of the IOP are described.
359 citations
TL;DR: In this paper, a numerical solution to the transport and energy degradation problem was proposed, verified by reproducing simplified problems to which analytic solutions exist, internal self-consistency tests, comparison with laboratory experiments of electron beams penetrating a collision chamber, and by comparison with auroral observations, particularly the emission ratio of the N2 second positive to N+2 first negative emissions.
Abstract: . Auroral electron transport calculations are a critical part of auroral models. We evaluate a numerical solution to the transport and energy degradation problem. The numerical solution is verified by reproducing simplified problems to which analytic solutions exist, internal self-consistency tests, comparison with laboratory experiments of electron beams penetrating a collision chamber, and by comparison with auroral observations, particularly the emission ratio of the N2 second positive to N+2 first negative emissions. Our numerical solutions agree with range measurements in collision chambers. The calculated N22P to N+21N emission ratio is independent of the spectral characteristics of the incident electrons, and agrees with the value observed in aurora. Using different sets of energy loss cross sections and different functions to describe the energy distribution of secondary electrons that emerge from ionization collisions, we discuss the uncertainties of the solutions to the electron transport equation resulting from the uncertainties of these input parameters.
204 citations
TL;DR: In this article, a transect between the northern tip of the Antarctic Peninsula and Kapp Norvegia was used to determine the structure of the Weddell gyre and its associated transports.
Abstract: . A cyclonic gyre controls the advection of source waters into the formation areas of bottom water in the southern and western parts of the Weddell Sea and the subsequent transport of modified water masses to the north. Determination of the structure of the Weddell Gyre and of the associated transports was one of the objectives of the "Weddell Gyre Study" which began in September 1989 and ended in January 1993. The collected data set comprises records of moored current meters and profiles of temperature and salinity distributed along a transect between the northern tip of the Antarctic Peninsula and Kapp Norvegia. The circulation pattern on the transect is dominated by stable boundary currents of several hundred kilometers width at the eastern and western sides of the basin. They are of comparable size on both sides and provide nearly 90% of the volume transport of the gyre which amounts to 29.5 Sv. In the interior, a weak anticyclonic cell of 800 km diameter transports less than 4 Sv. Apart from the continental slopes, the near-bottom currents flow at some locations in an opposite direction to those in the water column above, indicating a significant baroclinic component of the current field. The intensity of the boundary currents is subject to seasonal fluctuations, whereas in the interior, time scales from days to weeks dominate. The large-scale circulation pattern is persistent during the years 1989 to 1991. The heat transport into the southern Weddell Sea is estimated to be 3.48×1013 W. This implies an equivalent heat loss through the sea surface of 19 W m-2, as an average value for the area south of the transect. The derived salt transport is not significantly different from zero; consequently, the salt gain by sea ice formation has to compensate almost entirely the fresh water gain from the melting ice shelves and from precipitation. Estimation of water mass formation rates from the thermohaline differences of the inflow and outflow through the transect indicates that 6.0 Sv of Warm Deep Water are transformed into 2.6 Sv of Weddell Sea Bottom Water, into 1.2 Sv of Weddell Sea Deep Water, and into 2.2 Sv of surface water.
192 citations
TL;DR: A survey of the state-of-the-art results from recent and ongoing research on the greenhouse effect of high-flying global civil subsonic air traffic can be found in this article.
Abstract: Emissions from aircraft engines include carbon dioxide, water vapour, nitrogen oxides, sulphur components and various other gases and particles. Such emissions from high-flying global civil subsonic air traffic may cause anthropogenic climate changes by an increase of ozone and cloudiness in the upper troposphere, and by an enhanced greenhouse effect. The absolute emissions by air traffic are small (a few percent of the total) compared to surface emissions. However, the greenhouse effect of emitted water and of nitrogen oxides at cruise altitude is potentially large compared to that of the same emissions near the earth's surface because of relatively large residence times at flight altitudes, low background concentrations, low temperature, and large radiative efficiency. Model computations indicate that emission of nitrogen oxides has doubled the background concentration in the upper troposphere between 40°N and 60°N. Models also indicate that this causes an increase of ozone by about 5-20%. Regionally, the observed annual mean change in cloudiness is 0.4%. It is estimated that the resultant greenhouse effect of changes in ozone and thin cirrus cloud cover causes a climatic surface temperature change of 0.01-0.1 K. These temperature changes are small compared to the natural variability. Recent research indicates that the emissions at cruise altitude may increase the amount of stratospheric aerosols and polar stratospheric clouds and thereby have an impact on the atmospheric environment. Air traffic is increasing about 5-6% per year, fuel consumption by about 3%, hence the effects of the related emissions are expected to grow. This paper surveys the state of knowledge and describes several results from recent and ongoing research.
