Radiation from sub-Larmor scale magnetic fields
  Spontaneous rapid growth of strong magnetic fields is rather ubiquitous in
high-energy density environments ranging from astrophysical sources (e.g.,
gamma-ray bursts and relativistic shocks), to reconnection, to laser-plasma
interaction laboratory experiments, where they are produced by kinetic
streaming instabilities of the Weibel type. Relativistic electrons propagating
through these sub-Larmor-scale magnetic fields radiate in the jitter regime, in
which the anisotropy of the magnetic fields and the particle distribution have
a strong effect on the produced radiation. Here we develop the general theory
of jitter radiation, which includes (i) anisotropic magnetic fields and
electron velocity distributions, (ii) the effects of trapped electrons and
(iii) extends the description to large deflection angles of radiating particles
thus establishing a cross-over between the classical jitter and synchrotron
regimes. Our results are in remarkable agreement with the radiation spectra
obtained from particle-in-cell simulations of the classical Weibel instability.
Particularly interesting is the onset of the field growth, when the transient
hard synchrotron-violating spectra are common as a result of the dominant role
of the trapped population. This effect can serve as a distinct observational
signature of the violent field growth in astrophysical sources and lab
experiments. It is also interesting that a system with small-scale fields tends
to evolve toward the small-angle jitter regime, which can, under certain
conditions, dominate the overall emission of a source.
