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The main feature of this storm is an observed large enhancement in the F region electron density above Millstone Hill, the so-called dusk effect. Results show that the effects of the electric fields and height variations in the neutral wind associated with a traveling atmospheric disturbance played a crucial role in the formation of the observed dusk effect. Advection of higher-density plasma from the south apparently also contributed. Vibrationally excited N2 and 02 are found to be important in simulating the electron density during these disturbed summer solar maximum conditions. It has not been found necessary to change the MSIS-86 neutral density in order to bring about good agreement between the observed and calculated NmF2 and HmF2.

Abstract. This presentation concentrates on solar occultation measurements with the spaceborne spectrometer SCIA-MACHY in the UV-Vis wavelength range. Solar occultation measurements provide unique information about the vertical distribution of atmospheric constituents. For retrieval of vertical trace gas concentration profiles, an algorithm has been developed based on the optimal estimation method. The forward model is capable of simulating the extinction signals of different species as they occur in atmospheric transmission spectra obtained from occultation measurements. Furthermore, correction algorithms have been implemented to address shortcomings of the tangent height pre-processing and inhomogeneities of measured solar spectra. First results of O 3 and NO 2 vertical profile retrievals have been validated with data from ozone sondes and satellite based occultation instruments. The validation shows very promising results for SCIAMACHY O 3 and NO 2 values between 15 to 35 km with errors of the order of 10% and 15%, respectively.

Abstract.The Millstone Hill Ionospheric Model is used to simulate the April 10-12, 1997 ionospheric storm. For the first time determinations of the heightvarying wind from incoherent scatter radar measurements are used to simulate the ionospheric plasma. Results indicate that the observed dusk effect is due to a rise in HmF2 combined with an enhanced downward flux of ionization, associated with an altitude variation in the neutral wind. The negative phase on April 11 is fully explained by increased recombination due to the increased neutral composition ratios N2/O and 02/0 given by the MSIS model, except that reproduction of the observed G-condition during the morning hours requires these ratios to be larger than those in MSIS-86. This is consistent with the concept of a composition disturbance zone rotating into the morning sector following a storm which commenced during the nighttime hours.

Abstract.ElSCAT observations of thin layers of enhanced ionization and elevated electron temperature during auroral precipitations, similar to the phenomenon of the 'Enhanced Aurora', are presented and described. Being well narrower than those predicted by the conventional collisional approach, the layers are likely caused by plasma turbulence generated by precipitating electrons and field-aligned currents in the altitude range 110-170 km. The results are in a qualitative agreement with the model of the plasma turbulence layer which takes the influence of collisions upon the beam-plasma interaction into account.