158 citations
TL;DR: In this article, the probability distributions of field differences were calculated from time series of Helios data obtained in 1976 at heliocentric distances near 0.3 AU, and the relevance of these observations to the interpretation and understanding of the nature of solar wind magnetohydrodynamic (MHD) turbulence was pointed out.
Abstract: The probability distributions of field differences x( )=x(t+ )-x(t), where the variable x(t) may denote any solar wind scalar field or vector field component at time t, have been calculated from time series of Helios data obtained in 1976 at heliocentric distances near 0.3 AU. It is found that for comparatively long time lag , ranging from a few hours to 1 day, the differences are normally distributed according to a Gaussian. For shorter time lags, of less than ten minutes, significant changes in shape are observed. The distributions are often spikier and narrower than the equivalent Gaussian distribution with the same standard deviation, and they are enhanced for large, reduced for intermediate and enhanced for very small values of x. This result is in accordance with fluid observations and numerical simulations. Hence statistical properties are dominated at small scale by large fluctuation amplitudes that are sparsely distributed, which is direct evidence for spatial intermittency of the fluctuations. This is in agreement with results from earlier analyses of the structure functions of x. The non-Gaussian features are differently developed for the various types of fluctuations. The relevance of these observations to the interpretation and understanding of the nature of solar wind magnetohydrodynamic (MHD) turbulence is pointed out, and contact is made with existing theoretical concepts of intermittency in fluid turbulence.
152 citations
TL;DR: In this article, the authors describe characteristics and distribution patterns of snow and meteoric ice and assess their importance for the mass balance of sea ice, based on snow and ice-thickness measurements at >11 000 points augmented by snow- and icecore studies during 4 expeditions from 1986 - 92 in the Weddell Sea, and assess the importance of sea-ice mass balance.
Abstract: . Based on snow- and ice-thickness measurements at >11 000 points augmented by snow- and icecore studies during 4 expeditions from 1986 - 92 in the Weddell Sea, we describe characteristics and distribution patterns of snow and meteoric ice and assess their importance for the mass balance of sea ice. For first-year ice (FY) in the central and eastern Weddell Sea, mean snow depth amounts to 0.16 m (mean ice thickness 0.75 m) compared to 0.53 m (mean ice thickness 1.70 m) for second-year ice (SY) in the northwestern Weddell Sea. Ridged ice retains a thicker snow cover than level ice, with ice thickness and snow depth negatively correlated for the latter, most likely due to aeolian redistribution. During the different expeditions, 8, 15, 17 and 40% of all drill holes exhibited negative freeboard. As a result of flooding and brine seepage into the snow pack, snow salinities averaged 4‰. Through 18O measurements the distribution of meteoric ice (i.e. precipitation) in the sea-ice cover was assessed. Roughly 4% of the total ice thickness consist of meteoric ice (FY 3%, SY 5%). With a mean density of 290 kg/m3, the snow cover itself contributes 8% to total ice mass (7% FY, 11% SY). Analysis of ∆18O in snow indicates a local maximum in accumulation in the 65 to 75°S latitude zone. Hydrogen peroxide in the snow has proven useful as a temporal tracer and for identification of second-year floes. Drawing on accumulation data from stations at the Weddell Sea coast, it becomes clear that the onset of ice growth is important for the evolution of ice thickness and the interaction between ice and snow. Loss of snow to leads due to wind drift may be considerable, yet is reduced owing to metamorphic processes in the snow column. This is confirmed by a comparison of accumulation data from coastal stations and from snow depths over sea ice. Temporal and spatial accumulation patterns of snow are shown to be important in controlling the sea-ice cover evolution.
141 citations
TL;DR: An artificial feed-forward neural network with one hidden layer and error back-propagation learning is used to predict the geomagnetic activity index (Dst) one hour in advance, independent of whether the raw parameters are used, or the electric field and square root of the dynamical pressure.
