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Bruno Bavassano is a solar wind researcher whose publications connect spacecraft observations with the statistical behavior of magnetohydrodynamic turbulence. A 2003 study with Roberto Bruno, Vincenzo Carbone and Luca Sorriso-Valvo compared velocity and magnetic field fluctuations between $0.3$ and $1$ astronomical unit from the Sun. It separated fast from slow solar wind, and changes in field magnitude from changes in field direction.^[ref]

The diagnostic was scale-dependent kurtosis. For a fluctuation $X$ with mean zero, $K=\mathbb{E}[X^4]/\mathbb{E}[X^2{]}^2$ weights rare large events strongly. A Gaussian distribution has $K=3$; a rising value as the separation shrinks indicates that small-scale changes are becoming more unevenly distributed. The study found stronger intermittency in compressive magnetic fluctuations than in the corresponding velocity fluctuations, and a radial increase in fast-wind intermittency.^[ref]

For intuition, replacing one ordinary increment by an increment ten times larger multiplies its contribution to the fourth moment by $10^4$, rather than $10^2$ for its squared contribution. This is why a sparse set of sharp changes can matter even when most samples look modest.

By Luca Sorriso-Valvo, Raffaele Marino, Vincenzo Carbone, Fabio Lepreti, Pierluigi Veltri, Alain Noullez, Roberto Bruno, Bruno Bavassano, Ermanno Pietropaolo.

Physical Review Letters 99, 115001 (2007), DOI doi.org/10.1103/PhysRevLett.99.115001 . Primary byline as listed in the archive. Abstract and mixed third-order structure functions: Ulysses velocity/magnetic observations and evidence of a magnetohydrodynamic inertial energy cascade.