Observation and theoretical modeling of electron scale solar wind turbulence (original) (raw)
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Electron scale solar wind turbulence: cluster observations and theoretical modeling
Turbulence at MagnetoHydroDynamics (MHD) scales of the solar wind has been studied for more than three decades, using data analyzes, theoretical and numerical modeling. However smaller scales have not been explored until very recently. Here, we review recent results on the first observation of cascade and dissipation of the solar wind turbulence at the electron scales. Thanks to the high resolution magnetic and electric field data of the Cluster spacecraft, we computed the spectra of turbulence up to ∼ 100 Hz (in the spacecraft reference frame) and found two distinct breakpoints in the magnetic spectrum at 0.4 Hz and 35 Hz, which correspond, respectively, to the Doppler-shifted proton and electron gyroscales, fρ p and fρ e . Below fρ p the spectrum follows a Kolmogorov scaling f −1.62 , typical of spectra observed at 1 AU. Above fρ p a second inertial range is formed with a scaling f −2.3 down to fρ e . Above fρ e the spectrum has a steeper power law ∼ f −4.1 down to the noise level of the instrument. Solving numerically the linear Maxwell-Vlasov equations combined with recent theoretical predictions of the Gyro-Kinetic theory, we show that the present results are fully consistent with a scenario of a quasi-two-dimensional cascade into Kinetic Alfvén modes (KAW).
Scaling of the Electron Dissipation Range of Solar Wind Turbulence
The Astrophysical Journal, 2013
Electron scale solar wind turbulence has attracted great interest in recent years. Clear evidences have been given from the Cluster data that turbulence is not fully dissipated near the proton scale but continues cascading down to the electron scales. However, the scaling of the energy spectra as well as the nature of the plasma modes involved at those small scales are still not fully determined. Here we survey 10 years of the Cluster search-coil magnetometer (SCM) waveforms measured in the solar wind and perform a statistical study of the magnetic energy spectra in the frequency range [1, 180]Hz. We show that a large fraction of the spectra exhibit clear 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 will be discussed in detail. We compare our results to those reported in other studies and discuss their implication on the physical mechanisms and the theoretical modeling of energy dissipation in the SW.
Universality of Solar-Wind Turbulent Spectrum from MHD to Electron Scales
Physical Review Letters, 2009
To investigate the universality of magnetic turbulence in space plasmas we analyze seven time periods in the free solar wind under different plasma conditions. Three instruments on Cluster spacecraft operating in different frequency ranges give us the possibility to resolve spectra up to 300 Hz. We show that the spectra form a quasiuniversal spectrum following the Kolmogorov's law ∼ k −5/3 at MHD scales, a ∼ k −2.8 power-law at ion scales and an exponential ∼ e −(kρe) 1/2 at scales kρe ∼ [0.1, 1], where ρe is the electron gyroradius. This is the first observation of an exponential magnetic spectrum in space plasmas, that may indicate the onset of dissipation. We distinguish for the first time between the role of different spatial kinetic plasma scales and show that the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence. PACS numbers: 52.35.Ra,94.05.-a,96.60.Vg,95.30.Qd
Fluid-like dissipation of magnetic turbulence at electron scales in the solar wind
arXiv (Cornell University), 2011
The turbulent spectrum of magnetic fluctuations in the solar wind displays a spectral break at ion characteristic scales. At electron scales the spectral shape is not yet completely established. Here, we perform a statistical study of 102 spectra at plasma kinetic scales, measured by the Cluster/STAFF instrument in the free solar wind. We show that the magnetic spectrum in the high frequency range, [1, 400] Hz, has a form similar to what is found in hydrodynamics in the dissipation range ∼ Ak −α exp (−kℓ d). The dissipation scale ℓ d is found to be correlated with the electron Larmor radius ρe. The spectral index α varies in the range [2.2, 2.9] and is anti-correlated with ℓ d , as expected in the case of the balance between the energy injection and the energy dissipation. The coefficient A is found to be proportional to the ion temperature anisotropy, suggesting that local ion instabilities may play some rôle for the solar wind turbulence at plasma kinetic scales. The exponential spectral shape found here indicates that the effective dissipation of magnetic fluctuations in the solar wind has a wave number dependence similar to that of the resistive term in collisional fluids ∼ △δB ∼ k 2 δB.
Small-Scale Energy Cascade of the Solar Wind Turbulence
Astrophysical Journal, 2008
Magnetic fluctuations in the solar wind are distributed according to Kolmogorov's power law f −5/3 below the ion cyclotron frequency f ci . Above this frequency, the observed steeper power law is usually interpreted in two different ways: a dissipative range of the solar wind turbulence or another turbulent cascade, the nature of which is still an open question. Using the Cluster magnetic data we show that after the spectral break the intermittency increases toward higher frequencies, indicating the presence of non-linear interactions inherent to a new inertial range and not to the dissipative range. At the same time the level of compressible fluctuations raises. We show that the energy transfer rate and intermittency are sensitive to the level of compressibility of the magnetic fluctuations within the small scale inertial range. We conjecture that the time needed to establish this inertial range is shorter than the eddy-turnover time, and is related to dispersive effects. A simple phenomenological model, based on the compressible Hall MHD, predicts the magnetic spectrum ∼ k −7/3+2α , which depends on the degree of plasma compression α.
