Topic/Type: 1.1 Space & astrophysical plasmas, Oral

Magnetic field amplification and electron acceleration to a near ion energy equipartition by a mildly relativistic shock

G. C. Murphy1, M. E. Dieckmann2, L. O. Drury1

1 Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin, Ireland
2 Dept of Science and Technology, Linkoping University, Norrkoping, Sweden

The energetic electromagnetic eruptions observed during the prompt phase of gamma-ray bursts
are attributed to synchrotron (jitter) emissions. The internal shocks moving through the
ultrarelativistic jet, which is ejected by an imploding supermassive star, are the likely source of this
radiation.
Synchrotron (jitter) emissions at the observed strength require the simultaneous presence of
powerful magnetic fields and highly relativistic electrons. It is not yet fully understood how both
components are produced. It is, however, becoming increasingly evident that their source must be
the plasma processes and instabilities within the shock transition layer.
We explore with one and two-dimensional relativistic particle-in-cell simulations the transition layer
of a shock, that evolves out of the collision of two asymmetric plasma clouds at a speed 0.9c and
in the presence of a quasi-parallel magnetic field with a significant strength. The cloud densities vary
by the factor 10 and we consider equal number densities of ions and electrons in each cloud. We
employ an ion-to-electron mass ratio of 250. The peak Lorentz factor of the electrons is determined,
as well as the orientation and the strength of the magnetic field at the interface of the two colliding
plasma clouds. We find a strong plasma filamentation behind the otherwise planar shock front
as well as signatures of orthogonal magnetic field striping, indicative of the filamentation instability.
The magnetic field component orthogonal to the initial field direction is amplified to values that exceed
those expected from the shock compression by over an order of magnitude. The forming shock
is quasi-perpendicular due to this amplification. These powerful magnetic fields convect away from
the shock boundary and their energy density exceeds by far the thermal pressure of the plasma.
Localized magnetic bubbles form. The relativistic masses of the electrons and ions close to the shock
transition layer are comparable. The simultaneous presence of highly relativistic electrons and strong
magnetic fields will give rise to synchrotron emissions at a significant intensity.