Speaker
Description
P. Buratti (1, 2), E. Menegoni (3), V. Vittorini (1), M. Tavani (1), D. Borgogno (1), F. Pucci (1) and L. Foffano (1)
(1) INAF-IAPS Roma, via Fosso del Cavaliere 100, I-00133 Rome, Italy
(2) ENEA, NUC Department, via E. Fermi 45, 00044 Frascati, Italy
(3) ASI, via del Politecnico, Roma
Magnetic reconnection in collisionless current layers embedded in tenuous background plasmas is studied by means of the Zeltron relativistic particle-in-cell (PIC) code [1].
A strongly unstable initial equilibrium is considered by choosing the current layer width not much larger than the collisionless skin depth. The assumed aspect ratio of the current layer is relatively small (400). Our conditions are complementary to the ones in large scale simulations [2], where reconnection is triggered artificially introducing a local loss of equilibrium. On the contrary, the work we present here is focused on the spontaneous onset of reconnection, and its short term evolution.
The development of the tearing instability breaks the current layer into a chain of plasmoids. The initial instability growth rate as a function of wavenumber is consistent with linear theoretical estimates in which the effect of the background plasma is kept into account. The initial number of magnetic islands formed by the tearing instability corresponds to the wavelength for which the growth rate is maximum. Later on, the plasma current progressively becomes concentrated around islands o-points, forming plasmoid structures. Plasmoids attract each other and for this reason tend to form larger and larger structures by a sequence of mergers. The first generation of mergers occurs after abount 0.25 global light crossing times. Coalescing plasmoids approach each other at relativistic velocities. A single plasmoid filling the simulation box along the initial neutral line direction is left in about two global light-crossing times.
Electron-positron plasmas are considered in the simulations. The initially maxwellian background plasma populations develop hard power-law tails with sharp cutoff. Particle acceleration occurs in lockstep with plasmoid mergers. The cutoff shifts to higher energies the higher the magnetization.
[1] B. Cerutti et al., 2013, The Astrophysical Journal, 770, 147
[2] L. Sironi et al., 2016, MNRAS 462, 48