THE DISSIPATION OF SOLAR WIND TURBULENT FLUCTUATIONS AT ELECTRON SCALES (2011, ApJ, 730, 114)
The Astrophysical Journal, 2011
We present two-dimensional fully-kinetic Particle-in-Cell simulations of decaying electromagnetic fluctuations. The computational box is such that wavelengths ranging from electron to ion gyroradii are resolved. The parameters used are realistic for the solar wind, and the ion to electron mass ratio is physical. The understanding of the dissipation of turbulent fluctuations at small scales is thought to be a crucial mechanism for solar wind acceleration and coronal heating. The computational results suggest that a power law cascade of magnetic fluctuations could be sustained up to scales of the electron Larmor radius and smaller. We analyse the simulation results in the light of the Vlasov linear theory, and we comment on the particle heating. The dispersion curves of lightly damped modes in this regime suggest that a linear mechanism could be responsible for the observed steepening of power spectra at electron scales, but a straightforward identification of turbulent fluctuations as an ensemble of linear modes is not possible.
Frontiers in Astronomy and Space Sciences, 2020
We investigate the transition of the solar wind turbulent cascade from MHD to sub‐ion range by means of a detailed comparison between in situ observations and hybrid numerical simulations. In particular, we focus on the properties of the magnetic field and its component anisotropy in Cluster measurements and hybrid 2D simulations. First, we address the angular distribution of wave vector in the kinetic range between ion and electron scales by studying the variance anisotropy of the magnetic field components. When taking into account a single-direction sampling, like that performed by spacecraft in the solar wind, the main properties of the fluctuations observed in situ are also recovered in our numerical description. This result confirms that solar wind turbulence in the sub‐ion range is characterized by a quasi-2D gyrotropic distribution of k-vectors around the mean field. We then consider the magnetic compressibility associated with the turbulent cascade and its evolution from lar...
Solar wind turbulent spectrum from MHD to electron scales
AIP Conference Proceedings, 2010
Turbulent spectra of magnetic fluctuations in the free solar wind are studied from MHD to electron scales using Cluster observations. We discuss the problem of the instrumental noise and its influence on the measurements at the electron scales. We confirm the presence of a curvature of the spectrum ∼ exp kρ e over the broad frequency range ∼ [10, 100] Hz, indicating the presence of a dissipation. Analysis of seven spectra under different plasma conditions show clearly the presence of a quasi-universal power-law spectrum at MHD and ion scales. However, the transition from the inertial range ∼ k −1.7 to the spectrum at ion scales ∼ k −2.7 is not universal. Finally, we discuss the role of different kinetic plasma scales on the spectral shape, considering normalized dimensionless spectra.
Solar Wind Turbulent Spectrum at Plasma Kinetic Scales
The Astrophysical Journal, 2012
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 ∼ 10 2 km to ∼ 300 m. We show that for k ⊥ ρ e ∈ [0.03, 3] (that corresponds approximately to the frequency in the spacecraft frame f ∈ [3, 300] Hz), all the observed spectra can be described by a general law E(k ⊥) ∝ k −8/3 ⊥ exp (−k ⊥ ρ e), where k ⊥ 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.
The kinetic nature of turbulence at short scales in the solar wind
Planetary and Space Science, 2011
The analysis of the transition from the large-scale fluid regime to the short-scale kinetic range of wavelengths in the development of the turbulent cascade of energy is nowadays subject of fervent discussion in the space plasmas scientific community. We make use of Hybrid Vlasov-Maxwell simulations where the full kinetic dynamics of ions is taken into account, while electrons are treated as a fluid. We investigate the development of turbulence in the solar wind, in 1D-3V phase space configuration and in the frequency range across the ion cyclotron frequency. These simulations allow for the analysis of the role of kinetic effects in the short-scale region of the energy spectra in the direction parallel to the background magnetic field. Our numerical results show the presence of a significant electrostatic activity at small wavelengths, triggered by the resonant interaction of ions with longitudinal waves. Our model does not allow to take into account the evolution of the turbulent spectra in the plane perpendicular to the ambient field, due to limited dimensionality in phase space. On the other hand, this model permits to isolate and study the possibility of transferring the electromagnetic large-scale energy on the small-scale kinetic electrostatic component of the spectrum. Peculiar features observed in the spacecraft data in the solar wind are qualitatively reproduced within the hybrid-Vlasov model, such as the generation of perpendicular temperature anisotropy and accelerated longitudinal beams of ions in the distribution of particle velocities as well as the appearance of a marked peak of electrostatic activity in the short-scale termination of the turbulent spectra.