2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/183574Using a 2-1/2-dimensional particle-in-cell (PIC) code to simulate the relativistic expansion of a magnetized collisionless plasma into a vacuum, we report a new mechanism in which the magnetic energy is efficiently converted into the directed kinetic energy of a small fraction of surface particles. We study this mechanism for both electron-positron and electron-ion (mi/me=100, me is the electron rest mass) plasmas. For the electron-positron case the pairs can be accelerated to ultra-relativistic energies. For electron-ion plasmas most of the energy gain goes to the ions.7 pages text plus 5 figures, accepted for publication by Physical Review LettersAstrophysicstext