I. Zouganelis

Dust Detection by the Wave Instrument on STEREO: Nanoparticles Picked up by the Solar Wind?

The STEREO wave instrument (S/WAVES) has detected a very large number of intense voltage pulses. We suggest that these events are produced by impact ionisation of nanoparticles striking the spacecraft at a velocity of the order of magnitude of the solar wind speed. Nanoparticles, which are half-way between micron-sized dust and atomic ions, have such a large charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. Since the voltage produced by dust impacts increases very fast with speed, such nanoparticles produce signals as high as do much larger grains of smaller speeds. The flux of 10-nm radius grains inferred in this way is compatible with the interplanetary dust flux model. The present results may represent the first detection of fast nanoparticles in interplanetary space near Earth orbit.

Asteroid colors: a novel tool for magnetic field detection? The case of Vesta

Aims. Vestas surface is surprisingly pristine. Although its basaltic surface is roughly similar to the lunar surface, which is intensely space weathered, its surface remains unaltered. It has been shown recently that solar-wind irradiation dominates asteroidal space weathering with a timescale on the order 10 4 -10 6 years. Recent ion irradiation experiments on pyroxenes have shown significant reddening and darkening of the collected spectra with progressive irradiation. Since pyroxene is a major surface component of Vesta as determined by spectroscopy, we aimed to test whether the solar wind irradiation alters significantly the optical properties of the surface of Vesta. Methods. Consequently, we performed an ion irradiation experiment on a eucrite meteorite, which characterizes the surface of Vesta well, in order to simulate the solar wind irradiation on this asteroid. Results. Our result implies that, if solar wind ions do reach the surface of Vesta, its reflectance spectrum should be much redder and its albedo lower. Indeed, this implies that solar wind particles can not have reached the asteroid surface. This strongly suggests the presence of a magnetic field shielding the surface from solar wind ions. This is the first remote detection of the magnetic field of an asteroid based on its color.

A TRANSONIC COLLISIONLESS MODEL OF THE SOLAR WIND

Because of the semicollisional nature of the solar wind, the collisionless or exospheric approach and the hydrodynamic one are both inaccurate. However, the advantage of simplicity makes them useful for enlightening us on some basic mechanisms of solar wind acceleration. Previous exospheric models have been able to reproduce winds that were already nearly supersonic at the exobase, the altitude above which there are no collisions. In order to allow transonic solutions, a lower exobase has to be considered, in which case the protons are experiencing a nonmonotonic potential energy profile. This is done in the present work. In this model, the electron velocity distribution in the corona is assumed to be nonthermal. Parametric results are presented and show that the high acceleration obtained does not depend on the details of the nonthermal distributions. This acceleration seems, therefore, to be a robust result produced by the presence of a sufficient number of suprathermal electrons. A method for improving the exospheric description is also given, which consists of mapping particle orbits in terms of their invariants of motion.

Acceleration of weakly collisional solar-type winds

One of the basic properties of the solar wind, the high speed of the fast wind, is still not satisfactorily explained. This is mainly due to the theoretical difficulty of treating weakly collisional plasmas. The fluid approach implies that the medium is collision dominated and that the particle velocity distributions are close to Maxwellian. However, the electron velocity distributions observed in the solar wind depart significantly from Maxwellian. Recent kinetic collisionless models (called exospheric) using velocity distributions with a suprathermal tail have been able to reproduce the high speeds of the fast solar wind. In this Letter we present new developments of these models by generalizing them over a large range of corona conditions. We also present new results obtained by numerical simulations that include collisions. Both approaches calculate the heat flux self-consistently without any assumption on the energy transport. We show that both approaches—exospheric and collisional—yield a similar variation of the wind speed with the basic parameters of the problem; both produce a fast wind speed if the coronal electron distribution has a suprathermal tail. This suggests that exospheric models contain the necessary ingredients for powering a transonic stellar wind, including the fast solar wind.

Interplanetary Nanodust Detection by the Solar Terrestrial Relations Observatory/WAVES Low Frequency Receiver

New measurements using radio and plasma-wave instruments in interplanetary space have shown that nanometer-scale dust, or nanodust, is a significant contributor to the total mass in interplanetary space. Better measurements of nanodust will allow us to determine where it comes from and the extent to which it interacts with the solar wind. When one of these nanodust grains impacts a spacecraft, it creates an expanding plasma cloud, which perturbs the photoelectron currents. This leads to a voltage pulse between the spacecraft body and the antenna. Nanodust has a high charge/mass ratio, and therefore can be accelerated by the interplanetary magnetic field to the speed of the solar wind: significantly faster than the Keplerian orbital speeds of heavier dust. The amplitude of the signal induced by a dust grain grows much more strongly with speed than with mass of the dust particle. As a result, nanodust can produce a strong signal despite its low mass. The WAVES instruments on the twin Solar TErrestrial RElations Observatory spacecraft have observed interplanetary nanodust particles since shortly after their launch in 2006. After describing a new and improved analysis of the last five years of STEREO/WAVES Low Frequency Receiver data, we present a statistical survey of the nanodust characteristics, namely the rise time of the pulse voltage and the flux of nanodust. We show that previous measurements and interplanetary dust models agree with this survey. The temporal variations of the nanodust flux are also discussed.

