Interview of the investigators
Nicole Meyer-Vernet, an international team, led by yourself and LESIA, discovered a new type of dusts in the interplanetary medium, what is the peculiarity of these dusts?
They are nanoparticles. Their size ranges from 1 to 100 nanometers. One nanometer represents one billionth of a meter, i.e. approximately one millionth times smaller than a pinhead. These nanoparticles are between atomic structures and macroscopic objects. This size gives them specific properties compared to macroscopic materials. Into space, they are difficult to detect because they are outside the standard dust detectors' calibration range.
If they just got discovered in the interplanetary medium, 1 AU away from the Sun (approximately 150 million kilometers), it is because of their very high speed: around 300 km/s, i.e. approximately the solar wind speed and 10 times that of the standard micro-dusts at such distance from the Sun.
Where does this speed come from?
Under the effect of solar radiation, dust grains tend to knock much more electrons out than they collect ambient loads of plasma. So they are electrically charged and then submitted to the electromagnetic force resulting from the solar wind-carried magnetic field. For nanoparticles, this electromagnetic force is much higher than the Sun's gravitational tug and other forces such as the radiation pressure. The load/mass ratio of nanoparticles, which translates the importance of the electromagnetic forces compared to gravitational ones, is not as big as the one of atomic ions but it allows the nanoparticles to have a Larmor frequency much higher than their Sun orbital frequency. They thus tend to move around the force lines of the solar wind-carried magnetic field and become faster with speeds up to several hundreds of kilometers per second.
Could you explain the method you used to detect these dust particles?
When a dust grain crashes into a space probe at such speeds, it creates a microcrater by ejecting and ionizing the material which form an expanding plasma cloud. These plasma clouds produce electric field pulses. A radio receptor placed at the power antenna terminals of the S/WAVES experiment detect these pulses. Since the plasma quantity increases very fast depending on the impact speed, the power load detected is as important as that of much bigger and slower dust grains.
This method is as a complement for standard dust detectors because the detection surface is much wider (it is generally that of the probe itself) and usually does not require any specific orientation. This method provided the first in situ diagnosis of the dust rings of Saturn, Uranus and Neptune on board the Voyager probes.
Could this quick flux of nanoparticles have an negative impact on orbiting satellites?
This discovery is far too recent to can answer this question with certainty. Nanoparticles seem to have been observed in the near terrestrial environment by the International Space Station but their impact is probably too low to present a risk for the surfaces, even if the electric shock production cannot be excluded.
J.L. Bougeret, could you remind us what is the STEREO mission and what is the role of the WAVES instrument?
The STEREO mission is part of the STP (Solar Terrestrial Probes Programme) international science programme to study the Earth-Sun relations. It is a set of two NASA's satellites the instruments of which were designed and developed by American and European scientists. It was launch on October 26th, 2006.
The STEREO mission provides information on solar flares and their effects on the Earth's environment. In order to obtain stereoscopic images of the solar flares, one of the probes orbits the Sun ahead the Earth while the other orbits our star behind the Earth. The images obtained by the satellites are then combined with in situ data as well as ground observation and low orbiting satellite data to make the 3D-monitoring of the magnetic energy storage, ejection as well as the coronal matter path in the interplanetary medium possible.
Dust detection by the wave instrument on STEREO: nanoparticles picked up by the solar wind? N. Meyer-Vernet1, M. Maksimovic1, A. Czechowski3, I. Mann4, I. Zouganelis1 2, K. Goetz5, M.L. Kaiser6, O.C. St. Cyr6, J. L. Bougeret1, S. D. Bale7, Solar Physics, 11 april 2009.
1 LESIA France
2 LPP France
3 Space Research Centre Pologne
4School of Science and Engineering Japon
5 School of Physics and Astronomy Usa
6 NASA Usa
7 Space Sciences Laboratory Usa