Proceedings with Extended Abstracts (single PDF file) - Radio ...

PHASE DIFFUSION FORMULATION OF TURBULENT SCATTERSPECTRAHasan Bahcivan 1David L. Hysell 21 Department of Electrical and Computer Engineering, Cornell University, Ithaca, NY2 Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NYAbstractCoherent radiowave backscattering from plasma irregularities in the ionosphere providesinformation on the characteristic motion of the scatterers. The average trajectory of aparticle in the scattering medium represents diffusion and what a radar at a certaindiagnostic wavenumber measures is defined as phase diffusion. While we cannot obtainthe extended diffusion observations with a radar, the phase diffusion manifests itself asthe relaxation function of the backscattered field. In an attempt to understand theturbulent scattering processes, the characteristic relaxation of the backscattered signal canbe understood as the scattered field from a spontaneous fluctuation undergoing turbulentand thermal mixing. In this extended abstract, we suggest a relationship between thedecay function in a subsiding turbulence and the backscatter spectral observations fromdriven steady-state turbulent plasmas. Finally, we provide a range of anomalous spectrabased on different diffusion schemes modeled using random walk concepts, which couldprovide a framework for investigating anomalous scattering processes.IntroductionRefractive index fluctuations associated with plasma turbulence in the ionosphere givesrise to coherent scattering of radiowaves. Just like density fluctuations in the neutralatmosphere create refractive index fluctuations, electron density fluctuations are thesource of the scattered field from the ionosphere. The density fluctuations in a mediumare a result of certain characteristic motion of scatterers, where a stochastic model candescribe the characteristic motion. The average trajectory of a particle in the scatteringmedium is termed “diffusion”, which reflects, to a certain degree, underlying physicalprocesses.In the radiowave backscattering, the diffusion quantity we are concerned about is thecomplex amplitude of the scattered field. This quantity represents the motion of scatterersin a restricted space, called “phase space”. Therefore, the diffusional spread of thisquantity cannot go on indefinitely. When observed at a fixed wavenumber in k -space,the motion of particles can be visualized as a mixing event. For example, consider inertialrange turbulence where energy cascades toward higher wavenumbers. When such systemis closed, all the energy will eventually be dissipated due to molecular viscosity ordiffusion. The coherent scattered field of a particular wavelength in the inertial range willeventually vanish in the absence of processes creating the density irregularities.How do the time behavior of the observed complex amplitude of the scatter field, assuperposition of the fields from each scatterer, relate to the stochastic motion of110