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  • Open Access

Kinetic route to helicity-constrained decay

Dion Li*

  • *Contact author: dionli@psfc.mit.edu
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APS Open Sci. 1, 000055 – Published 6 July, 2026

DOI: https://doi.org/10.1103/j5p4-jj3d

Abstract

Through two-dimensional, three-velocity-component particle-in-cell simulations of freely decaying subion turbulence, intermittent localized regions with E·B≠0 are found, in the early electron-scale interaction phase, to be statistically associated with decreases in |HVs|, the fixed-gauge structure-integrated magnetic-helicity diagnostic. This structure-level behavior coincides with a decline of the Saffman helicity-variance plateau value IH. Motivated by these observations, we propose a source-compensated, history-dependent helicity density that satisfies an exact local balance identity by construction, enabling Saffman-type two-point correlation integrals, which, under standard flux-decorrelation assumptions, can exhibit intermediate-scale plateaus that are roughly time independent. In the simulations, such plateaus are observed to remain approximately invariant over the measured kinetic interval even as IH evolves during the early kinetic stage. Under approximate single-scale self-similarity, the plateau behavior of the magnetic integral is consistent with the two-dimensional decay constraint BL∼const. For initially net-helical configurations, we observe rapid development of mixed-signed magnetic-helicity patches and a decrease of the global fractional helicity, such that the decay over the kinetic interval is again most consistent with the cancellation-dominated scaling constraint.

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APS Open Sci. 1, 000055– Published 6 July, 2026
Vol. 1
  • Received 19 February 2026
  • Revised 2 May 2026
  • Accepted 15 June 2026
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