Energetic particles accelerated via turbulent magnetic reconnection in protoplanetary discs -- I. Ionisation rates

TL;DR

This paper investigates how energetic particles from turbulent magnetic reconnection in protoplanetary discs can significantly enhance ionisation rates, surpassing traditional sources like X-rays and cosmic rays, especially in the disc's atmosphere and inner regions.

Contribution

It introduces a model for particle acceleration via turbulent reconnection in discs and quantifies their ionisation impact, highlighting their importance in disc chemistry and physics.

Findings

EPs can dominate ionisation beyond standard sources at certain energy fractions.

Even low fractions of energy channeled to EPs have notable effects.

EPs are a robust internal ionisation mechanism in protoplanetary discs.

Abstract

Context. Ionisation controls the chemistry, thermal balance, and magnetic coupling in protoplanetary discs. However, standard ionisation vectors such as stellar UV, X-rays, Galactic cosmic rays (GCRs) might not be efficient enough, as UV/X-rays are attenuated rapidly with depth, while GCRs are modulated. Turbulence-induced magnetic reconnection in disc atmospheric layers offers a physically motivated, in situ source of energetic particles (EPs) that has never been considered. Aims. We quantify the ionisation and heating produced by EPs accelerated by turbulent reconnection, identify where they dominate over X-rays and GCRs, and determine energetic thresholds for their relevance. We provide scalable diagnostics tied to the local energy budget. Methods. We adopt a Fermi-like acceleration model with parameters linked to a turbulent reconnection geometry trigger by the magneto-rotational instability, yielding a steady-state energy distribution of the EP forming a power-law of index $p=2.5$. We propagate electrons and protons through the disc and compute primary and secondary ionisation and associated heating on a fiducial T Tauri disc model background. The non-thermal normalisation is set by the fraction of local viscous accretion energy dissipation channelled to EPs, parametrised by $\kappa$. Results. For $\kappa\gtrsim 0.4\%$, EPs ionisation overpass standard ionisation sources in the disc atmosphere and intermediate/deep layers out to radii of a few tens of astronomical units. Even at $\kappa\sim 0.025\%$, EPs contribute at the few-percent level, thus are chemically and dynamically relevant. These results identify EPs accelerated by turbulence-induced magnetic reconnection as a rather robust, disc-internal ionisation channel that should be included in thermo-chemical models of protoplanetary discs.