arXiv subjects

Andre Mangeney

Publications and source records attributed to Andre Mangeney.

6 recordsLinked to original sources

arXiv2021

Spectrum of kinetic plasma turbulence at 0.3-0.9 astronomical units from the Sun

We investigate spectral properties of turbulence in the solar wind that is a weakly collisional astrophysical plasma, accessible to in-situ observations. Using the Helios search coil magnetometer measurements in the fast solar wind, in the inner heliosphere, we focus on properties of the turbulent magnetic fluctuations at scales smaller than the ion characteristic scales, the so-called kinetic plasma turbulence. At such small scales, we show that the magnetic power spectra between 0.3 and 0.9 AU from the Sun have a generic shape $\sim f^{-8/3}\exp{(-f/f_d)}$ where the dissipation frequency $f_d$ is correlated with the Doppler shifted frequency $f_{ρe}$ of the electron Larmor radius. This behavior is statistically significant: all the observed kinetic spectra are well described by this model, with $f_d = f_{ρe}/1.8$. Our results indicate that the electron gyroradius plays the role of the dissipation scale and marks the end of the electromagnetic cascade in the solar wind.

physics.space-ph↗

arXiv2014

Whistler mode waves and the electron heat flux in the solar wind: Cluster observations

The nature of the magnetic field fluctuations in the solar wind between the ion and electron scales is still under debate. Using the Cluster/STAFF instrument, we make a survey of the power spectral density and of the polarization of these fluctuations at frequencies $f\in[1,400]$ Hz, during five years (2001-2005), when Cluster was in the free solar wind. In $\sim 10\%$ of the selected data, we observe narrow-band, right-handed, circularly polarized fluctuations, with wave vectors quasi-parallel to the mean magnetic field, superimposed on the spectrum of the permanent background turbulence. We interpret these coherent fluctuations as whistler mode waves. The life time of these waves varies between a few seconds and several hours. Here we present, for the first time, an analysis of long-lived whistler waves, i.e. lasting more than five minutes. We find several necessary (but not sufficient) conditions for the observation of whistler waves, mainly a low level of the background turbulence, a slow wind, a relatively large electron heat flux and a low electron collision frequency. When the electron parallel beta factor $β_{e\parallel}$ is larger than 3, the whistler waves are seen along the heat flux threshold of the whistler heat flux instability. The presence of such whistler waves confirms that the whistler heat flux instability contributes to the regulation of the solar wind heat flux, at least for $β_{e\parallel} \ge$ 3, in the slow wind, at 1 AU.

astro-ph.SR↗

arXiv2012

Solar wind turbulent spectrum at plasma kinetic scales

The description of the turbulent spectrum of magnetic fluctuations in the solar wind in the kinetic range of scales is not yet completely established. Here, we perform a statistical study of 100 spectra measured by the STAFF instrument on the Cluster mission, which allows to resolve turbulent fluctuations from ion scales down to a fraction of electron scales, i.e. from $\sim 10^2$ km to $\sim 300$ m. We show that for $k_{\perp}ρ_e \in[0.03,3]$ (that corresponds approximately to the frequency in the spacecraft frame $f\in [3,300]$ Hz), all the observed spectra can be described by a general law $E(k_\perp)\propto k_\perp^{-8/3}\exp{(-k_\perp ρ_e)}$, where $k_{\perp}$ is the wave-vector component normal to the background magnetic field and $ρ_e$ the electron Larmor radius. This exponential tail found in the solar wind seems compatible with the Landau damping of magnetic fluctuations onto electrons.

