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Efforts have been made to extend the MF radar tidal profiles to E-region heights. The totally reflected MF radar echoes from E-region heights during daytime are known to be group-retarded and the corresponding wind and tidal data will have associated height discrepancies. The estimation of the E-region real heights (Namboothiri <i>et al.</i>, 1993), and the elimination of the data for which the group retardation is significant, are selected as the basic criteria to extend the tidal profiles to 100-125 km. The analysis of the quiet (A<sub>p</sub><19) days of the winter and summer seasons of 1988/89 shows that the tidal propagation continues to higher altitudes with some changes in their pattern, e.g. longer wavelengths, compared to that in the lower altitudes. Comparison with the model profiles shows some resemblance and some disagreements. The reliability of the MF radar tidal measurements of E-region heights and the propagation of tides in this region have been discussed in the light of existing theories and other experimental observations. It is concluded that, based on the initial studies with UHF and MF systems and within the limits of the available theories, the information on tides presented here for the 100-125 km region using the MF radar observations is useful. Suggestions for future work in this direction are also given.
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During the last 30 years, significant effort has been made to improve air quality through legislation for emissions reduction. Global three-dimensional chemistry-transport simulations of atmospheric composition over the past 3 decades have been performed to estimate what the air quality levels would have been under a scenario of stagnation of anthropogenic emissions per capita as in 1980, accounting for the population increase (BA1980) or using the standard practice of neglecting it (AE1980), and how they compare to the historical changes in air quality levels. The simulations are based on assimilated meteorology to account for the year-to-year observed climate variability and on different scenarios of anthropogenic emissions of pollutants. The ACCMIP historical emissions dataset is used as the starting point. Our sensitivity simulations provide clear indications that air quality legislation and technology developments have limited the rapid increase of air pollutants. The achieved reductions in concentrations of nitrogen oxides, carbon monoxide, black carbon, and sulfate aerosols are found to be significant when comparing to both BA1980 and AE1980 simulations that neglect any measures applied for the protection of the environment. We also show the potentially large tropospheric air quality benefit from the development of cleaner technology used by the growing global population. These 30-year hindcast sensitivity simulations demonstrate that the actual benefit in air quality due to air pollution legislation and technological advances is higher than the gain calculated by a simple comparison against a constant anthropogenic emissions simulation, as is usually done. Our results also indicate that over China and India the beneficial technological advances for the air quality may have been masked by the explosive increase in local population and the disproportional increase in energy demand partially due to the globalization of the economy.
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Fluid theories explain the origin of region-2 field-aligned currents as the closure of the ring current, driven itself by the azimuthal pressure gradients generated in the magnetospheric ring plasma by the sunward convection. Although the structure of pressure gradients appears experimentally complex, observations confirm that a close connection exists between the region-2 field-aligned currents and the ring current. The fluid linear theory of the adiabatic transport by convection of the ring plasma gives a first estimate of this process, and leads ultimately to phase quadrature (in terms of magnetic local time) between the region-2 field-aligned currents and the convection potential. When significant non-adiabatic processes are taken into account, such as precipitations at auroral latitudes, the theoretical phase difference rotates toward opposition. We determine experimentally the phase relationship between the region-2 field-aligned currents and the convection potential from recent statistics, depending on the magnetic activity index <i>K<sub>p</sub></i>, and performed from the EISCAT data base. For geometrical reasons of sufficient probing of region 2, it is only computed in the case of a moderate magnetic activity corresponding to 2 ≤ <i>K<sub>p</sub></i><4. Region-2 field-aligned currents are found to be in phase opposition with the convection electrostatic potential at auroral latitudes. This confirms the importance of non adiabatic processes, especially ion losses, in the generation of region-2 field-aligned currents, as theoretically suggested.
