Simultaneity and generalized connections in general relativity

Stationary extended frames in general relativity are considered. The requirement of stationarity allows to treat the spacetime as a principal fiber bundle over the one-dimensional group of time translations. Over this bundle a connection form establishes the simultaneity between neighboring events accordingly with the Einstein synchronization convention. The mathematics involved is that of gauge theories where a gauge choice is interpreted as a global simultaneity convention. Then simultaneity in non-stationary frames is investigated: it turns to be described by a gauge theory in a fiber bundle without structure group, the curvature being given by the Fr\"olicher-Nijenhuis bracket of the connection. The Bianchi identity of this gauge theory is a differential relation between the vorticity field and the acceleration field. In order for the simultaneity connection to be principal, a necessary and sufficient condition on the 4-velocity of the observers is given.Comment: RevTeX, 9 pages, 2 figures, 1 table. Previous title "The gauge nature of simultaneity". Classical and Quantum Gravity http://www.iop.org/EJ/journal/CQ

European Research on Magnetic Nanoparticles for Biomedical Applications: Standardisation Aspects

Magnetic nanoparticles have many applications in biomedicine and other technical areas. Despite their huge economic impact, there are no standardised procedures available to measure their basic magnetic properties. The International Organization for Standardization is working on a series of documents on the definition of characteristics of magnetic nanomaterials. We review previous and ongoing European research projects on characteristics of magnetic nanoparticles and present results of an online survey among European researchers

Contribution of Ionospheric Sounding to the Study of Large Subduction Earthquakes, the Case of the Mw 7.8 Pedernales Earthquake (2016, Ecuador)

International audienceLarge earthquakes (Mw > 6.5) in the shallow subduction zone are powerful sources of destructive tsunamis. The uplift at the source related to those events also triggers coseismic ionospheric disturbances (CIDs) that are routinely detected using Global Navigation Satellite Systems (GNSS) multi-frequency measurements. Developed over the last two decades, this novel observation method offers a unique view of the rupture history of subduction earthquakes, as it sounds the area directly above the rupture. In this presentation, we first review the historical research linking the seismic source characteristics to the CIDs observed using GNSS-derived Total Electron Content (TEC) measurements. We then report on CIDs associated with the Mw 7.8 Pedernales megathrust earthquake, April 2016 in Ecuador. Using a model based on acoustic ray tracing and ionospheric coupling, we simulate the observations and show that the best fitting model allows to localize the maximum of uplift and thus, the tsunami initiation zone consecutive to the offshore rupture at depth. Finally, we highlight how ionospheric measurements by multi-GNSS receivers can contribute to better and more rapidly assess the tsunamigenic potential of an underwater earthquake