Anisotropy evolution of magnetic field fluctuation through the bow shock

Abstract

Measurement of energy distributions in the wave vector domain reveals how anisotropy of turbulent magnetic field fluctuations evolves as the solar wind encounters the terrestrial bow shock and the magnetosphere. While fluctuations in the solar wind, the magnetosheath, and the magnetospheric cusp regions are characterized by the perpendicular wave vector geometry to the mean magnetic field direction, that in the foreshock region is characterized by the parallel wave vector geometry. Linear and nonlinear plasma processes are discussed for the anisotropy evolution. Key words: Wave-vector spectra, magnetic field, bow shock, multi-point measurements.Many in-situ spacecraft observations suggest that shock waves in the interplanetary space such as planetary bow shocks and traveling shocks in the co-rotating interaction regions are often accompanied by turbulent fluctuations, and furthermore, there are indications that interstellar shocks or supernova remnants may be associated with turbulence (Hester et al., 1994;Spitler and Spangler, 2005). How turbulence evolves as it encounters the shock wave in a collisionless plasma is an interesting problem and of particular importance in space physics and astrophysics. Earth's bow shock, a standing shock wave located at about 20 Earth radii in front of the Earth, serves as an ideal, natural laboratory for studying turbulence evolution across the shock. Many spacecraft visited the Earth's bow shock and observed various kinds of electrostatic and electromagnetic fluctuations near the shock, e.g., Shin et al. (2007). Spatial properties of waves or turbulence in the surroundings of the bow shock can be extensively studied by the Cluster mission (Escoubet et al., 2001), as it provides four-point measurements in the near-Earth space.Here we present a measurement of energy distributions of magnetic field fluctuations in the wave vector domain using the Cluster fluxgate magnetometer data . We use the concept of two distinct fluctuation geometries to study anisotropy: parallel and perpendicular wave vector geometries (Fig. 1). The idea of the two fluctuation geometries is motivated by long-standing questions about the nature of symmetries of plasma turbulence, viz., whether wave vectors in plasma turbulence prefer parallel or perpendicular directions to the mean magnetic field (Matthaeus et al., 1990;Carbone et al., 1995).We investigate the magnetic field data for two Cluster orCopy right

Wavevector anisotropy of plasma turbulence at ion kinetic scales: solar wind observations and hybrid simulations

“…The perpendicular extension of the spectrum indicates that the spectral energy transfer or cascade is anisotropic accordingly. There are spacecraft observations of the parallel extension of the energy spectrum, but only under limited conditions, e.g., high-speed solar wind streams (Dasso et al, 2005) or shock-upstream region (Narita and Glassmeier, 2010). Development in wavevector anisotropy leads to a structure formation while the plasma evolves into a turbulent state, which is markedly different from fluid turbulence.…”

Effect of Upstream ULF Waves on the Energetic Ion Diffusion at the Earth's Foreshock. I. Theory and Simulation

“…From the above, the diffusion coefficient is written as k r vK 2 for K<1, where the dimensionless parameter r =K bL , and the wave amplitude s =b . Here, the wave amplitude, σ ⊥ , from Table 1 is used, and a perpendicular scale length of the turbulence is roughly assumed to be L ⊥ ; 1000 km from the foreshock wave observations (Narita & Glassmeier 2010). In the present study, K<1 throughout the energy range from 11.5 to 123 keV.…”

The Wave Telescope Technique

“…The fluctuation energy is obtained in the three‐dimensional wavevector domain in the wave telescope analysis, and the wavevector dependence is useful to study the spectral anisotropy with respect to the parallel and perpendicular directions to the mean magnetic field (Narita et al., 2014). A study of energy spectra along the Cluster orbit through the magnetosheath, bow shock, and foreshock regions exhibits a transition of the spectral anisotropy (Narita & Glassmeier, 2010). The spectrum is extended parallel to the mean magnetic field in the foreshock region, indicating that that the waves are excited at various wavelengths along the mean field by the linear instabilities (particularly by the right‐hand ion beam instability), the wave‐wave couplings, and the wave‐particle interactions.…”