[1] We proposed to identify the sources of desert dust aerosols with local maxima of the TOMS aerosol index distribution averaged for the long period. Being simpler than the approach based on a dusty days occurrence, our method gives the same results. It was first shown that in spring-summer, the flux of dust from the sources located at latitude $16°N and longitude $16°E and around latitude $19°N and longitude $6°W exceed the sinks due to settling and transport. As a result the atmosphere over North Africa is almost permanently loaded with a significant amount of mineral desert dust in spring and in summer. It is also shown that the Chad basin source located around latitude 16°N and longitude 16°E is relatively more stable with a maximum activity around April. The region around latitude 19°N and longitude 6°W appears as a more variable source with maximum in July. Low pressure systems, called Sharav cyclones, mobilize the already suspended mineral dust and transport it eastward and northward along the Mediterranean basin. A new method for description of dust plumes propagation was applied to the study of dust events in the Mediterranean Sea and enabled us to follow their dynamics. Identifiable dust plumes appear first in the western sector of the sea and then move eastward with a speed of about 7 to 8 degrees per day. In spring, this motion continues at least up to the eastern coast of the Mediterranean. In summer the dustplume is prevented from penetrating further east of about 15°E.
[1] We use monthly data of aerosol optical thickness (AOT) from the Moderate Resolution Imaging Spectroradiometer on board the seven NASA Terra and Aqua satellites for a 10 year period (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) in order to determine seasonal variations of Saharan dust transport over the Mediterranean toward Europe. The maxima of AOT are used to visualize the transport paths. Saharan dust reaches Europe over the Mediterranean and also by looping back over the Atlantic. In spring, aerosols are observed within a wide range of longitudes in Europe, with the highest AOT over western Europe. This may be partially explained by dust transport to western Europe via the Atlantic route, while to central and eastern Europe, dust is transported over the Mediterranean. During all seasons, dust is transported over the Mediterranean to Europe. In the summer months, aerosols are observed predominantly in central Europe. In autumn, aerosol activity is strongest in eastern Europe. We show that there are local AOT maxima over north Italy, in the Alps, in Spain, southeast of the Pyrenees and Sierra Nevada, and in the Rila Mountains in Bulgaria. We suggest that these maxima of aerosol concentration appear as the dust-carrying airflow reaches the mountains and slows.
The annual variation of the desert dust aerosol loading above the eastern Mediterranean is studied. Three periods are identified, March–May, July–August, and September–November, for which the properties of dust particles are distinctly different. The dust layers are at higher altitudes and consist of larger particles in summer and autumn than in spring. The real part of the refractive index of the particles is the same for summer and autumn periods and exceeds the real part of the refractive index measured during the spring. The imaginary part of the refractive index is negligible both in spring and in summer, whereas the imaginary refractive index becomes significant in September–November, indicating the presence of absorbing aerosols. The difference is attributed to different sources and desert dust trajectories in these periods. In spring the desert aerosol from the source in Chad is transported to the eastern Mediterranean predominantly along the North African coast. This is the motion associated with Sharav cyclones. The aerosols come to the eastern Mediterranean via Egypt from the sources near the Red Sea in July–August. In autumn the dust arrives to the eastern Mediterranean from the Libyan coast.
[1] We present the results of space-based observations of sprites obtained during the Mediterranean Israeli Dust Experiment (MEIDEX) sprite campaign conducted on board the space shuttle Columbia during its STS-107 mission in January 2003. A total of $6 hours of useful data were saved from 21 different orbits, of which 1/5 contained lightning. We imaged sprites from an altitude of 280 km using a calibrated multispectral camera above thunderstorms in various geographical locations, mainly in central Africa, northern Australia, and South America, and also over the Pacific and Indian Oceans. In this paper we report on sprites observed from ranges 1600-2000 km from the shuttle, at altitudes of 40-90 km above the ground. Their brightness was in the range of 0.3-1.7 mega-Rayleighs (MR) in the 665 nm filter and 1.44-1.7 MR in the 860 nm filter. On the basis of the frequency of observed events and the number of tropical thunderstorms, we estimate the sprite rate in the tropics to be of the order of several per minute.
Abstract. The results of our three-dimensional multiscale magnetohydrodynamic (MHD) model describing the interaction of Saturn's magnetosphere with Titan's upper atmosphere and exosphere are presented. The effects of a conducting ionosphere, exospheric mass loading and ion-neutral charge exchange are taken into consideration. Because the ion gyroradius, in general, is not small compared to the radius of Titan the MHD approximation is not strictly applicable. However, it is still useful for the understanding of the general large-scale features of the magnetospheric interaction. The upstream parameters are set for the typical case when Titan is inside the Saturnian magnetosphere. Although Titan certainly does not have a strong intrinsic magnetic field, some observed structures in Titan's wake might be explained by assuming the existence of a small intrinsic magnetic field. Thus we also considered the possibility that Titan has a small intrinsic dipole moment. The results of the simulations are compared with Voyager measurements and provide estimates of the total mass loading and the rate of ion-neutral charge exchange. The simulation results reproduce the overall behavior of the observations, but it is clear that non-MHD effects also play a role in the interaction between Saturn's magnetosphere and Titan.
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