The ambipolar diffusion approximation is used to
model partially ionised plasma dynamics in a single
fluid setting. To correctly apply the commonly used
version of ambipolar diffusion, a set of criteria
should be satisfied including the requirement that
the difference in velocity between charges and
neutral species (known as drift ...
The ambipolar diffusion approximation is used to
model partially ionised plasma dynamics in a single
fluid setting. To correctly apply the commonly used
version of ambipolar diffusion, a set of criteria
should be satisfied including the requirement that
the difference in velocity between charges and
neutral species (known as drift velocity) is much
smaller than the thermal velocity, otherwise the
drift velocity will drive a non-negligible level of
further collisions between the two species. In this
paper we explore the consequence of relaxing this
assumption. We show that a new induction equation
can be formulated without this assumption. This
formulation reduces to the ambipolar induction
equation in the case the drift velocity is small.
In the large drift velocity limit, the feedback
of the drift velocity on the collision frequency
results in decreased diffusion of the magnetic field
compared with the standard ambipolar diffusion
approximation for the same parameters. This has a
natural consequence of reducing the frictional heating
that can occur. Applying this to results from flux
emergence simulations where the expansion of the
magnetic field leads to strong adiabatic cooling of
the partially ionised chromosphere resulted in a
noticeable reduction in the magnitude of the predicted
drift velocities.