G. Belmont

NEW INSIGHT INTO SHORT-WAVELENGTH SOLAR WIND FLUCTUATIONS FROM VLASOV THEORY

The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is being investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfven waves (KAWs) at sub-ion scales with ω ωci) is more relevant. Here, we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically β i β e ~ 1 and for high oblique angles of propagation 80° ≤ Θ kB ωci) or KAW (ω < ωci), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW.

Scaling of the Electron Dissipation Range of Solar Wind Turbulence

Electron scale solar wind (SW) turbulence has attracted great interest in recent years. Considerable evidence exists that the turbulence is not fully dissipated near the proton scale, but continues cascading down to electron scales. However, the scaling of the magnetic energy spectra as well as the nature of the plasma modes involved at those small scales are still not fully determined. Here we survey 10xa0yr of the Cluster STAFF search-coil magnetometer waveforms measured in the SW and perform a statistical study of the magnetic energy spectra in the frequency range [1, 180]xa0Hz. We found that 75% of the analyzed spectra exhibit breakpoints near the electron gyroscale ρ e , followed by steeper power-law-like spectra. We show that the scaling below the electron breakpoint cannot be determined unambiguously due to instrumental limitations that we discuss in detail. We compare our results to those reported in other studies and discuss their implications for the physical mechanisms involved and for theoretical modeling of energy dissipation in the SW.

Proton acceleration in antiparallel collisionless magnetic reconnection: Kinetic mechanisms behind the fluid dynamics

[1]xa0This paper investigates the proton kinetic mechanisms leading to the formation of plasma jets in antiparallel magnetic reconnection. In particular, the interaction of the protons with the Hall electric field in the proton non-ideal region is discussed. The study, based on a two-dimensional hybrid simulation, details the important role of the proton pressure force in the acceleration process and its role in maintaining open and steady the proton outflow channel. When no fluid closure is assumed, it is found that this force arises from a strong anisotropy in velocity space which comes from kinetic effect. By analyzing the distribution functions and the individual particle dynamics, it is shown that the mixing of protons bouncing in a divergent electrostatic potential well associated to the Hall effect statistically couples the two in-plane velocity components of the particles. This coupling results, from the macroscopic point of view, in off-diagonal components of the pressure tensor.

Kinetic equilibrium for an asymmetric tangential layer

Finding kinetic (Vlasov) equilibria for tangential current layers is a long standing problem, especially in the context of reconnection studies, when the magnetic field reverses. Its solution is of pivotal interest for both theoretical and technical reasons when such layers must be used for initializing kinetic simulations. The famous Harris equilibrium is known to be limited to symmetric layers surrounded by vacuum, with constant ion and electron flow velocities, and with current variation purely dependent on density variation. It is clearly not suited for the “magnetopause-like” layers, which separate two plasmas of different densities and temperatures, and for which the localization of the current density j=nδv is due to the localization of the electron-to-ion velocity difference δv and not of the density n. We present here a practical method for constructing a Vlasov stationary solution in the asymmetric case, extending the standard theoretical methods based on the particle motion invariants. We show ...

Energy budgets in collisionless magnetic reconnection: Ion heating and bulk acceleration

This paper investigates the energy transfer in the process of collisionless antiparallel magnetic reconnection. Using two-dimensional hybrid simulations, we measure the increase of the bulk and thermal kinetic energies and compare it to the loss of magnetic energy through a contour surrounding the ion decoupling region. It is shown, for both symmetric and asymmetric configurations, that the loss of magnetic energy is not equally partitioned between heating and acceleration. The heating is found to be dominant and the partition ratio depends on the asymptotic parameters, and future investigations will be needed to understand this dependence.