Abstract: . An artificial feed-forward neural network with one hidden layer and error back-propagation learning is used to predict the geomagnetic activity index (Dst) one hour in advance. The Bz-component and ΣBz, the density, and the velocity of the solar wind are used as input to the network. The network is trained on data covering a total of 8700 h, extracted from the 25-year period from 1963 to 1987, taken from the NSSDC data base. The performance of the network is examined with test data, not included in the training set, which covers 386 h and includes four different storms. Whilst the network predicts the initial and main phase well, the recovery phase is not modelled correctly, implying that a single hidden layer error back-propagation network is not enough, if the measured Dst is not available instantaneously. The performance of the network is independent of whether the raw parameters are used, or the electric field and square root of the dynamical pressure.
117 citations
TL;DR: In this paper, the average contrail cover exhibits maximum values along the transatlantic flight corridor around 50 °N (of almost 2%) and over western Europe resulting in 0.5% contrail cloudiness on average.
Abstract: . Contrail cloudiness over Europe and the eastern part of the North Atlantic Ocean was analyzed for the two periods September 1979 - December 1981 and September 1989 - August 1992 by visual inspection of quicklook photographic prints of NOAA/AVHRR infrared images. The averaged contrail cover exhibits maximum values along the transatlantic flight corridor around 50 °N (of almost 2%) and over western Europe resulting in 0.5% contrail cloudiness on average. A strong yearly cycle appears with a maximum (
99 citations
TL;DR: In this article, an experimental study of two different aircraft turbine engines was conducted to determine detailed organic emissions, as well as emissions of inorganic gases, at several engine power settings.
Abstract: . Assessment of the environmental impact of aircraft emissions is required by planners and policy makers. Seveal areas of concern are: 1. exposure of airport workers and urban residents to toxic chemicals emitted when the engines operate at low power (idle and taxi) on the ground; 2. contributions to urban photochemical air pollution of aircraft volatile organic and nitrogen oxides emissions from operations around airports; and 3. emissions of nitrogen oxides and particles during high-altitude operation. The environmental impact of chemicals emitted from jet aircraft turbine engines has not been firmly established due to lack of data regarding emission rates and identities of the compounds emitted. This paper describes an experimental study of two different aircraft turbine engines designed to determine detailed organic emissions, as well as emissions of inorganic gases. Emissions were measured at several engine power settings. Measurements were made of detailed organic composition from C 1 through C 17 , CO, CO 2 , NO, NO x , and polycyclic aromatic hydrocarbons. Measurements were made using a multi-port sampling pro be positioned directly behind the engine in the exhaust exit plane. The emission measurements have been used to determine the organic distribution by carbon number and the distribution by compound class at each engine power level. The sum of the organic species was compared with an independent measurement of total organic carbon to assess the carbon mass balance. A portion of the exhaust was captured and irradiated in outdoor smog chambers to assess the photochemical reactivity of the emissions with respect to ozone formation. The reactivity of emissions from the two engines was apportioned by chemical compound class.
92 citations
TL;DR: In this paper, the authors used the median hourly electron content-latitude profiles obtained in South East Asia under solar minimum and maximum conditions to establish seasonal and solar differences in the diurnal variations of the ionospheric equatorial anomaly (EIA).
Abstract: . Median hourly, electron content-latitude profiles obtained in South East Asia under solar minimum and maximum conditions have been used to establish seasonal and solar differences in the diurnal variations of the ionospheric equatorial anomaly (EIA). The seasonal changes have been mainly accounted for from a consideration of the daytime meridional wind, affecting the EIA diffusion of ionization from the magnetic equator down the magnetic field lines towards the crests. Depending upon the seasonal location of the subsolar point in relation to the magnetic equator diffusion rates were increased or decreased. This led to crest asymmetries at the solstices with (1) the winter crest enhanced in the morning (increased diffusion rate) and (2) the same crest decaying most rapidly in the late afternoon (faster recombination rate at lower ionospheric levels). Such asymmetries were also observed, to a lesser extent, at the equinoxes since the magnetic equator (located at about 9°N lat) does not coincide with the geographic equator. Another factor affecting the magnitude of a particular electron content crest was the proximity of the subsolar point, since this increased the local ionization production rate. Enhancements of the EIA took place around sunset, mainly during the equinoxes and more frequently at solar maximum, and also there was evidence of apparent EIA crest resurgences around 0300 LST for all seasons at solar maximum. The latter are thought to be associated with the commonly observed, post-midnight, ionization enhancements at midlatitudes, ionization being transported to low latitudes by an equatorward wind. The ratio increases in crest peak electron contents from solar minimum to maximum of 2.7 at the equinoxes, 2.0 at the northern summer solstice and 1.7 at northern winter solstice can be explained, only partly, by increases in the magnitude of the eastward electric field E overhead the magnetic equator affecting the [E×B] vertical drifts. The most important factor is the corresponding increase in ionization production rate due to the increase in solar radiation flux. The EIA crest asymmetries observed at solar maximum were less significant, and this is probably due to the corresponding increase in ionization densities leading to an increase of the retarding effect of ion-drag on the daytime meridional winds.
