Coherence and Intermittency of Electron Density in Small-scale Interstellar Turbulence

Spatial intermittency in decaying kinetic Alfvén wave turbulence is investigated to determine if it produces nonGaussian density fluctuations in the interstellar medium. Non-Gaussian density fluctuations have been inferred from pulsar scintillation scaling. Kinetic Alfvén wave turbulence characterizes density evolution in magnetic turbulence at scales near the ion gyroradius. It is shown that intense localized current filaments in the tail of an initially Gaussian probability distribution function possess a sheared magnetic field that strongly refracts the random kinetic Alfvén waves responsible for turbulent decorrelation. The refraction localizes turbulence to the filament periphery, and hence it avoids mixing by the turbulence. As the turbulence decays, these long-lived filaments create a non-Gaussian tail. A condition related to the shear of the filament field determines which fluctuations become coherent and which decay as random fluctuations. The refraction also creates coherent structures in electron density. These structures are not localized. Their spatial envelope maps into a probability distribution that decays as density to the power 3. The spatial envelope of density yields a Levy distribution in the density gradient. Subject headinggs: ISM: general — MHD — turbulence