Quasi-thermal noise in space plasma: kappa distributions

The transport of energy in collisionless plasmas, especially in space plasmas, is far from being understood. Measuring the temperature of the electrons and their nonthermal properties can give important clues to understand the transport properties. Quasi-thermal noise (QTN) spectroscopy is a reliable tool for measuring accurately the electron density and temperature since it is less sensitive to the spacecraft perturbations than particle detectors. This work models the plasma QTN using a generalized Lorentzian (“kappa”) distribution function for the electrons. This noise is produced by the quasi-thermal fluctuations of the electrons and by the Doppler-shifted thermal fluctuations of the ions. A sum of two Maxwellian functions has mainly been used for modeling the QTN of the electrons, but the observations have shown that the electrons are better fitted by a kappa distribution function. Pioneer work on QTN calculation only considered integer values of κ. This paper extends these calculations to real values...

Interplanetary coronal mass ejection observed at STEREO-A, Mars, comet 67P/Churyumov-Gerasimenko, Saturn, and New Horizons en route to Pluto: Comparison of its Forbush decreases at 1.4, 3.1, and 9.9 AU

We discuss observations of the journey throughout the Solar System of a large interplanetary coronal mass ejection (ICME) that was ejected at the Sun on 14 October 2014. The ICME hit Mars on 17 October, as observed by the Mars Express, MAVEN, Mars Odyssey and MSL missions, 44 hours before the encounter of the planet with the Siding-Spring comet, for which the space weather context is provided. It reached comet 67P/Churyumov-Gerasimenko, which was perfectly aligned with the Sun and Mars at 3.1 AU, as observed by Rosetta on 22 October. The ICME was also detected by STEREO-A on 16 October at 1 AU, and by Cassini in the solar wind around Saturn on the 12 November at 9.9 AU. Fortuitously, the New Horizons spacecraft was also aligned with the direction of the ICME at 31.6 AU. We investigate whether this ICME has a non-ambiguous signature at New Horizons. A potential detection of this ICME by Voyager-2 at 110-111 AU is also discussed. The multi-spacecraft observations allow the derivation of certain properties of the ICME, such as its large angular extension of at least 116°, its speed as a function of distance, and its magnetic field structure at four locations from 1 to 10 AU. Observations of the speed data allow two different solar wind propagation models to be validated. Finally, we compare the Forbush decreases (transient decreases followed by gradual recoveries in the galactic cosmic ray intensity) due to the passage of this ICME at Mars, comet 67P and Saturn.

A new exospheric model of the solar wind acceleration: the transsonic solutions

This paper presents basic issues for the solar wind acceleration with a collisionless model when the base of the wind is sufficiently low for the potential energy of the protons to have a maximum, thereby producing a transsonic wind. Using a formulation in terms of the particle invariants of motion, we study the existence of different categories of ion orbits and the consequences on the wind acceleration. We also study how a suprathermal tail in the electrons velocity distribution enhances the wind acceleration and makes the electron temperature increase within a few solar radii.

A Short Review of Passive R. F. Electric Antennas as In Situ Detectors of Space Plasmas

We review the basic principles and recent or planned applications of passive, radio frequency electric antennas for in situ measurements of dusty plasmas in space. Electric antennas as passive wave detectors are reliable and versatile tools for such measurements, with the technique of Quasi Thermal Noise Spectroscopy and its generalization to dusty plasmas. The technique has been applied in the interplanetary medium, cometary plasma and dust tails, plasma environments of the Earth, Venus, Jupiter (including the Io plasma torus), Saturn (including the plasma torus and the E ring), with antennas of various shape aboard a number of spacecraft, including, most recently, Cassini. The technique is in the course of use on STEREO (NASA) in interplanetary magnetic clouds, is selected on MMO/Bepi‐Colombo (JAXA‐ESA) for in situ plasma measurements at Mercury, and planned on the Solar Orbiter (ESA) and other spacecraft projects. The diagnostic is based on the spectral analysis of the electric potential induced by the...

Quasi‐thermal noise spectroscopy: preliminary comparison between kappa and sum of two Maxwellian distributions

Quasi‐thermal noise spectroscopy has been intensively used to measure in situ the solar wind electron density and core temperature in space with various spacecraft. This method allowed study of the large‐scale properties of the solar wind. This paper reminds theoretical tools to compute the quasi‐thermal noise spectroscopy using a superposition of two Maxwellian distributions to describe the electrons, and the ones using a kappa distribution, which has been recently extended to non integer values of kappa. This paper presents an example of Ulysses data fitted with quasi‐thermal noise using a kappa and the sum of two Maxwellians. We make a preliminary comparison of the two results.