78 citations
TL;DR: The National Oceanic and Atmospheric Administration's (NOAA) Wind Profiler Demonstration Network consists of 32 wind profiling radars, based primarily in the central United States as discussed by the authors.
Abstract: . The National Oceanic and Atmospheric Administration's (NOAA) Wind Profiler Demonstration Network consists of 32 wind profiling radars, based primarily in the central United States. The network is being used to determine the operational feasibility and characteristics of a possible future nationwide profiler network. Data processing is performed both at the individual profiler sites and at a central hub processing system. This paper documents the algorithms used at the profilers to produce profiles of the moments of the velocity spectrum every 6 minutes, as well as those used on the hub to produce quality-controlled hourly winds.
TL;DR: In this article, the sensitivity of snow cover to changes in meteorological variables at a regional scale, a numerical snow model and an analysis system of the meteorological conditions adapted to relief were used.
Abstract: . In order to study the sensitivity of snow cover to changes in meteorological variables at a regional scale, a numerical snow model and an analysis system of the meteorological conditions adapted to relief were used. This approach has been successfully tested by comparing simulated and measured snow depth at 37 sites in the French Alps during a ten year data period. Then, the sensitivity of the snow cover to a variation in climatic conditions was tested by two different methods, which led to very similar results. To assess the impact of a particular "doubled CO2" scenario, coherent perturbations were introduced in the input data of the snow model. It was found that although the impact would be very pronounced, it would also be extremely differentiated, dependent on the internal state of the snow cover. The most sensitive areas are the elevations below 2400 m, especially in the southern part of the French Alps.
TL;DR: In this paper, the authors investigated the growth rate of irregularities by the generalized collisional Rayleigh-Taylor (GRT) instability mechanism, including the gravitational and cross-field instability terms.
Abstract: . The occurrence of spread-F at Trivandrum (8.5°N, 77°E, dip 0.5°N) has been investigated on a seasonal basis in sunspot maximum and minimum years in terms of the growth rate of irregularities by the generalized collisional Rayleigh-Taylor (GRT) instability mechanism which includes the gravitational and cross-field instability terms. The occurrence statistics of spread-F at Trivandrum have been obtained using quarter hourly ionograms. The nocturnal variations of the growth rate of irregularities by the GRT mechanism have been estimated for different seasons in sunspot maximum and minimum years at Trivandrum using h'F values and vertical drift velocities obtained from ionograms. It is found that the seasonal variation of spread-F occurrence at Trivandrum can, in general, be accounted for on the basis of the GRT mechanism.
TL;DR: In this paper, a thermal low-pressure system over the Iberian Peninsula is analyzed using the potential vorticity approach and the authors show that its main characteristic is the existence of a negative potential-vorticity (PV) dome and a funnel-like structure for potential temperature, both located at the centre of the low.
Abstract: During the summer a thermal low-pressure system is locked over the Iberian Peninsula. We present a first analysis of such a system using the potential vorticity approach. Our results show that its main characteristic is the existence of a negative potential vorticity (PV) dome and a funnel-like structure for potential temperature, both located at the centre of the low. The build-up and evolution of this PV dome can be understood in terms of the dot products of the absolute vorticity and the gradient of diabetic heating vectors and the curl of the friction forces and the gradient of potential temperature vectors. The inhibition of the Algerian Mediterranean cyclogenesis during the summer seems to bear some relation to the existence of this kind of low-pressure disturbance over the Iberian Peninsula.
TL;DR: In this article, the electron density and temperature during the ionospheric storms (15-16 June 1965, 29-30 September 1969 and 17-18 August 1970) are compared with model results.
