Turbulent energy dissipation and intermittency in ambipolar diffusion
  magnetohydrodynamics
  The dissipation of kinetic and magnetic energy in the interstellar medium
(ISM) can proceed through viscous, Ohmic or ambipolar diffusion (AD). It occurs
at very small scales compared to the scales at which energy is presumed to be
injected. This localized heating may impact the ISM evolution but also its
chemistry, thus providing observable features. Here, we perform 3D spectral
simulations of decaying magnetohydrodynamic turbulence including the effects of
AD. We find that the AD heating power spectrum peaks at scales in the inertial
range, due to a strong alignment of the magnetic and current vectors in the
dissipative range. AD affects much greater scales than the AD scale predicted
by dimensional analysis. We find that energy dissipation is highly concentrated
on thin sheets. Its probability density function follows a lognormal law with a
power-law tail which hints at intermittency, a property which we quantify by
use of structure function exponents. Finally, we extract structures of high
dissipation, defined as connected sets of points where the total dissipation is
most intense and we measure the scaling exponents of their geometric and
dynamical characteristics: the inclusion of AD favours small sizes in the
dissipative range.
