Solar Wind Turbulence and the Role of Ion Instabilities (1306.5336v1)

Abstract: Solar wind is probably the best laboratory to study turbulence in astrophysical plasmas. In addition to the presence of magnetic field, the differences with neutral fluid isotropic turbulence are: weakness of collisional dissipation and presence of several characteristic space and time scales. In this paper we discuss observational properties of solar wind turbulence in a large range from the MHD to the electron scales. At MHD scales, within the inertial range, turbulence cascade of magnetic fluctuations develops mostly in the plane perpendicular to the mean field. Solar wind turbulence is compressible in nature. The spectrum of velocity fluctuations do not follow magnetic field one. Probability distribution functions of different plasma parameters are not Gaussian, indicating presence of intermittency. At the moment there is no global model taking into account all these observed properties of the inertial range. At ion scales, turbulent spectra have a break, compressibility increases and the density fluctuation spectrum has a local flattening. Around ion scales, magnetic spectra are variable and ion instabilities occur as a function of the local plasma parameters. Between ion and electron scales, a small scale turbulent cascade seems to be established. Approaching electron scales, the fluctuations are no more self-similar: an exponential cut-off is usually observed indicating an onset of dissipation. The nature of the small scale cascade and a possible dissipation mechanism are still under debate.

• The paper demonstrates that solar wind turbulence at MHD scales follows power-law spectra with clear anisotropy and residual velocity effects.
• The paper identifies ion-scale transitions marked by spectral breaks and enhanced compressibility due to Hall effects and instabilities like mirror and firehose.
• The paper reveals a secondary inertial range and electron-scale dissipation, implicating kinetic Alfvén turbulence and electron Landau damping in energy loss.

Overview of "Solar Wind Turbulence and the Role of Ion Instabilities"

The paper "Solar Wind Turbulence and the Role of Ion Instabilities" presents a comprehensive examination of turbulence in solar wind, accompanied by the involvement of ion instabilities. The paper encompasses a vast range of scales from magnetohydrodynamic (MHD) to electron kinetic scales, emphasizing the observational characteristics, theoretical implications, and potential dissipation mechanisms of solar wind turbulence.

Key Observations and Results

1. MHD Scale Turbulence:
   • The paper identifies the presence of power-law energy spectra, non-Gaussian probability distribution functions, and linear scaling of third-order moments—haLLMarks of fully developed turbulence.
   • There is a notable anisotropy with respect to the magnetic field observed as $k_{\perp} \gg k_{\|}$, wherein the perpendicular magnetic spectrum follows a $k_{\perp}^{-5/3}$ law, whereas the parallel spectrum aligns with a $k_{\|}^{-2}$ law.
   • Despite the dominance of Alfvénic fluctuations, velocity spectra deviate from magnetic spectra with a characteristic $-3/2$ slope, implying a form of residual energy.
2. Ion Scale Transition:
   • At ion scales, turbulence exhibits spectral breaks and increased variability, potentially attributed to the Hall effect and the presence of plasma instabilities such as mirror and firehose instabilities influenced by ion temperature anisotropy.
   • A significant increase in compressibility is observed, along with spectral changes that suggest a transition from a fluid to kinetic description of the plasma.
3. Small Scale Cascade:
   • Between ion and electron scales, the observed spectra suggest a secondary inertial range characterized by $k_{\perp}^{-2.8}$ scaling and enhanced compressibility.
   • This turbulent range is associated with the presence of kinetic Alfvén turbulence and possible dissipation mechanisms that lead to an exponential cut-off at electron scales.
4. Dissipation at Electron Scales:
   • The dissipation mechanism is inferred to feature an exponential function $\sim k_{\perp}^{-8/3}\exp(-k_{\perp}\rho_e)$, implicating the electron gyro-radius $\rho_e$ as a crucial scale for describing the dissipation range.
   • The findings posit electron Landau damping as a potential dissipation mechanism in the collisionless solar wind environment.

Implications and Future Directions

The research establishes a foundational understanding of solar wind turbulence, highlighting the significance of different scales and the role of ion instabilities. It explores the complex transition from macroscopic fluid dynamics to microscopic kinetic processes.

• Theoretical Significance: The paper reinforces the universality of the Kolmogorov scaling in magnetized environments and emphasizes the need to integrate kinetic effects, particularly around ion scales, into turbulence models.
• Practical Implications: Understanding solar wind turbulence facilitates insights into space weather phenomena, which can impact satellite operations and communication systems.
• Future Research: Future efforts should focus on resolving the debate between wave-like vs. coherent structure-based dissipation mechanisms, with emphasis on detailing the topological characteristics of intermittent structures.

The paper paves the way for further investigation into the nonlinear interactions in solar wind turbulence and their impact on energy transfer and dissipation, bridging the gap between observational data and theoretical models.