Abstract: . Millstone Hill ionospheric storm time measurements of the electron density and temperature during the ionospheric storms (15-16 June 1965; 29-30 September 1969 and 17-18 August 1970) are compared with model results. The model of the Earth's ionosphere and plasmasphere includes interhemispheric coupling, the H + , O + ( 4 S), O + ( 2 D), O + ( 2 P), NO + , O + 2 and N + 2 ions, electrons, photoelectrons, the electron and ion temperature, vibrationally excited N 2 and the components of thermospheric wind. In order to model the electron temperature at the time of the 16 June 1965 negative storm, the heating rate of the electron gas by photoelectrons in the energy balance equation was multiplied by the factors 5-30 at he altitude above 700 km for the period 4.50-12.00 LT, 16 June 1965. The [O]/[N 2 ] MSIS-86 decrease and vibrationally excited N 2 effects are enough to account for the electron density depressions at Millstone Hill during the three storms. The factor of 2 (for 27-30 September 1969 magnetic storm) and the & actor 2.7 (for 16-18 August 1970 magnetic storm) reduction in the daytime peak density due to enhanced vibrationally excited N 2 is brought about by the increase in the O + +N 2 rate factor.
TL;DR: In this paper, the authors evaluated the 30-year period beginning in 1957 by calculating the number of occurrences where the observed visibility was reduced below threshold values of 10 km and 5 km, and found that the frequency of low visibility was several times greater for the 1977-1986 period than for the 1957-1966 period.
Abstract: The dust haze conditions, typical of the African atmosphere south of the Sahara, are a result of wind-generated dust from arid lands. The magnitude of the dust haze is evaluated for the 30-year period beginning in 1957 by calculating the number of occurrences where the observed visibility was reduced below threshold values of 10 km and 5 km. The frequency of low visibility was several times greater for the 1977–1986 period than for the 1957–1966 period. Large decreases in visibility are observed after the severe droughts of 1972–1973 and 1982-1984. Contrasting regional differences of the dustiness evolution are noticed. These differences are closely related to the differences in the regional rainfall evolution. The increase in dustiness is believed to arise from dust produced in new desertic areas which result from rainfall shortages along the southern border of the Sahara.
TL;DR: In this article, the mesoscale structure of local physical oceanographic conditions in the Bransfield Strait during the Austral summer was derived from the BIOANTAR 93 cruise and auxiliary remote sensing data.
Abstract: . The Bransfield Strait is one the best-known areas of Antarctica's oceanic surroundings. In spite of this, the study of the mesoscale variability of its local circulation has been addressed only recently. This paper focuses on the mesoscale structure of local physical oceanographic conditions in the Bransfield Strait during the Austral summer as derived from the BIOANTAR 93 cruise and auxiliary remote sensing data. Moreover, data recovered from moored current meters allow identification of transient mesoscale phenomena.
TL;DR: In this paper, the scaling exponents of the velocity structure functions, obtained from the velocity fluctuations measured in the interplanetary space plasma, were analyzed in the absence and in the presence of the large-scale magnetic field decorrelation effect.
Abstract: We analyze the scaling exponents of the velocity structure functions, obtained from the velocity fluctuations measured in the interplanetary space plasma. Using the expression for the energy transfer rate which seems the most relevant in describing the evolution of the pseudo-energy densities in the interplanetary medium, we introduce an energy cascade model derived from a simple fragmentation process, which takes into account the intermittency effect. In the absence and in the presence of the large-scale magnetic field decorrelation effect the model reduces to the fluid and the hydromagnetic p-model, respectively. We show that the scaling exponents of the q-th power of the velocity structure functions, as obtained by the model in the absence of the decorrelation effect, furnishes the best-fit to the data analyzed from the Voyager 2 velocity field measurements at 8.5 AU. Our results allow us to hypothesize a new kind of scale-similarity for magnetohydrodynamic turbulence when the decorrelation effect is at work, related to the fourth-order velocity structure function.
TL;DR: In this article, the presence of polar patches as observed simultaneously in the same magnetic meridian of opposite nightside ionospheres by coherent and incoherent scatter radars is described, and the patches appear to be related to variations either in the Bz or By component of the interplanetary magnetic field which cause transient merging on the dayside magnetopause.
Abstract: . The presence of polar patches as observed simultaneously in the same magnetic meridian of opposite nightside ionospheres by coherent and incoherent scatter radars are described. The patches appear to be related to variations either in the Bz or By component of the interplanetary magnetic field which cause transient merging on the dayside magnetopause. The passage and characteristics of polar patches as they traverse the polar cap into the nightside auroral oval are not significantly affected by the occurrence of small substroms. This study illustrates how the observations of polar patches in the nightside high-latitude ionosphere could be of great value in determining their formation process.