astro-ph.SR↗

arXiv2009

Universality of solar wind turbulent spectrum from MHD to electron scales

In order to investigate the universality of magnetic turbulence in space plasmas we analyze seven time periods in the free solar wind of different origin, slow or fast, and under different plasma conditions. The orientation of magnetic field to the flow velocity was always quasi-perpendicular. Unique combination of three instruments on Cluster spacecraft which operate in different frequency ranges give us the possibility to resolve spectra up to 300 Hz. We show that spectra measured under different plasma conditions have a similar shape. Such a quasi-universal spectrum consists of three parts: two power laws and an exponential domain. At MHD scales, Kolmogorov's law $\sim k^{-5/3}$ is found. At scales smaller than the ion characteristic scales, a $k^{-2.8}$ law is observed. At scales $kρ_e\sim (0.1-1)$, where $ρ_e$ is the electron gyroradius, the magnetic spectrum follows an exponential law $\exp(-k^{1/2})$, indicating the onset of dissipation. This is the first observation of an exponential magnetic spectrum in space plasmas. We show that among several spatial kinetic plasma scales, the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence.

physics.plasm-ph↗

arXiv2008

Spectra and anisotropy of magnetic fluctuations in the Earth's magnetosheath: Cluster observations

We investigate the spectral shape, the anisotropy of the wave vector distributions and the anisotropy of the amplitudes of the magnetic fluctuations in the Earth's magnetosheath within a broad range of frequencies. We present the first observations of a Kolmogorov-like inertial range of Alfvenic fluctuations in the magnetosheath flanks, below fci. In the vicinity of fci, a spectral break is observed, like in solar wind turbulence. Above the break, the energy of compressive and Alfvenic fluctuations generally follow a power law with a spectral index between -3 and -2. Concerning the anisotropy of the wave vector distribution, we observe a change in its nature in the vicinity of ion characteristic scales: if at MHD scales there is no evidence for a dominance of a slab (k||>kperp) or 2D (kperp>k||) turbulence, above the spectral break, (f>fci, kc/wpi>1) the 2D turbulence dominates. This 2D turbulence is observed in six selected one-hour intervals among which the average proton beta varies from 0.8 to 9. It is observed for both the transverse and compressive magnetic fluctuations, independently on the presence of linearly unstable modes at low frequencies or Alfven vortices at the spectral break. We then analyse the anisotropy of the magnetic fluctuations in a time dependent reference frame based on the field B and the flow velocity V directions. Within the range of the 2D turbulence, at scales [1,30]kc/wpi, and for any beta we find that the magnetic fluctuations at a given frequency in the plane perpendicular to B have more energy along the BxV direction. This non-gyrotropy of the fluctuations is consistent with gyrotropic fluctuations at a given wave vector, with kperp>k||, which suffer a different Doppler shift along and perpendicular to V in the plane perpendicular to B.

astro-ph↗

arXiv2005

On spectral scaling laws for incompressible anisotropic MHD turbulence

A heuristic model is given for anisotropic magnetohydrodynamics (MHD) turbulence in the presence of a uniform external magnetic field $B_0 {\bf {\hat e}_{\pa}}$. The model is valid for both moderate and strong $B_0$ and is able to describe both the strong and weak wave turbulence regimes as well as the transition between them. The main ingredient of the model is the assumption of constant ratio at all scales between \add{the} linear wave period and \add{the} nonlinear turnover timescale. Contrary to the model of critical balance introduced by Goldreich and Sridhar [P. Goldreich and S. Sridhar, ApJ {\bf 438}, 763 (1995)], it is not assumed in addition that this ratio be equal to unity at all scales which allows us to use the Iroshnikov-Kraichnan phenomenology. It is then possible to recover the widely observed anisotropic scaling law $\kpa \propto \kpe^{2/3}$ between parallel and perpendicular wavenumbers (with reference to $B_0 {\bf {\hat e}_{\pa}}$) and to obtain the universal prediction, $3α+ 2β= 7$, for the total energy spectrum $E(\kpe,\kpa) \sim \kpe^{-α} \kpa^{-β}$. In particular, with such a prediction the weak Alfvén wave turbulence constant-flux solution is recovered and, for the first time, a possible explanation to its precursor found numerically by Galtier et al [S. Galtier et al., J. Plasma Phys. {\bf 63}, 447 (2000)] is given.

physics.plasm-ph↗