Vincent Génot

Multispacecraft observation of magnetic cloud erosion by magnetic reconnection during propagation

During propagation, Magnetic Clouds (MC) interact with their environment and, in particular, may reconnect with the solar wind around it, eroding away part of its initial magnetic flux. Here we quantitatively analyze such an interaction using combined, multipoint observations of the same MC flux rope by STEREO A, B, ACE, WIND and THEMIS on November 19-20, 2007. Observation of azimuthal magnetic flux imbalance inside a MC flux rope has been argued to stem from erosion due to magnetic reconnection at its front boundary. The present study adds to such analysis a large set of signatures expected from this erosion process. (1) Comparison of azimuthal flux imbalance for the same MC at widely separated points precludes the crossing of the MC leg as a source of bias in flux imbalance estimates. (2) The use of different methods, associated errors and parametric analyses show that only an unexpectedly large error in MC axis orientation could explain the azimuthal flux imbalance. (3) Reconnection signatures are observed at the MC front at all spacecraft, consistent with an ongoing erosion process. (4) Signatures in suprathermal electrons suggest that the trailing part of the MC has a different large-scale magnetic topology, as expected. The azimuthal magnetic flux erosion estimated at ACE and STEREO A corresponds respectively to 44% and 49% of the inferred initial azimuthal magnetic flux before MC erosion upon propagation. The corresponding average reconnection rate during transit is estimated to be in the range 0.12-0.22 mV/m, suggesting most of the erosion occurs in the inner parts of the heliosphere. Future studies ought to quantify the influence of such an erosion process on geo-effectiveness. ©2012. American Geophysical Union. All Rights Reserved.

Electron acceleration by Alfvén waves in density cavities

A new electromagnetic two-dimensional guiding center particle in cell (PIC) code is used to investigate the propagation of an Alfven wave in the perpendicular density gradients that characterize the edges of the auroral density cavities. It is shown that the wave planes are strongly distorted by the inhomogeneities of the plasma and that a significant electric field develops in the direction parallel to the background magnetic field during the propagation. This field efficiently accelerates the electrons in the parallel direction, and the incident wave is thus strongly absorbed. The associated dissipation rate is sufficiently strong to explain a complete wave absorption on the density gradients over a fraction of wavelength. The electron parallel acceleration is also characterized. It corresponds to a global parallel acceleration of the electron population. These PIC simulations suggest that the perpendicular density gradients corresponding to the auroral plasma cavities play an important role in the auroral particle acceleration.

Currents and associated electron scattering and bouncing near the diffusion region at Earth's magnetopause

Based on high-resolution measurements from NASAs Magnetospheric Multiscale mission, we present the dynamics of electrons associated with current systems observed near the diffusion region of magnetic reconnection at Earths magnetopause. Using pitch angle distributions (PAD) and magnetic curvature analysis, we demonstrate the occurrence of electron scattering in the curved magnetic field of the diffusion region down to energies of 20 eV. We show that scattering occurs closer to the current sheet as the electron energy decreases. The scattering of inflowing electrons, associated with field-aligned electrostatic potentials and Hall currents, produces a new population of scattered electrons with broader PAD which bounce back and forth in the exhaust. Except at the center of the diffusion region the two populations are collocated and appear to behave adiabatically: the inflowing electron PAD focuses inward (toward lower magnetic field), while the bouncing population PAD gradually peaks at 90° away from the center (where it mirrors owing to higher magnetic field and probable field-aligned potentials).

Kinetic study of the mirror mode

The linear Vlasov dispersion is solved to reveal the kinetic properties of the mirror mode. The existence of the torsional component of both the magnetic and velocity perturbations are kinetic features that are not explained by MHD theory. A parameter study is then employed to clarify the behavior of these particular components under different plasma conditions. The key parameters are the ion temperature anisotropy and the plasma to magnetic pressure ratio which act in similar ways to control the value of the torsional (or Alfvenic) components. Data from the Active Magnetospheric Particle Tracer Explorers mission show that under most magnetosheath plasma conditions the magnetic torsional component is negligible, whereas the velocity one is always larger or comparable to the other components. This experimental illustration also shows that there exist elliptically polarized mirror modes, contrary to what our, and all previous, analytical treatments predict.

