  Hopping conductivity is enhanced when exposed to microwave fields (Phys. Rev.
Lett., 102, 206601, 2009). Data taken on a variety of Anderson-localized
systems are presented to illustrate the generality of the phenomenon. Specific
features of these results lead us to conjecture that the effect is due to a
field-enhanced hopping, which is the high frequency version of the non-Ohmic
effect, well known in the dc transport regime. Experimental evidence in support
of this scenario is presented and discussed. It is pointed out that existing
models for non-Ohmic behavior in the hopping regime may, at best, offer a
qualitative explanation to experiments. In particular, they cannot account for
the extremely low values of the threshold fields that mark the onset of
non-Ohmic behavior.
