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The contribution of resonant wave-particle interactions to the formation and decay of the magnetospheric ring current is analysed in the framework of a self-consistent set of equations which take into account azimuthal plasmasphere asymmetry. It is shown that the cyclotron interaction of westward drifting energetic protons with Alfven waves in the evening-side plasmaspheric bulge region leads to the formation of a ring current asymmetry located near 18:00 MLT. The time-scale of this asymmetry is determined by the proton drift time through the plasmaspheric bulge and is about 1 - 3 h. A symmetrical ring current decays mainly due to charge exchange processes. The theory is compared with known experimental data on ions and waves in the ring current and on low-latitude magnetic disturbances. New low-latitude magnetometer data on the magnetic storm of 24 - 26 July 1986 are also discussed. The model presented explains the observed localization of an asymmetrical ring current loop in the evening sector and the difference in relaxation time-scales of the asymmetry and the <i>D<sub>st</sub></i> index. It also explains measured wave turbulence levels in the evening-side plasmasphere and wave observation statistics.
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With the atmosphere general circulation model ECHAM the passive transport of NO<sub>x</sub> emitted from global subsonic air traffic and the NO<sub>x</sub> concentration change due to these emissions are investigated. The source of NO<sub>x</sub> is prescribed according to an aircraft emission data base. The sink of NO<sub>x</sub> is parameterized as an exponential decay process with globally constant lifetime. Simulations in perpetual January and July modes are performed. Both the resulting mean and the standard deviation of the NO<sub>x</sub> mass mixing ratio are analysed. In January horizontal dispersion is more pronounced and vertical mixing is smaller than in July. In both cases the resulting quasi-stationary fields of the mass mixing ratio display a pronounced zonal asymmetry. The variability accounts up to 30% of the mean field.
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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 NO<sub>x</sub>) 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 NO<sub>x</sub> 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.
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As an introduction to this special issue of Annales Geophysicae a review is given of the applicability and performance of wind profilers. In meteorology, wind profilers play an increasingly important role. They can provide upper-wind measurements with an accuracy comparable to radiosonde data, but with a significantly higher temporal resolution. Height coverage and vertical resolution depend on the operating frequency, which, depending on the application, is usually chosen to be around 50, 400 or 1000 MHz. Measurements from stand-alone profilers in sparse data areas as well as data from wind profiler networks - which have increased the spatial resolution of an existing radiosonde network - have shown a positive impact on numerical weather forecasting. Information from nearby profilers can help the individual meterologist to optimize local short-term weather forecasts. In atmospheric research, wind profilers have been used for various boundary layer studies. In several mesoscale monitoring experiments wind profilers have played an important role. In Europe the development and implementation of wind profiler networks have been supported since 1987 by the CEC-sponsored COST-74 project. A follow-up project will probably start in 1994. Several topics deserve attention. A further assessment of the quality of wind profiler measurements is important. Automatic quality control procedures should be refined. Improved data assimilation techniques in numerical models will enable a better use of the high temporal resolution of wind profiler data. For a cost-effective development and use of wind profilers, the realization of frequency allocations, as globally harmonized as possible, is essential. Finally, the integration of wind profilers with other complementary measuring techniques is important.
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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 (A<sub>p</sub>=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<sup>-1</sup> and it increases with height, while it is about 1.0 mV m<sup>-1</sup> 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.
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A discussion is given of plasma flows in the dawn and nightside high-latitude ionospheric regions during substorms occurring on a contracted auroral oval, as observed using the EISCAT CP-4-A experiment. Supporting data from the PACE radar, Greenland magnetometer chain, SAMNET magnetometers and geostationary satellites are compared to the EISCAT observations. On 4 October 1989 a weak substorm with initial expansion phase onset signatures at 0030 UT, resulted in the convection reversal boundary observed by EISCAT (at ~0415 MLT) contracting rapidly poleward, causing a band of elevated ionospheric ion temperatures and a localised plasma density depletion. This polar cap contraction event is shown to be associated with various substorm signatures; Pi2 pulsations at mid-latitudes, magnetic bays in the midnight sector and particle injections at geosynchronous orbit. A similar event was observed on the following day around 0230 UT (~0515 MLT) with the unusual and significant difference that two convection reversals were observed, both contracting poleward. We show that this feature is not an ionospheric signature of two active reconnection neutral lines as predicted by the near-Earth neutral model before the plasmoid is "pinched off", and present two alternative explanations in terms of (1) viscous and lobe circulation cells and (2) polar cap contraction during northward IMF. The voltage associated with the anti-sunward flow between the reversals reaches a maximum of 13 kV during the substorm expansion phase. This suggests it to be associated with the polar cap contraction and caused by the reconnection of open flux in the geomagnetic tail which has mimicked "viscous-like" momentum transfer across the magnetopause.
