Non-stationary Magnetic Microstructures in Stellar Thin Accretion Discs

TL;DR

This paper studies the formation and evolution of magnetic microstructures in stellar accretion discs, highlighting their transient nature due to resistivity and their dependence on temperature and scale.

Contribution

It extends a stationary ideal MHD model to include time-dependent visco-resistive effects, analyzing the lifetime of magnetic microstructures in stellar discs.

Findings

Microstructures exhibit exponential damping over time due to resistivity.

Their lifetime depends on midplane temperature and structure scale.

Microstructures can persist for minutes to hours within certain temperature ranges.

Abstract

We examine the morphology of magnetic structures in thin plasma accretion discs, generalizing a stationary ideal MHD model to the time-dependent visco-resistive case. Our analysis deals with small scale perturbations to a central dipole-like magnetic field, which give rise -- as in the ideal case -- to the periodic modulation of magnetic flux surfaces along the radial direction, corresponding to the formation of a toroidal current channels sequence. These microstructures suffer an exponential damping in time because of the non-zero resistivity coefficient, allowing us to define a configuration lifetime which mainly depends on the midplane temperature and on the length scale of the structure itself. By means of this lifetime we show that the microstructures can exist within the inner region of stellar discs in a precise range of temperatures, and that their duration is consistent with local transient processes (minutes to hours).