Herve Lamy

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.

Dust in the interplanetary medium

The mass density of dust particles that form from asteroids and comets in the interplanetary medium of the solar system is, near 1 AU, comparable to the mass density of the solar wind. It is mainly contained in particles of micrometer size and larger. Dust and larger objects are destroyed by collisions and sublimation and hence feed heavy ions into the solar wind and the solar corona. Small dust particles are present in large number and as a result of their large charge to mass ratio deflected by electromagnetic forces in the solar wind. For nanodust particles of sizes 1–10 nm, recent calculations show trapping near the Sun and outside from about 0.15 AU ejection with velocities close to solar wind velocity. The fluxes of ejected nanodust are detected near 1 AU with the plasma wave instrument onboard the STEREO spacecraft. Although such electric signals have been observed during dust impacts before, the interpretation depends on several different parameters and data analysis is still in progress.

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.

Complexity Phenomena and ROMA of the Earth's Magnetospheric Cusp, Hydrodynamic Turbulence, and the Cosmic Web

Abstract“Dynamic complexity” is a phenomenon observed for a nonlinearly interacting system within which multitudes of different sizes of large scale coherent structures emerge, resulting in a globally nonlinear stochastic behavior vastly different from that which could be surmised from the underlying equations of interaction. A characteristic of such nonlinear, complex phenomena is the appearance of intermittent fluctuating events with the mixing and distribution of correlated structures on all scales. We briefly review here a relatively recent method, ROMA (rank-ordered multifractal analysis), explicitly developed for analysis of the intricate details of the distribution and scaling of such types of intermittent structure. This method is then used for analysis of selected examples related to the dynamic plasmas of the cusp region of the Earth’s magnetosphere, velocity fluctuations of classical hydrodynamic turbulence, and the distribution of the structures of the cosmic gas obtained by use of large-scale, moving mesh simulations. Differences and similarities of the analyzed results among these complex systems will be contrasted and highlighted. The first two examples have direct relevance to the Earth’s environment (i.e., geoscience) and are summaries of previously reported findings. The third example, although involving phenomena with much larger spatiotemporal scales, with its highly compressible turbulent behavior and the unique simulation technique employed in generating the data, provides direct motivation for applying such analysis to studies of similar multifractal processes in extreme environments of near-Earth surroundings. These new results are both exciting and intriguing.

Poly-SiGe-based MEMS Xylophone Bar Magnetometer

This paper presents the design, fabrication and preliminary characterization of highly sensitive MEMS-based Xylophone Bar Magnetometers (XBMs) realized in imecs poly-SiGe MEMS technology. Key for our Lorentz force driven capacitively sensed resonant sensor are the combination of reasonably high Q-factor and conductivity of imecs poly-SiGe, our optimized multiphysics sensor design targeting the maximization of the Q-factor in a wide temperature range as well as our proprietary monolithic above-CMOS integration and packaging schemes. Prototypes 3-axis devices were fabricated and characterized. We present optical vibrometer and electrical S-parameter measurements of XBMs performed in vacuum with a reference magnet at increasing sensor separation. The optical oscillation amplitude is well correlated with the magnetic field amplitude. The electrical 2-port measurements, 1st port as Lorentz force actuator and 2nd port as capacitive sensor, also reproduces the designed magnetic field dependence. This opens the way towards the on-chip integration of small footprint extremely sensitive magnetometers.

Influence of multiphysics couplings on the performance of a MEMS magnetometer

In this paper, several physical phenomena that are usually not taken into account in MEMS simulations are considered for the simulation of a MEMS xylophone bar magnetometer. These phenomena are the temperature dependency of the material properties, the strong coupling between various fields of physics (thermal, electric and mechanical) and the stress produced by the change of temperature inside the structure. It is shown that the temperature dependency of the material properties has a relatively small influence whereas the pre-stress has a significant one. Because of the pre-stressed state, the deformation of the bar at the fundamental frequency is not a typical first mode vibration but it exhibits additional waves between the linkages, where the bar is stressed, which significantly decrease the amplitude of the deflection.

Instrumentation of the Belgian RAdio Meteor Stations (BRAMS)

BRAMS (Belgian RAdio Meteor Stations) is a network of radio receiving stations using forward scatter techniques to study the meteoroid population. The main goals are to compute meteoroid flux rates and mass indexes, determine individual meteor trajectories and orbital parameters of meteoroids, and to analyse meteor profiles in order to retrieve physical parameters of the meteoroids. For that purpose, a dedicated beacon is used together with 26 single-antenna receiving stations spread over Belgium, one dual antenna station and one interferometer. The instrumentation of this network is described in this paper.

The thermospheric auroral red line Angle of Linear Polarization

The auroral red line at 630 nm is linearly polarized. Up to now, only its Degree of Linear Polarization had been studied. In this article, we examine for the first time the Angle of Linear Polarization (AoLP) and we compare the measurements to the apparent angle of the magnetic field at the location of the red line emission. We show that the AoLP is a tracer of the magnetic field configuration. This opens new perspectives, both in the frame of space weather and in the field of planetology.

Determination of precipitating electron fluxes from inversion of coordinated ALIS/EISCAT observations

In March 2008, discrete stable auroral arcs were investigated during a coordinated observational campaign between the European Incoherent Scatter Radar (EISCAT) and the Auroral Large Imaging System (ALIS). The two sets of data are inverted to retrieve fluxes of precipitating electrons. Both results are matching in energy and shape with a typical average precipitation energy of a few keV. Finally, by using the average energy spectrum of electrons deduced from ALIS data as input for the TRANS4 kinetic/fluid electron transport 1D model, computed auroral emissions are compared to ALIS observations and electron density profiles are compared to EISCAT profiles.

BRAMS: The Belgian RAdio Meteor Stations

BRAMS (Belgian RAdio Meteor Stations) is a new facility currently developed in Belgium to detect and study meteors with radio forward scattering techniques. The transmitter is a dedicated beacon and 25 receiving stations (including one interferometer) are spread all over Belgium. The characteristics of BRAMS and the scientific goals of the project will be discussed. Preliminary results for the automatic detection and counting of meteor echoes will be presented as well as initial results obtained with the interferometer. The methods planned to calculate meteoroid flux densities and trajectories of individual meteoroids will be discussed.