  Null and X-points are not themselves directly important to magnetic
reconnection because distinguishable field lines do not approach them closely.
Even in a collision-free plasma, magnetic field lines that approach each other
on a scale $c/\omega_{pe}$ become indistinguishable during an evolution. What
is important is the different regions of space that can be explored by magnetic
field lines that pass in the vicinity of null and X-points. Traditional
reconnection theories made the assumption that the reconnected magnetic flux
must be dissipated or diffused by an electric field. This assumption is false
in three dimensional systems because an ideal evolution can cause magnetic
field lines that cover a large volume to approach each other within the
indistinguishability scale $c/\omega_{pe}$. When the electron collision time
$\tau_{ei}$ is short compared to the evolution time of the magnetic field
$\tau_{ev}$, the importance of $c/\omega_{pe}$ is replaced by the resistive
time scale $\tau_\eta=(\eta/\mu_0)L^2$ with $L$ the system scale. The magnetic
Reynolds number is $R_m\equiv\tau_\eta/\tau_{ev}$ is enormous in many
reconnection problems of interest. Magnetic flux diffusion implies the current
density required for reconnection to compete with evolution scales as $R_m$
while flux mixing implies the required current density to compete scales as
$\ln R_m$.
