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

TL;DR

This paper identifies a temperature-dependent conductivity instability in thermally ionized plasmas, which causes localized, intermittent heating in protoplanetary disks by focusing currents into narrow, high-temperature sheets.

Contribution

It introduces a novel instability mechanism driven by thermal ionization, explaining intermittent heating events observed in protoplanetary disks.

Findings

The instability leads to focused current sheets with higher currents and temperatures.

It provides a potential explanation for observed heating events in protoplanetary disks.

The mechanism contrasts with traditional magnetic reconnection processes.

Abstract

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.