Electron and Proton Heating in Transrelativistic Magnetic Reconnection

Abstract

Hot collisionless accretion flows, such as the one in Sgr A* at our Galactic center, provide a unique setting for the investigation of magnetic reconnection. Here protons are nonrelativistic, while electrons can be ultrarelativistic. By means of 2D particle-in-cell simulations, we investigate electron and proton heating in the outflows of transrelativistic reconnection (i.e., σ w ∼ 0.1 – 1 , where the magnetization σ w is the ratio of magnetic energy density to enthalpy density). For both electrons and protons, we find that heating at high β i (here β i is the ratio of proton thermal pressure to magnetic pressure) is dominated by adiabatic compression (“adiabatic heating”), while at low β i it is accompanied by a genuine increase in entropy (“irreversible heating”). For our fiducial σ w = 0.1 , the irreversible heating efficiency at β i ≲ 1 is nearly independent of the electron-to-proton temperature ratio T e / T i (which we vary from 0.1 up to 1), and it asymptotes to ∼ 2 % of the inflowing magnetic energy in the low- β i limit. Protons are heated more efficiently than electrons at low and moderate β i (by a factor of ∼7), whereas the electron and proton heating efficiencies become comparable at β i ∼ 2 if T e / T i = 1 , when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection ( σ w ≳ 1 ). Our results have important implications for the two-temperature nature of collisionless accretion flows and may provide the subgrid physics needed in general relativistic MHD simulations.