Short Circuits in Thermally Ionized Plasmas: A Mechanism for Intermittent Heating of Protoplanetary Disks

摘要

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