  Many astrophysical systems of interest, including protoplanetary accretion
disks, are made of turbu- lent magnetized gas with near solar metallicity.
Thermal ionization of alkali metals in such gas exceeds non-thermal ionization
when temperatures climb above roughly 1000 K. As a result, the conductiv- ity,
proportional to the ionization fraction, gains a strong, positive dependence on
temperature. In this paper, we demonstrate that this relation between the
temperature and the conductivity triggers an exponential instability that acts
similarly to an electrical short, where the increased conductivity concentrates
the current and locally increases the Ohmic heating. This contrasts with the
resistiv- ity increase expected in an ideal magnetic reconnection region. The
instability acts to focus narrow current sheets into even narrower sheets with
far higher currents and temparatures. We lay out the basic principles of this
behavior in this paper using protoplanetary disks as our example host system,
motivated by observations of chondritic meteorites and their ancestors, dust
grains in protoplanetary disks, that reveal the existence of strong, frequent
heating events that this instability could explain.
