2025A&A...696A..25N

The toroidal magnetic field is a key ingredient of relativistic jets launched by certain accreting astrophysical black holes, and of plasmoids emerging from the tearing instability during magnetic reconnection, which is a candidate dissipation mechanism in jets. Tension of the toroidal field is an anisotropic force that can compress local energy and momentum densities. We investigate this effect in plasmoids produced during relativistic reconnection initiated from a Harris layer by means of kinetic particle-in-cell numerical simulations, varying the system size (including 3D cases), magnetisation, or guide field. We find that: (1) plasmoid cores are dominated by plasma energy density for guide fields up to B_z ∼ B₀; (2) relaxed 'monster' plasmoids compress plasma energy density only modestly (by a factor of ∼3 above the initial level for the drifting particle population); (3) energy density compressions by factors ≳10 are achieved during plasmoid mergers, especially with the emergence of secondary plasmoids. This kinetic-scale effect can be combined with a global focusing of the jet Poynting flux along the quasi-cylindrical bunched spine (a proposed jet layer adjacent to the cylindrical core) due to poloidal line bunching (a prolonged effect of tension in the jet toroidal field) to enhance the luminosity of rapid radiation flares from blazars. The case of M87 as a misaligned blazar is discussed.