Self‐Reformation of the Quasi‐Perpendicular Shock: CLUSTER Observations

Among several mechanisms issued from simulation and theoretical studies proposed to account for the nonstationarity of quasi-perpendicular supercritical shocks, one process—the so-called self-reformation—driven by the accumulation of reflected ions in the foot has been intensively analyzed with simulations. Present results based on experimental CLUSTER mission clearly evidence signatures of this self-reformation process for the terrestrial bow shock. The study based on magnetic field measurements includes two parts: (i) a detailed analysis of one typical shock crossing for almost perpendicular shock directions where the risk of pollution by other nonstationarity mechanisms is minimal. A special attention is drawn on appropriate treatment of data to avoid any wrong interpretation. One key result is that the ramp width can reach a very narrow value covering a few electron inertial lengths only; (ii) a statistical analysis allows relating the signatures of this nonstationarity with different plasma conditions and shock regimes. Present results are discussed in comparison with previous simulation works.

Tracing solar wind plasma entry into the magnetosphere using ion-to-electron temperature ratio

When the solar wind Mach number is low, typically such as in magnetic clouds, the physics of the bow shock leads to a downstream ion-to-electron temperature ratio that can be notably lower than usual. We utilize this property to trace solar wind plasma entry into the magnetosphere by use of Cluster measurements in the vicinity of the dusk magnetopause during the passage of a magnetic cloud at Earth on November 25, 2001. The ion-to-electron temperature ratio was indeed low in the magnetosheath (Ti/Te ∼ 3). In total, three magnetopause boundary layer intervals are encountered on that day. They all show that the low ion-to-electron temperature ratio can be preserved as the plasma enters the magnetosphere, and both with and without the observation of Kelvin-Helmholtz activity. This suggests that the ion-to-electron temperature ratio in the magnetopause boundary layer, which is usually high, is not prescribed by the heating characteristics of the plasma entry mechanism that formed these boundary layers. In the future, this property may be used to (1) further trace plasma entry into inner regions and (2) determine the preferred entry mechanisms if other theoretical, observational and simulation works can give indications on which mechanisms may alter this ratio.

Fast evolving spatial structure of auroral parallel electric fields

We use a new nonlinear guiding center particle in cell code to investigate the role of Alfven waves in the auroral particle acceleration. The propagation of an Alfven wave in an inhomogeneous auroral plasma, described by a density cavity, is considered. Parallel electric fields are generated on the edges of the cavity, leading to an efficient electron acceleration and to the formation of electron beams. A beam-plasma instability takes place. It evolves nonlinearly, and small-scale electrostatic structures are created and propagate at a velocity close to the Alfven velocity. A first category of these structures could be assimilated to weak double layers, whereas a second may be related to electron phase space holes. This study demonstrates the potential importance of Alfven waves in the auroral acceleration and the coupling between large-scale electromagnetic and small-scale electrostatic phenomena.

Mirror and Firehose Instabilities in the Heliosheath

We investigate the nature of the heliosheath plasma behind the termination shock across which jump relations in anisotropic MHD are formulated. Along side analytical results for downstream parameters in the strictly parallel and perpendicular cases we numerically solve the Rankine-Hugoniot relations for arbitrary shock angle and strength. We then focus on two temperature anisotropy driven instabilities which have attracted attention in many other astrophysical situations, namely the mirror and firehose instabilities. It is revealed that the firehose instability is mainly controlled by the shock strength with little influence of the shock angle contrary to the mirror instability for which both parameters intervene. We confirm results showing that the heliosheath plasma observed by Voyager 1 immediately behind the termination shock is mirror unstable. Similar conditions are probable in the heliosheath recently encountered by Voyager 2. Finally, by comparison with studies in the Earths magnetosheath context, we formulate predictions on the shapes of mirror-associated magnetic fluctuations in the heliosheath. Both hole and peak magnetic structures were indeed observed by Voyager 1 and these shapes correspond to different stages of the mirror instability.

Particle acceleration linked to Alfvén wave propagation on small scale density gradients

Abstract We study how Alfven waves propagate in the presence of sharp density gradients in the direction perpendicular to the ambient magnetic field. A fully electromagnetic electron guiding centre code is used for the simulation. During the propagation, initially parallel ( k ∥ = 0), transverse scales of the order of c/ω pe are quickly reached which contributes to the creation of a significant parallel component of the electric field in the region of density inhomogeneity. The effects of this field on the velocity distribution functions are then discussed. In particular, we show that they can present a strong deviation from their initial Gaussian shape (global shift in energy) due to the action of the parallel electric field. Evidences are then given for a net energy gain of the electrons, to the expense of the wave, during this process. This energy transfer mechanism may be relevant in order to explain the particle acceleration in the auroral plasma cavities.

Cross-comparison of spacecraft-environment interaction model predictions applied to Solar Probe Plus near perihelion

Five spacecraft-plasma models are used to simulate the interaction of a simplified geometry Solar Probe Plus (SPP) satellite with the space environment under representative solar wind conditions ne ...