POSSIBLE ROLE OF ELECTROMAGNETIC LOW FREQUENCY WAVES IN THE IO TORUS IN THE PRODUCTION OF JOVIAN AURORAE

Abstract The possible role of ULF waves observed in the Io torus in the generation of Jovian autorae is investigated. Aaroral emissions have been observed, not only at high latitude, but also at the footprint of magnetic field lines connected to the Io torus in a limited loagitudinal range around 150°. As Ulysses passed by Jupiter, it explored the Io torus in latitude and longituds, allowing to test x possible localization of the waves on field lines mapping down into the surors. A maximum intensity of the waves is observed in the high latitude part of the torus around the longitude where the surorae were observed. Another maximum is also observed at the magnetic equator. The propagation mode of the waves is magnetic equator. The propagation mode of the values necessary for scattering ions, showing that the ULF waves could possibly be at the origin of the aurorae.

Power of magnetopause low‐frequency waves: A statistical study

[1]xa0ULF fluctuations at the Earth magnetopause have been the subject of a long-standing interest because of their noticeable amplitude and their potential role in plasma penetration through the magnetosphere boundary. Some information on the nature of these fluctuations has already been provided thanks to GEOS and ISEE, and more recently Cluster, case studies and from a few statistical studies on magnetopause crossings. Here we present a new statistical study concerning the power of the ultralow frequency fluctuations based on the data of the Cluster-STAFF experiment. New insight in the properties of turbulence is obtained, because of the orbit of the Cluster spacecraft, which gives access to regions far from the subsolar point. The correlation of the wave power with solar wind and magnetosheath properties indicates that these fluctuations are controlled and possibly driven by external sources. Its correlation with the magnetopause properties (namely the magnetic shear angle) provides an indication of an amplification process at the boundary.

On the relationship between quadrupolar magnetic field and collisionless reconnection

Using hybrid simulations, we investigate the onset of fast reconnection between two cylindrical magnetic shells initially close to each other. This initial state mimics the plasma structure in High Energy Density Plasmas induced by a laser-target interaction and the associated self-generated magnetic field. We clearly observe that the classical quadrupolar structure of the out-of-plane magnetic field appears prior to the reconnection onset. Furthermore, a parametric study reveals that, with a non-coplanar initial magnetic topology, the reconnection onset is delayed and possibly suppressed. The relation between the out-of-plane magnetic field and the out-of-plane electric field is discussed.

Landau and Non-Landau Linear Damping: Physics of the Dissipation

The linear Landau effect is revisited by the means of numerical simulations and analytical calculations. The existence of non-Landau solutions to the Vlasov-Poisson system is emphasized and the consistency of these solutions with respect to the arguments based on energy is investigated. The present article briefly summarizes the content of two articles already published on the subject and introduces a discussion based on the exchanges that occurred at Marseille during the Vlasovia meeting.

Correction to “ULF wave identification in the magnetosheath: The k‐filtering technique applied to Cluster II data”

[1] In the results presented in ‘‘ULF wave identification in the magnetosheath: The k-filtering technique applied to Cluster II data’’ by Sahraoui et al. (Journal of Geophysical Research, 108(A9), 1335, doi:10.1029/2002JA009587, 2003), there is an error in the matrix transformation from the GSE frame to the MFA (Magnetic Field-Aligned) one: the z axis of the adopted frame was not really aligned with the local magnetic field. The MFA frame has in fact to be corrected with respect to the old one by the following angles between the axes: (ex, ex 0 ) = 3 , (ey, ey 0 ) = 179 and (ez, ez 0 ) = 167 . Because of this angle shift, some of the characteristics of the identified waves are slightly modified. This concerns particularly the propagation angles with respect to B0. In the figure below, which is to be compared with Figure 7a in the original paper, we illustrate such a modification concerning the dominant mirror mode previously identified: the modulus of the wave vector remains unchanged k 0.012 rd/km; however, its direction is now 80 with respect to B0 instead of 62 given previously. Nevertheless, all the main physical conclusions brought by the paper (mixture of the LF linear modes with the predominance of the mirror mode, importance of the Doppler shift, . . .) remain fully valid. The error in the rotation matrix was discovered thanks to a comparative study done on the same data by S. Walker and M. A. Balikhin, to whom we are grateful.