MIST

By Alina Bendt (University of Warwick)

The solar wind may be heated by turbulence. Coherent structures which are one possible mediating mechanism of the turbulent cascade may dissipate energy. The partial variance increment (PVI) is routinely used to characterize and identify coherent structures. Previously the threshold beyond which fluctuations may be coherent structures was identified by comparisons to a Gaussian distribution. We compare wavelet decompositions with the Haar and 10th-order Daubechies wavelets to determine the threshold from the physical character of the fluctuations. These wavelets are sensitive to sharp changes and oscillations in the time series respectively. Comparisons of the fluctuation distributions obtained from these two wavelets reveal a core and tail., the latter is dominated by coherent structures. The transition from core to tail identifies the PVI threshold. This threshold coincides with the PVI value where the PVI distributions obtained from the Haar and Db10 wavelets start to depart from each other.  We find a single value for the threshold in each the kinetic and inertial ranges. This threshold is independent of heliocentric distance and solar wind conditions. The detailed behaviour of the fluctuations above the threshold varies, reflecting different ways that turbulence develops with distance from the sun. This suggests an underlying mechanism governing these coherent structures that is the same regardless of the specific plasma conditions.

Compensated Quantile-Quantile (QQ) plots discriminate where the Haar (sharp changes) and Db10 (wave-packet) decomposed PVI fluctuation distributions diverge (gray shading) for all solar wind intervals in the kinetic range. The upper panels [(a)–(c)] illustrate the compensated QQ plots. [(d)–(j)] are compensated QQ plots in the kinetic range overplotted for all intervals. These are divided into three categories based on heliocentric distance, (i) 0.3 R < 0.4 au, (ii) 0.4 < R < 0.8 au, and (iii) R > 0.8 au (rows), shown for all magnetic field components (columns). The different scales τ are color coded from dark blue (0.25 s) to light blue (2 s). The overplotted PVI threshold of 2.2 (marked by a black vertical line) is the same for all panels, capturing where the Haar and Db10 derived PVI distributions diverge. The gray shaded region determined from the variance of the threshold ranges from 2.1 to 2.7. This threshold is independent of the scale τ of the turbulence and heliocentric distance. Dashed horizontal lines are at PVI ±0.2 for reference.

See publication for details:
Bendt & Chapman (2025) Ubiquitous threshold for coherent structures in solar wind turbulence, Phys. Rev. Research, doi:10.1103/PhysRevResearch.7.023176