Time evolution of probability density functions observed in solar wind plasma densities

Mathematics – Probability

Scientific paper

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2109 Discontinuities, 2139 Interplanetary Shocks, 3379 Turbulence, 7807 Charged Particle Motion And Acceleration, 7863 Turbulence

Scientific paper

The variation of the solar wind densities observed by the ACE and Wind spacecraft is examined on time scales ranging from one minute to one week. The probability density function (PDF) of the differences of the logarithms of the plasma densities is well represented by a one-dimensional Kappa distribution. Kappa distributions of plasma velocities are found in various space environments including that near Earth, planetary magnetospheres and the solar corona. Compared to a Maxwellian velocity distribution which describes a plasma in a thermal equilibrium, a Kappa distribution is characterized by fat tails, frequently generated by anomalous Levy-type diffusion processes. For time lags ranging from minutes to hours we find highly leptokurtic PDFs with Kappa indices of about 2. The slope of the variance (2nd moment) vs. time lag, and the observed power spectra are consistent with those found in a turbulent fluid flow. The Kappa index increases with time lag, and the PDF converges toward a Gaussian distribution on a time scale of a few days. The evolution of the observed PDF and the possible physical causes are discussed.

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