Studies of compositional variations in the thermosphere and ionosphere using far-ultraviolet images from DE-1

Thesis (Ph.D.) University of Alaska Fairbanks, 1998The Dynamics Explorer mission returned a wealth of information from its two orbiting platforms. Of interest here are the three scanning photometers aboard the high-altitude platform DE-1, which obtained hundreds of thousands of global images of Earth, beginning in September of 1981, while using broad- and narrow-band filters to isolate particular terrestrial emissions. The far-ultraviolet (FUV) emissions include the line emissions of OI (130.4 and 135.6 nm) and the band emissions of \rm N\sb2 LBH, the brightness of each yielding information on the composition of the upper atmosphere. The OI emissions are related to the column density of atomic oxygen in the upper-atmosphere as well as the abundance of thermospheric \rm N\sb2, both of which are affected by geomagnetic processes. This thesis presents a model of the DE-1 response to the OI emissions during periods of low geomagnetic activity and uses this model for studies of thermospheric response to geomagnetic storms and substorms. Variations in brightness observed after geomagnetic events are most often seen as decreases corresponding to reduced thermospheric O column densities. The relation between compositional variations in the morning sector at middle latitudes and the orientation of the magnetic field embedded in the solar wind is investigated. The orientation, which strongly affects the circulation of the thermosphere at high latitudes where these variations originate, is shown to be a significant parameter. Variations in brightness within the southern polar cap are investigated in the first study of its kind, demonstrating 20-30% decreases in brightness with the onset of magnetic activity and revealing structure in composition over distances on the order of ${\sim}300$ km. Compositional disturbances are observed immediately after heating takes place, demonstrating for the first time that an FUV instrument can detect changes in thermospheric composition on time scales under one hour. During these events, mid-latitude composition often remains relatively unperturbed. The first survey of FUV images to include ground-based measurements of ionospheric properties demonstrates that decreases in OI brightness correspond to decreases in peak F2 electron densities, known to be related to the ratio of the densities of O and $\rm N\sb2.

Bactericidal effect of titanium dioxide nanoparticles: an overview of the toxicity mechanism

International audienceBactericidal effects of NP-TiO2 under dark condition are still the subject of debate. As an example, some authors found that NP-TiO2 have no effect on the laboratory model strain E. coli while others found for this strain EC50 values differing from 10 magnitude orders. Such contradictory results may be explained by multiple causal factors, including differences in the intrinsic characteristics of NP-TiO2 and bacterial cells used in these studies, exposure conditions for toxicity assessment and even the method used to assess the toxicity, the latter being often inappropriate for nanoparticle toxicity assessment. Increasing knowledge about exposition parameters which trigger NP toxicity and associated molecular mechanisms would contribute to properly evaluate and predict NP effects/fate in the environment.In our study, analysis of the interactions between nanoparticles (NPs) and bacteria highlighted the paramount role played by interfacial electrostatic interactions (NP-cell vs. NP-NP interactions) in determining the extent of NP toxicity and the importance of physico-chemical parameters such as pH and ionic strength in controlling these interactions. In condition of interaction, we showed that nanoparticles causes E.coli membrane depolarization and loss of membrane integrity leading to higher cell permeability. A transcriptomic analysis highlighted that deregulated genes are involved in the response to osmotic stress, metabolism of various cell envelope components and the uptake/metabolism of other endogenous and exogenous compounds. In addition, a significant number of deregulated genes encode proteins localized in the membrane and periplasmic space. All in all, results indicate that the primary effect of NP-TiO2 is initiated at the cell envelope level (membrane depolarization, loss of integrity) triggering an osmotic stress response in bacteria. These results are supported by the observed massive leakage of intracellular K+/Mg