Princeton University Astroplasmas Seminar

Magnetic reconnection is ubiquitous in space and astrophysical plasmas rapidly converting magnetic field energy into plasma particles. Among numerous candidate kinetic mechanisms, ion acoustic instabilities driven by the relative drift between ions and electrons, or electric current, have been long hypothesized to play a critical role in dissipating magnetic energy in collisionless plasmas, but their effectiveness and even existence during reconnection remain elusive due to ion Landau damping and difficulties in detecting on the Debye length scale in the laboratory. Here we report a clear identification of sudden onset of ion acoustic bursts by collective Thomson scattering diagnostics in the exhaust of anti-parallel reconnection magnetically driven in high-Z plasmas at low beta on a novel platform using high-power lasers. The ion acoustic bursts are followed by electron acoustic bursts with electron heating and bulk acceleration. These observations are successfully reproduced by 1D and 2D Particle-in-Cell simulations in which ion acoustic instabilities, driven by current due to electron jet in the reconnection exhaust, grow rapidly to form electrostatic double layers. These double layers in turn induce electron two-stream instability to generate electron acoustic bursts, during which electrons are heated and accelerated in accordance with the measurements. Our results demonstrate the importance of ion and electron acoustic dynamics during magnetic reconnection when ion Landau damping is ineffective, a condition that may exist in many plasmas including near-Earth space,  stellar flares, and black hole accretion engines.