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Since May 1992 a small, 915-MHz profiler has been operated continuously in downtown Montreal. It is a five-beam system employing a microstrip array antenna, located atop a 14-story office building that houses several academic departments of McGill University. The data are used for research on precipitation physics and the clear-air reflectivity in addition to wind profiling. We are especially interested in situations in which the reflectivities of the clear air and the precipitation are comparable. This permits the study of interactions between the precipitation and the clear air, a new area of research made possible by wind profilers. On clear days in the summer, 30-min consensus winds can often be measured to an altitude of 3 km, but ground clutter in the antenna sidelobes interferes with measurements below 600 m. Rain when present often permits wind profiling down to 100 m and up to 6 km or higher. On cold winter days there are some periods when the reflectivity is too weak at all levels to permit wind estimation. Falling snow, however, provides readily detectable echoes and serves as a good tracer of the wind and so allows profiling over its full altitude extent. The best conditions for observing interactions between precipitation and the clear air are when light rain falls through a reflective layer associated with a frontal surface or inversion. Unexpectedly, flocks of migrating birds sometimes completely dominate the signal at night in the spring and fall seasons.
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Radioacoustic sounding (RASS), normally used for temperature profiling, can also be applied for wind measurements. The RASS detects echoes of radar waves, which have been scattered at acoustic waves, and derives the sound velocity from the frequency shift. From the difference of sound velocities measured under different beam directions windprofiles can be determined. Ground clutter does not principally interfere with RASS echoes due to their big frequency shift. Therefore, RASS can supplement radar wind profilers at lower levels where clear-air echoes may be not detectable due to ground clutter. The upper measuring altitude of RASS is limited to a few thousand radar wavelengths by the sound absorption and by the drift of the focal spot of the RASS echo. A further alternative for low-level wind measurements is the Doppler sodar. It is less sensitive to ground clutter than radar, but the measuring height is also limited by sound absorption. It requires no frequency allocation and may therefore be the only choice at some locations. In Germany, Doppler sodars have been operating successfully on a routine basis for more than 10 years at several sites for environmental monitoring purposes.
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This paper presents a mechanism to explain the observed formation of a surface temperature minimum at tidal fronts in shelf seas. Tidal fronts mark the boundary between water which is kept vertically mixed by fast tidal currents and water which stratifies in summer. The fronts are associated with strong horizontal surface gradients of several water properties, including temperature. In the early studies of tidal fronts, a minimum in surface temperature was occasionally observed between the cool surface waters on the mixed side of the front and the warm surface waters on the stratified side. It was suggested that this was caused by upwelling of deep water at the front. In this paper we describe an alternative and simpler explanation based on the local balance of heating and stirring. The net heat flux into the sea in spring and early summer is greater on the mixed side of the front than on the stratified side. This happens because the heat loss mechanism is dependent on sea surface temperature and stratified waters, having a higher surface temperature, lose more heat. The stratified water near the front therefore has lower heat content (and lower depth-mean temperature) than the mixed water. If some of the stratified water becomes mixed, for example with increased tidal stirring at spring tides, a zone of minimum surface temperature will be formed at the front. A numerical model for the study of this mechanism shows that the temperature minimum at tidal fronts can be explained by the process described above. The minimum appears most clearly at spring tides, but can still be present in a weaker form at neap tides. A further prediction of the model is an increase of the horizontal temperature gradient at spring tides, which is in agreement with observations. An unexpected outcome of the modelling is the prediction of the formation of a marked sea surface temperature minimum, not yet observed, occurring in the autumn and located at the summer position of the tidal front.
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Ground-based microwave measurements of the diurnal and seasonal variations of ozoneat 42±4.5 and 55±8 km are validated by comparing with results from a zero-dimensional photochemical model and a two-dimensional (2D) chemical/radiative/dynamical model, respectively. O<sub>3</sub> diurnal amplitudes measured in Bordeaux are shown to be in agreement with theory to within 5%. For the seasonal analysis of O<sub>3</sub> variation, at 42±4.5 km, the 2D model underestimates the yearly averaged ozone concentration compared with the measurements. A double maximum oscillation (~3.5%) is measured in Bordeaux with an extended maximum in September and a maximum in February, whilst the 2D model predicts only a single large maximum (17%) in August and a pronounced minimum in January. Evidence suggests that dynamical transport causes the winter O<sub>3</sub> maximum by propagation of planetary waves, phenomena which are not explicitly reproduced by the 2D model. At 55±8 km, the modeled yearly averaged O<sub>3</sub> concentration is in very good agreement with the measured yearly average. A strong annual oscillation is both measured and modeled with differences in the amplitude shown to be exclusively linked to temperature fields.
