Does the helicity barrier control ion heating in imbalanced solar wind
In plain words
Near the Sun most solar-wind waves travel outward, an imbalance that theory says blocks the energy cascade at the scale of ion orbits. The blocked energy would then accumulate and heat the ions through resonant waves, and whether this happens in the real wind is under test.
Precise statement
In imbalanced Alfvenic turbulence (normalized cross helicity $\sigma_c$ near 1) at $\beta$ below about 1, conservation of generalized helicity blocks the cascade at $k\rho_i \sim 1$ (the helicity barrier), causing a steep transition range in the magnetic spectrum and ion heating by ion-cyclotron waves. Determine whether this mechanism sets the transition-range steepness, ion-cyclotron wave occurrence and $Q_i/Q_e$ as functions of $\sigma_c$ and $\beta_i$ in the solar wind, and its threshold in $\beta$. An answer is spacecraft statistics showing or excluding the predicted dependences.
What would settle it
Parker Solar Probe and Solar Orbiter statistics of the spectral transition, ion-cyclotron wave occurrence and ion heating versus $\sigma_{c}$ and $\beta_{i}$ matching hybrid-kinetic predictions.
Status in the literature
Unverified note
Proposed by Squire et al. (2022, https://doi.org/10.1038/s41550-022-01624-z); transition-range observations supporting it appeared in 2025 (McIntyre et al., https://doi.org/10.1103/PhysRevX.15.031008) and 2026 (Zhao et al., https://doi.org/10.3847/2041-8213/ae9cb4).