TL;DR: In this article, a numerical model called PICASSO [ Production d'Ions Corona Au Sol Sous Orage (French) and Production of Corona Ions at the Ground Beneath Thundercloud (English)], previously designed, is used to describe the evolution of the principal electrical parameters below a thunderstorm, taking into account the major part played by corona ions.
Abstract: . A numerical model called PICASSO [Production d'Ions Corona Au Sol Sous Orage (French) and Production of Corona Ions at the Ground Beneath Thundercloud (English)], previously designed, is used to describe the evolution of the principal electrical parameters below a thunderstorm, taking into account the major part played by corona ions. In order to improve the model restitution of a real situation, various improvements are performed: an initial vertical distribution of aerosol particles is introduced instead of the previously used uniform concentration; time and space calculation steps are adjusted according to the electric field variation rate; the upper boundary condition is improved; and the coefficients of ion attachment are reconsidered with an exhaustive bibliographic study. The influence of the ion attachment on aerosol particles, on the electric field and charge density aloft, is studied by using three different initial aerosol particle concentrations at ground level and two types of initial vertical distributions: uniform and non-uniform. The comparison between field data and model results leads to adjust the initial aerosol particle concentration over the experimental site at the value of 5000 cm-3 which appears to be highly realistic. The evolutions of the electric field and of the charge density at altitude are greatly influenced by the aerosol concentration. On the contrary, the surface intrinsic field, defined as the electric field that would exist underneath a thundercloud if there were no local charges, is weakly affected when the model is forced by the surface field. A good correlation appears between the success in the triggered lightning attempts and this intrinsic field evaluation. Therefore, when only the surface field is available, the model can be used in a triggered lightning experiment.
Journal Article•
TL;DR: In this article, simultaneous observations by EISCAT and the Kilpisjarvi all-sky camera revealed the presence of a band of enhanced electric field on one side only of an auroral arc.
Abstract: Simultaneous observations by EISCAT and the Kilpisjarvi all-sky camera revealed the presence of a band of enhanced electric field on one side only of an auroral arc. Both the arc and the enhanced electric field drifted equatorward at a velocity close to the prevailing convection velocity. The same drift speed was indicated by EISCAT measurements of ion-frictional heating: this descended in height as the magnetic field lines carrying the enhanced electric field cut across the EISCAT beam
TL;DR: The relationship between the auroral electrojet indices (AE) and the ring current magnetic field (DR) was investigated by observations obtained during the magnetic storm on 1-3 April 1973 as mentioned in this paper.
Abstract: The relationship between the auroral electrojet indices (AE) and the ring current magnetic field (DR) was investigated by observations obtained during the magnetic storm on 1–3 April 1973. During the storm main phase the DR development is accompanied by a shift of the auroral electrojets toward the equator. As a result, the standard AE indices calculated on the basis of data from auroral observatories was substantially lower than the real values (AE’). To determine AE’ during the course of a storm main phase data from subauroral magnetic observatories should be used. It is shown that the intensity of the indices (AE’) which take into account the shift of the electrojets is increased substantially relative to the standard indices during the storm main phase. AE’ values are closely correlated with geoeffective solar wind parameters. A high correlation was obtained between AE’ and the energy flux into the ring current during the storm main phase. Analysis of magnetic field variations during intervals with intense southward IMF components demonstrates a decrease of the saturation effect of auroral electrojet currents if subauroral stations magnetic field variations are taken into account. This applies both to case studies and statistical data. The dynamics of the electrojets in connection with the development of the ring current and of magnetospheric substorms can be described by the presence (absence) of saturation for minimum (maximum) AE index values during a 1-h interval. The ring current magnetic field asymmetry (ASY) was calculated as the difference between the maximum and minimum field values along a parallel of latitude at low latitudes. The ASY value is closely correlated with geoeffective solar wind parameters and simultaneously is a more sensitive indicator of IMF Bz variations than the symmetric ring current. ASY increases (decreases) faster during the main phase (the recovery phase) than DR. The magnetic field decay at low latitudes in the recovery phase occurs faster in the afternoon sector than at dusk.
TL;DR: In this paper, a 54.95 MHz coherent backscatter radar, an ionosonde and the magnetometer located at Trivandrum in India (8.5○N, 77○E, 0.5 nT dip angle) recorded large-amplitude ionospheric fluctuations and magnetic field fluctuations associated with a Pc5 micropulsation event, which occurred during an intense magnetic storm on 24 March 1991 (Ap=161).
