The energetics of relativistic magnetic reconnection: ion-electron repartition and particle distribution hardness

• Published in: Astronomy and astrophysics (Berlin) 2014-10, Vol.570, p.A112
• Main Authors: Melzani, Mickaël, Walder, Rolf, Folini, Doris, Winisdoerffer, Christophe, Favre, Jean M.
• Format: Article
• Language: English
• Subjects: instabilities; magnetic reconnection; methods: numerical; plasmas; relativistic processes
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201424193

Collisionless magnetic reconnection is a prime candidate to account for flare-like or steady emission, outflow launching, or plasma heating, in a variety of high-energy astrophysical objects, including ones with relativistic ion-electron plasmas. But the fate of the initial magnetic energy in a reconnection event remains poorly known. What are the amounts assigned to kinetic energy, the ion and electron distribution, and the hardness of the particle distributions? We explored these questions with 2D particle-in-cell simulations of ion-electron plasmas. We find that 45 to 75% of the total initial magnetic energy ends up in kinetic energy, this fraction increasing with the inflow magnetization. Depending on the guide field strength, ions get from 30% to 60% of the total kinetic energy. Particles can be separated into two populations that mix only weakly: (i) particles initially in the current sheet heated by its initial tearing and subsequent contraction of the islands, and (ii) particles from the background plasma that primarily gain energy via the reconnection electric field when passing near the X-point. Particles of (ii) tend to form a power law with an index p = −dlog n(γ)/dlog γ that depends mostly on the inflow Alfvén speed VA and magnetization σs of species s. For electrons p = 5 to 1.2 for increasing σe. The highest particle Lorentz factor for ions or electrons increases roughly linearly with time for all the relativistic simulations. This is faster, and the spectra can be harder, than for collisionless shock acceleration. We discuss applications to microquasar and AGN coronae, to extragalactic jets, and to radio lobes. We point out situations where effects, such as Compton drag or pair creation, are important.