Limits on the compression of magnetic islands in strongly radiative magnetic reconnection

The evolution of magnetic islands generated in a reconnecting relativistic pair plasma is investigated using 2D and 3D particle-in-cell simulations in strong magnetic fields. For sufficiently strong fields (and a weak guide field), radiation cooling leads to compression of the magnetic islands, which amplifies fields and plasma density [1]. The quantum electrodynamic (QED) module [2] of the OSIRIS framework allows us to model the radiation as either classical radiation reaction or the QED emission of discrete photons according to non-linear Compton scattering, as well as single photon decay into pairs (non-linear Breit-Wheeler). These QED effects are important for the field strengths close to the critical (Schwinger) field occurring in magnetar magnetospheres, where gamma-ray flares occur. We show that the measured increases in density n and magnetic fields B due to compression are limited by power-laws in n-B space. In 3D, the magnetic flux ropes become kink-unstable, which effectively limits the compression of density. However, increasing upstream plasma magnetization leads to stronger magnetic compression, which in turn leads to increased pair production. [1] K. Schoeffler et al., ApJ, 870, 1 (2019) [2] T. Grismayer et al., Phys. Plasmas 23, 056706 (2016)