Pulsar radio wave dispersion, intermittency, and kinetic Alfvén wave turbulence Paul Terry Stas Boldyrev

Intermittency in magnetic turbulence is emerging as an area of focus in Magnetic Chaos group Spatial intermittency in turbulence: Turbulence becomes spotty Initial stimulus: pulsar radio wave dispersion observations butDissipation ranges characterized by intermittency, intermittent structures in 2D MHD involve reconnection, intermittency invoked to save dynamo from back reaction, observed in lab plasmas Intermittency studies: good lab/astrophysics connections, application to multiple center focus areas, fundamental problem

Outline Paul Terry (20 min) Brief introduction and key questions Excitation of electron density in magnetic turbulence Overview of intermittency in dynamical models and expt. MHD, Drift Alfvén turbulence, lab experiments Studies at Wisconsin Stas Boldyrev (20 min) Details of pulsar radio wave dispersion Statistical background Proposed studies at LAPD

Pulsar signal broadening is consistent with intermittent turbulence in ISM Pulsar signals dispersed by transit through ISM Dispersion: scattering of radio wave pulses by electron density fluctuations Broadened signal width scales as R 4 ( R: distance from source) If distribution of electron density fluctuations is Gaussian, signal width scales as R 2 Observed scaling distribution with long tail (Levy) Spatial intermittency; High amplitude density fluctuations (blobs) in localized regions surrounded by weak fluctuations Distance to neighboring blobs >> size of blob Blobs at small scales produce dispersion

Key questions growing out of pulsar problem How does density evolve in magnetic turbulence? Can density fluctuations become large? At what scales? What do we know about intermittency of density in magnetic turbulence? Do dynamical models for magnetic turbulence evolve to produce density fluctuations consistent with Levy statistics? Is there intermittency in laboratory plasmas?

How does electron density evolve in magnetic turbulence? where Basic model: RMHD + compressible electron continuity Two regimes: 1) MHD: Large scales - k i << 1 Density passively advected Pressure, compressional coupling weak 2) Kinetic Alfvén: scales just under k i = 1 to k i >> 1 Pressure, compressional coupling strong

Electron density fluctuations become strong at scales slightly larger than i Spectral energy transfer: k i << 1 : transfer dominated by v B k i 1 : transfer dominated by n B Low k : v and B equipartitioned Density at level dictated by High k : n and B equipartitioned, even if no linear or external drive of density v and B decouple 1 Low k - high k crossover at k i < 1

High- k regime shows intermittency in decaying turbulence Studies underway to examine density structures and intermittency Low k MHD and high k kinetic Alfvén wave regimes Vary ratio of resistivity to diffusivity Examine stationary dissipation regime Determine radio wave dispersion associated with density structures Kurtosis of current Current contours Cuts across structure

New experimental evidence suggests that part of MST spectrum may be in dissipation range Higher intensity fluctuations: Left propagating Power law steepens with k Suggests dissipation Lower intensity fluctuations: Right propagating Power law constant with k Suggests inertial range Possible model: (Elsässer)

Spectra that steepen with higher k are characteristic of intermittent dissipation range Hydro: Dissipation range 1 decade before Kolmogorov scale Spectral index (instantaneous) increases with k Fluctuations are intermittent MST: Spectral index increases with k Dissipation range? Dissipation rate? Heating rate? Intermittency? Intermittent dissipation range (Frisch) MST