Abstract: A 54.95-MHz coherent backscatter radar, an ionosonde and the magnetometer located at Trivandrum in India (8.5○N, 77○E, 0.5○N dip angle) recorded large-amplitude ionospheric fluctuations and magnetic field fluctuations associated with a Pc5 micropulsation event, which occurred during an intense magnetic storm on 24 March 1991 (Ap=161). Simultaneous 100-nT-level fluctuations are also observed in the H-component at Brorfelde, Denmark (55.6○N gm) and at Narsarsuaq, Greenland (70.6○N gm). Our study of the above observations shows that the E-W electric field fluctuations in the E- and F-regions and the magnetic field fluctuations at Thumba are dominated by a near-sinusoidal oscillation of 10 min during 1730–1900 IST (1200-1330 UT), the amplitude of the electric field oscillation in the equatorial electrojet (EEJ) is 0.1-0.25 mV m−1 and it increases with height, while it is about 1.0 mV m−1 in the F-region, the ground-level H-component oscillation can be accounted for by the ionospheric current oscillation generated by the observed electric field oscillation in the EEJ and the H-component oscillations at Trivandrum and Brorfelde are in phase with each other. The observations are interpreted in terms of a compressional cavity mode resonance in the inner magnetosphere and the associated ionospheric electric field penetrating from high latitudes to the magnetic equator.
TL;DR: In this article, the spatial extent and temporal behaviour of quasi-periodic (QP) intensity modulations of 0.5-2 kHz ELF-VLF signals were investigated in a comparative study of data collected at the Antarctic stations of South Pole (L=14), Halley (L =4), and Siple(L=4).
Abstract: The spatial extent and temporal behaviour of quasi-periodic (QP) intensity modulations of 0.5-2 kHz ELF-VLF signals were investigated in a comparative study of data collected at the Antarctic stations of South Pole (L=14), Halley (L=4), and Siple (L=4). Frequently, the waveforms of ELF-VLF signals simultaneously received at each site were identical. Although of similar frequency structure, the waveforms of the accompanying Pc3 magnetic pulsations did not show a one-to-one association. Whereas both are dayside phenomena, QP emissions occur over a smaller range of local times, and have a maximum of occurrence later in the day closer to local noon. QP emissions are identified with the periodic modulation of the electron pitch-angle distribution by the propagation of ULF compressional fast-mode waves through a region. However, contrary to previous ideas, rising-tone emissions do not represent the frequency-time signatures of such waves. In addition to generation close to the equatorial plane, we propose an additional high-latitude source of QP emissions. These emissions are associated with regions of minimum B produced by the dayside compression of the magnetosphere close to the magnetopause. Model magnetic field calculations of these minimum-B regions as a function of magnetic local time and invariant latitude are presented.
TL;DR: In this paper, the authors focus on the tidal character of the currents and the relative importance of tidal flow in the general hydrodynamics of the Bransfield Strait, using a dense grid of hydrographic stations, completed during the BIOANTAR 93 cruise.
Abstract: . During the 1992-1993 oceanographic cruise of the Spanish R/V Hesperides, recording equipment was deployed in the Bransfield Strait. Six Aanderaa RCM7 current meters and three Aanderaa WLR7 tide gauges were successfully recovered after an operation period of 2.5 months. Relevant features of the time series obtained are presented and discussed in this paper. The emphasis is placed on the tidal character of the currents and the relative importance of tidal flow in the general hydrodynamics of the strait. For these purposes a dense grid of hydrographic stations, completed during the BIOANTAR 93 cruise, is used. Preliminary geostrophic calculations relative to a 400 m depth, yield current velocities of around 0.20 m s-1 in the study area, whereas the magnitude of tidal currents is seen to be 0.30-0.40 m s-1.
TL;DR: In this article, a tracer transport model coupled with the ECMWF analyses has been used to study the transport of trace gases in the atmosphere, and the model gives a reasonable description of their general transport.
Abstract: A three-dimensional off-line tracer transport model coupled to the ECMWF analyses has been used to study the transport of trace gases in the atmosphere. The model gives a reasonable description of their general transport in the atmosphere. The simulation of the transport of aircraft emissions (as NOx) has been studied as well as the transport of passive tracers injected at different altitudes in the North Atlantic flight corridor. A large zonal variation in the NOx concentrations as well as large seasonal and yearly variations was found. The altitude of the flight corridor influences the amount of tracers transported into the troposphere and stratosphere to a great extent.
