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The ion temperature during magnetic reconnections measured along the direction of the magnetic field at the MST Reverse Field Pinch has not yet received a satisfactory theoretical explanation. In this work we argue that it is consistent with a picture of ion energization by the parallel electric fields generated by the plasma during reconnection, and thermalization due to the chaotic ion dynamics. Three possible sources of randomness are pointed out: particle motion along static stochastic magnetic field lines, breakup of the adiabatic ion dynamics caused by the strong gradient nonlinearity of the magnetic field, parallel acceleration along stochastic time-dependent electric fields. The three mechanisms are likely operative simultaneously; regardless the specific mechanism active, the correct scaling laws with ion parameters are recovered. Furthermore, we argue that quantitative agreement can be obtained by feeding the model with realistic values for the plasma conditions.

Stochastic heating is a well-known mechanism through which magnetized particles may be energized by low-frequency electromagnetic waves. In its simplest version, under spatially homogeneous conditions, it is known to be operative only above a threshold in the normalized wave amplitude, which may be a demanding requisite in actual scenarios, severely restricting its range of applicability. In this work we show, by numerical simulations supported by inspection of the particle Hamiltonian, that allowing for even a very weak spatial inhomogeneity completely removes the threshold, trading the requirement upon the wave amplitude with a requisite upon the duration of the interaction between wave and particle. The thresholdless chaotic mechanism considered here is likely to be applicable to other inhomogeneous systems., Comment: V2. Some minor mistakes corrected