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A numerical procedure has been developed to deduce the plasma pressure and anisotropy from the Tsyganenko magnetic field model. The Tsyganenko empirical field model, which is based on vast satellite field data, provides a realistic description of magnetic field configuration in the magnetosphere. When the force balance under the static condition is assumed, the electromagnetic <strong>J</strong>×<strong>B</strong> force from the Tsyganenko field model can be used to infer the plasma pressure and anisotropy distributions consistent with the field model. It is found that the <strong>J</strong>×<strong>B</strong> force obtained from the Tsyganenko field model is not curl-free. The curl-free part of the <strong>J</strong>×<strong>B</strong> force in an empirical field model can be balanced by the gradient of the isotropic pressure, while the nonzero curl of the <strong>J</strong>×<strong>B</strong> force can only be associated with the pressure anisotropy. The plasma pressure and anisotropy in the near-Earth plasma sheet are numerically calculated to obtain a static equilibrium consistent with the Tsyganenko field model both in the noon-midnight meridian and in the equatorial plane. The plasma pressure distribution deduced from the Tsyganenko 1989 field model is highly anisotropic and shows this feature early in the substorm growth phase. The pressure anisotropy parameter α<i><sub>P</sub></i>, defined as α<i><sub>P</sub></i>=1-<i>P</i><sub>Vert</sub><i>P</i><sub>&bottom;</sub>, is typically ~0.3 at <i>x</i> ≈ -4.5<i>R<sub>E</sub></i> and gradually decreases to a small negative value with an increasing tailward distance. The pressure anisotropy from the Tsyganenko 1989 model accounts for 50% of the cross-tail current at maximum and only in a highly localized region near <i>x</i>sim-10<i>R<sub>E</sub></i>. In comparison, the plasma pressure anisotropy inferred from the Tsyganenko 1987 model is much smaller. We also find that the boundary conditions have significant effects on the plasma pressure distributions and have to be considered carefully.
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An analysis of interplanetary magnetic field (IMF) and plasma data taken near 1 AU during solar activity cycle 21 reveals the following. 1. The yearly averaged spiral angle shows a solar cycle dependence. 2. The spiral angle north of the current sheet is 2.4° higher than south of it during both epochs of positive and negative polarities. 3. The included angle is 4.8° higher during the epoch of positive polarity than during the epoch of negative polarity. 4. The asymmetries in the number of away and toward IMF days are correlated with the asymmetries in solar activity. 5. The solar plasma north of the current sheet is hotter, faster and less dense than south of it during the epoch of negative polarity. 6. An asymmetry in the averaged filed magnitude is absent for solar cycle 21.
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Results from a mathematical model provide a description of the mid-latitude, low <i>L</i>-shell ionosphere and plasmasphere. Variations in the composition and dynamics of the plasmasphere and changes in the nature of the coupling between the plasmasphere and the ionosphere are studied for moderately disturbed conditions. Modelled results are compared to group delay and Doppler shift measurements of whistler mode signals at Faraday, Antarctica (<i>L</i> ≈ 2.5), to investigate the effects of disturbed time electric fields on the inner plasmasphere and ionosphere. The disturbed time electric field causes a rapid outward drifting of the plasma leading to a decrease in modelled plasmaspheric electron density at a fixed <i>L</i>-value, which agrees with experimental observations. During the periods of outward drift, the modelled coupling flux is upwards to the plasmasphere which can lead to a significant depletion of <i>N</i><sub>m</sub>F2 values.
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The local effects of the emission of a solid-fuelled rocket on the stratospheric ozone concentration have been investigated by photochemical model calculations. A one-dimensional horizontal model has been applied which calculates the trace gas composition at a single atmospheric altitude spatially resolved around the exhaust plume. Different cases were tested for the emissions of the Space Shuttle concerning the composition of the exhaust and the effects of heterogeneous reactions on atmospheric background aerosol. <p style="line-height: 20px;">The strongest depletion of ozone is achieved when a high amount of the emitted chlorine is Cl<sub>2</sub>. If it is purely HCl, the effect is smallest, though in this case the heterogeneous reactions show their largest influence. From the results it may be estimated whether ozone depletion caused by rocket launches can be detected by satellite instruments. It appears that the chance of coincidental detection of such an event is rather small.
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Recent discussions on high-speed civil transport (HSCT) systems have renewed the interest in the chemistry of supersonic-aircraft plumes. The engines of these aircraft emit large concentrations of radicals like O, H, OH, and NO. In order to study the effect of these species on the composition of the atmosphere, the detailed chemistry of an expanding and cooling plume is examined for different expansion models. <p style="line-height: 20px;">For a representative flight at 26 km the computed trace gas concentrations do not differ significantly for different models of the expansion behaviour. However, it is shown that the distributions predicted by all these models differ significantly from those adopted in conventional meso-scale and global models in which the plume chemistry is not treated in detail. This applies in particular to the reservoir species HONO and H<sub>2</sub>O<sub>2</sub>.
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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 CO<sub>2</sub>" 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.
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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.