Journal Article•
TL;DR: In this article, the authors define the edges of the polar vortices during one Antarctic and two Arctic winters an two stratospheric levels with the help of ECMWF global initialized analysis fields, based on isentropic total wind maxima at successive 1.5 degrees longitude bins encircling the respective pole.
Abstract: The dynamically induced stratospheric polar vortices are major factors in the formation and evolution of large chemicallyperturbed regions in both hemispheres during respective winter months. For a proper understanding of thephysicochemical processes in the polar stratospheres, it is necessary to be able to keep track of the extents andpositions of the polar vortices. This has often been done with the help of the maximum gradient of Ertel's potential vorticity(PV) on isentropic surfaces, which should bring out the edges of a polar vortex. The approaches have, however, beenmostly qualitative in nature. In this work, we define the edges of the polar vortices during one Antarctic and two Arcticwinters an two stratospheric levels with the help of ECMWF global initialized analysis fields. Our definition of a polar vortexis based on isentropic total wind maxima at successive 1.5 degrees longitude bins encircling the respective pole. Aretrieval of the isentropic PV values along the diagnosed vortex edges permits us to subsequently perform a quantitativecomparison between vortex edge defined by the total wind maxima, and the variability of isentropic PV values there. Itappears that one is able to assign discrete ranges of PV for diagnosing the edges of the polar vortices. However, suchranges exhibit variability on time scales longer than a few months, as well as between individual winters. This should betaken into account when using PV in the interpretation of experimental data.
TL;DR: Theoretical model calculations along with ground-based observations from Huancayo ionosonde station and ESRO-4 gas analyzer data were used to estimate the contribution of neutral gas composition changes and E×B vertical plasma drift to the observed F2-layer storm effects at the geomagnetic equator as mentioned in this paper.
Abstract: Theoretical model calculations along with ground-based observations from Huancayo ionosonde station and ESRO-4 gas analyzer data, were used to estimate the contribution of neutral gas composition changes and E×B vertical plasma drift to the observed F2-layer storm effects at the geomagnetic equator. Atomic oxygen concentration increase may give the main contribution to the positive NmF2 effect when drift velocity changes are small, but negative storm effects, on the other hand, are related mostly to vertical drift variations.
TL;DR: In this paper, the Schumann-Runge bands of molecular oxygen between 175 nm and 205 nm cover a spectral region of great importance for the effect of solar radiation in the middle atmosphere.
Abstract: The Schumann-Runge bands of molecular oxygen between 175 nm and 205 nm cover a spectral region of great importance for the effect of solar radiation in the middle atmosphere. The highly structured absorption cross sections that can be computed with good accuracy lead to prohibitive computer resources, however, when they have to be used in complex atmospheric models. Although various approximations have been developed, another approach is proposed to avoid spurious effects, even when the new approximations are used outside of their validity range, corresponding to a solar attenuation of the order of 10−10. Comparisons between exact computations and present approximations lead to satisfactory agreement.
TL;DR: In this article, the finite element ocean tide model was applied to the simulation of the M2 and K1 components over the South Atlantic Ocean, and the results showed that the results were in very good agreement with in situ data.
Abstract: . The finite element ocean tide model of Le Provost and Vincent (1986) has been applied to the simulation of the M2 and K1 components over the South Atlantic Ocean. The discretisation of the domain, of the order of 200 km over the deep ocean, is refined down to 15 km along the coasts, such refinement enables wave propagation and damping over the continental shelves to be correctly solved. The marine boundary conditions, from Dakar to Natal, through the Drake passage and from South Africa to Antarctica, are deduced from in situ data and from Schwiderski's solution and then optimised following a procedure previously developed by the authors. The solutions presented are in very good agreement with in situ data: the root mean square deviations from a standard subset of 13 pelagic stations are 1.4 cm for M2 and 0.45 cm for K1, which is significantly better overall than solutions published to date in the literature. Zooms of the M2 solution are presented for the Falkland Archipelago, the Weddell Sea and the Patagonian Shelf. The first zoom allows detailing of the tidal structure around the Falklands and its interpretation in terms of a stationary trapped Kelvin wave system. The second zoom, over the Weddell Sea, reveals for the first time what must be the tidal signal under the permanent ice shelf and gives a solution over that sea which is generally in agreement with observations. The third zoom is over the complex Patagonian Shelf. This zoom illustrates the ability of the model to simulate the tides, even over this area, with a surprising level of realism, following purely hydrodynamic modelling procedures, within a global ocean tide model. Maps of maximum associated tidal currents are also given, as a first illustration of a by-product of these simulations.