  We present the results of three-dimensional kinetic particle-in-cell (PIC)
simulations of isotropic periodic relativistically magnetized pair-plasma
equilibria known as the ABC fields. We performed several simulations for
initial wavenumbers k_ini = 2 or k_ini = 4, different efficiencies of radiative
cooling (including radiation reaction from synchrotron and inverse Compton
processes), and different mean magnetization values. These equilibria evolve by
means of ideal coalescence instability, the saturation of which generates ab
initio localized kinetically-thin current layers -- sites of magnetic
reconnection and non-thermal particle acceleration -- eventually relaxing to a
state of lower magnetic energy at conserved total magnetic helicity. We
demonstrate that magnetic relaxation involves in addition localized collapses
of magnetic minima and bulk mergers of current layer pairs, which represents a
novel scenario of spontaneous magnetic dissipation with application to the
rapid gamma-ray flares of blazars and of the Crab Nebula. Particle acceleration
under strong radiative losses leads to formation of power-law indices N(gamma)
~ gamma^(-p) up to p ~= -2.3 at mean hot magnetization values of <sigma_hot> ~
6. Individual energetic particles can be accelerated within one light-crossing
time by electric fields that are largely perpendicular to the local magnetic
fields. The energetic particles are highly anisotropic due to the kinetic
beaming effect, implying complex patterns of rapid variability. A significant
fraction of the initial total energy can be radiated away in the overall
process of magnetoluminescence.
