Nanoparticles in the solar wind
1er mars 2009
An international team, led by astronomers from Paris Observatory has found evidence for a new population of particles in the interplanetary medium : nanoparticles, accelerated at several hundred kilometres per second by the magnetic field carried by the solar wind. This serependipitous discovery is based on data from the
Nanoparticules - of size between 1 and 100 nanometres (1), lie at the frontier between macroscopic objects and atomic structures. Their small size gives them a privileged role, in particular for surface interactions, since they have a high surface area to volume ratio so that they may behave differently from bulk materials. They are difficult to detect in space because they are outside the calibration domain of conventional dust detectors. Their first detection in the interplanetary medium at 1 AU (2) from the Sun was made possible by their high speed : about 300 km/s, of the order of the solar wind speed, and roughly 10 times more than that of microsized dust at this distance from the Sun. The SWAVES instrument (P.I. J.-L. Bougeret) on the STEREO mission is a joint project of LESIA (INSU-CNRS and Universities of Paris 6 and Paris 7) at Paris Observatory, NASA/GSFC, University of Minnesota (USA) and University of California (USA). It is dedicated to the study of solar radio emissions and to in situ measurement of electrostatic fields. The wave receivers were designed, built, and tested at LESIA in Paris Observatory, with the support of CNES and CNRS. (1) A nanometer (nm) is equal to one billionth of a meter. This is the scale of very large molecules, roughly one million times smaller than the head of a pin. (2) The astronomic unit (AU) is the mean distance from the Earth to the Sun, nearly 150 millions km. (3) The electromagnetic force of a dust grain is proportional to its electric charge and to the product of its velocity in the solar wind frame by the magnetic field component normal to this velocity. The electric charge of a dust particle is roughly proportional to its surface. The charge-to-mass ratio, which determines the ratio between electromagnetic and gravitational forces is therefore inversely proportional to the size. It is thus much greater for nanoparticles than for micro-sized dust. Dust detection by the wave instrument on STEREO : nanoparticles picked up by the solar wind ? N. Meyer-Vernet, M. Maksimovic, A. Czechowski, I. Mann, I. Zouganelis, K. Goetz, M. L. Kaiser, O. C. St. Cyr, J.-L. Bougeret, S. D. Bale, Solar Phys. 2009 (in press). Detecting nanoparticles at radiofrequencies : Jovian dust stream impacts on Cassini/RPWS N. Meyer-Vernet, A. Lecacheux, M. L. Kaiser, D.A. Gurnett, 2009, Geophys. Res. Lett. 36, L03103 Contact Nicole Meyer-Vernet, (Observatoire de Paris, LESIA, and CNRS)
Dernière modification le 4 mars 2013