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Peter H. Yoon studies the kinetic theory of plasmas, including the electron velocity distributions observed in the solar wind. His 2014 paper relates the Kappa distribution, with its comparatively large high-velocity tail, to a self-consistent description of weak plasma turbulence.

The paper extends the calculation to electrostatic fluctuations associated with Langmuir waves, often called quasi-thermal noise. It thereby connects a particle distribution to the fluctuations with which the particles interact, rather than simply choosing an empirical tail and leaving the field unspecified. The author affiliations identify the University of Maryland and Kyung Hee University.^[ref]

Plasma turbulence is irregular, interacting multiscale motion of plasma particles and electromagnetic fields. At large scales a magnetohydrodynamics model can describe coupled velocity and magnetic fluctuations; near ion and electron scales their separate kinetic responses become important. A broadband spectrum alone is insufficient to establish how energy moves between scales.

A useful measurement combines spacecraft magnetometer data with plasma velocity and density measurements. Mixed third-order statistics of velocity and magnetic-field increments can test an energy-cascade relation. Ulysses observations found the predicted approximately linear scale dependence over an inertial range, providing evidence of transfer between scales rather than merely a noisy signal.^[ref]

For an idealized constant-density model, write the Elsässer fields $z^{\pm }=u\pm B/\sqrt{{\mu }_0\rho }$. Their nonlinear coupling shows why magnetic and velocity fluctuations should be measured together. Interpreting a spacecraft time lag as a spatial separation additionally requires a flow-advection approximation. At small scales, explaining plasma heating requires identifying how fluctuating field energy reaches particle distributions, beyond observing a spectral slope.