Nanoparticles in the solar wind |
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Nanoparticles in the solar wind
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 S/WAVES instrument on board the STEREO spacecraft, using a radio receiver built by the Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique in Paris Observatory.
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.
What is the origin of this high speed? Dust grains carry an electric charge in the interplanetary medium because they eject much more photoelectrons due to solar ionising radiation than they collect charges from the ambient plasma. They are thus subjected to the electromagnetic force due to the magnetic field carried by the solar wind. For nanoparticles, this Lorentz force is much greater than the solar gravitational attraction and other forces. The charge-to-mass ratio of nanoparticules (3), which determines the importance of electromagnetic forces compared to gravitational ones, is not as high as for atomic ions, but it is high enough to make the Larmor frequency of nanoparticles much greater than their orbital frequency around the Sun. As a result, they tend to gyrate around the lines of force of the magnetic field carried by by the solar wind, which accelerates them to a speed of several hundred kilometres per second.
When a dust grain hits a spacecraft at such a high speed, it produces a microcrater whose material is vaporised and ionised, producing an expanding plasma cloud (Figure 1).
Figure 1: Sketch of the expanding plasma cloud produced by a nanoparticle impacting the spacecraft STEREO A at a speed of several hundred kilometres per second.
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These transient plasma clouds produce electric pulses which are detected by the radio receiver S/WAVES at the ports of the electric antennas (Figure 2). Because the amount of produced plasma increases very fast with the impact speed, the detected power is as high as that produced by dust grains which are much larger but move more slowly. Furthermore, the configuration of the STEREO antennas favors this detection.
Figure 2: Power spectral density measured in several frequency bands of the receiver S/WAVES, produced by impacts of nanoparticles (in red). The spectral amplitude is much greater than the quasi-thermal noise of the solar wind plasma (in blue, from Meyer-Vernet and Perche 1989), and has a very different shape. The insert shows the transient voltage produced by an individual impact.
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Similar electric signals were measured on board several space probes, in the dusty rings of Saturn, Uranus and Neptune, and in the environment of comet Halley, and were produced by microsized dust impacting spacecraft at several tens of kilometres per second. But in the interplanetary medium there is not enough microsized dust to explain STEREO observations. In contrast, since the dust flux increases with decreasing mass, these observations are consistent with the flux of nanoparticles implied by dust flux models (Figure 3).
Streams of fast nanoparticles ejected by Jupiter and Saturn were detected near these planets by conventional dust detectors (below their calibration range), and we have shown that these streams are detected, too, by the radio instrument (RPWS) on board the spacecraft Cassini. The present results are the first detection in the solar wind around 1 AU (2) from the Sun of nanoparticules, presumably originating from the inner solar system.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.
References
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)
