Subresolution Activity in Solar and Stellar Coronae from Magnetic Field Line Tangling

TL;DR

This study investigates how magnetic field line tangling influences coronal heating and radiative emissions in solar and stellar coronae, revealing key dependencies on the ratio of forcing timescale to Alfvén crossing time.

Contribution

It demonstrates the impact of the photospheric forcing timescale on coronal heating rates and emissions, providing a scaling law for volumetric heating based on magnetic and convective parameters.

Findings

Heating rate peaks for high t_p/t_A ratios, leading to strong X-ray and EUV emission.

Lower t_p/t_A ratios result in reduced heating and radiative output.

The volumetric heating rate scales with convective and magnetic parameters as predicted.

Abstract

The heating of coronal loops is investigated to understand the observational consequences in terms of the thermodynamics and radiative losses from the Sun as well as the magnetized coronae of stars with an outer convective envelope. The dynamics of the Parker coronal heating model are studied for different ratios of the photospheric forcing velocity timescale $t_p$ to the Alfv\'en crossing time along a loop $t_A$. It is shown that for $t_p/t_A \gtrsim$ 10--24 the heating rate and maximum temperature are largest and approximately independent of $t_p/t_A$, leading to a strong emission in X-rays and EUV. On the opposite decreasing $t_p/t_A$ to smaller values leads to lower heating rates and plasma temperatures, and consequently fading high-energy radiative emission once $t_p/t_A \lesssim$ 1--3. The average volumetric loop heating rate is shown to scale as $\ell_p u_p B_0^2/4\pi L^2$, where $\ell_p$ and $u_p$ are respectively the convective granule length-scale and velocity, $B_0$ is the intensity of the strong magnetic field threading the loop, and $L$ the loop length. These findings support a recent hypothesis explaining ultracool dwarf observations of stars with similar magnetic field strength but radically different topologies displaying different radiative emission.