Ion-to-electron heating ratio in collisionless turbulence
In plain words
Turbulence in a collision-free plasma heats ions and electrons by different amounts. The ratio sets what telescopes see from hot astrophysical gas, and it is not yet pinned down.
Precise statement
For Alfvenic turbulence cascading to kinetic scales, determine the ion-to-electron heating ratio $Q_{i}/Q_{e}$ as a function of ion $\beta_{i}$, $T_{i}/T_{e}$ and the ratio of compressive to Alfvenic energy injection. An answer is a validated fit $Q_{i}/Q_{e}(\beta_{i}, T_{i}/T_{e})$ consistent with Parker Solar Probe and Solar Orbiter radial temperature profiles.
What would settle it
Kinetic simulations in agreement with heating rates inferred from spacecraft temperature profiles across the measured $\beta$ range.
Status in the literature
Hybrid gyrokinetic simulations (Kawazura and collaborators, 2019-2020) gave a parametric fit with $Q_{\mathrm{i}}/Q_{\mathrm{e}}$ rising steeply with $\beta_{\mathrm{i}}$; observational confirmation is incomplete.