Radiating Current Sheets in the Solar Chromosphere

TL;DR

This study models current sheets in the solar chromosphere using an advanced MHD approach, revealing their properties, radiative flux, and heating mechanisms, which improve understanding of chromospheric heating and emission processes.

Contribution

It introduces a self-consistent MHD model of chromospheric current sheets incorporating NLTE ionization and radiative cooling, providing detailed physical and radiative properties of these sheets.

Findings

Current sheets have magnetic profiles similar to Harris sheets with 25-150 G fields.

Radiative flux from sheets is 4.9e5 to 4.5e6 ergs/cm^2/s, close to observed values.

Sheets are 0.5-13 km thick with densities 10^13-10^15 cm^-3, higher ionization and temperature.

Abstract

An MHD model of a Hydrogen plasma with flow, an energy equation, NLTE ionization and radiative cooling, and an Ohm's law with anisotropic electrical conduction and thermoelectric effects is used to self-consistently generate atmospheric layers over a $50$ km height range. A subset of these solutions contain current sheets, and have properties similar to those of the lower and middle chromosphere. The magnetic field profiles are found to be close to Harris sheet profiles, with maximum field strengths $\sim 25-150$ G. The radiative flux $F_R$ emitted by individual sheets is $\sim 4.9 \times 10^5 - 4.5 \times 10^6$ ergs-cm$^{-2}$-s$^{-1}$, to be compared with the observed chromospheric emission rate of $\sim 10^7$ ergs-cm$^{-2}$-s$^{-1}$. Essentially all emission is from regions with thicknesses $\sim 0.5 - 13$ km containing the neutral sheet. About half of $F_R$ comes from sub-regions with thicknesses 10 times smaller. A resolution $\lesssim 5-130$ m is needed to resolve the properties of the sheets. The sheets have total H densities $\sim 10^{13}-10^{15}$ cm$^{-3}$. The ionization fraction in the sheets is $\sim 2-20$ times larger, and the temperature is $\sim 2000-3000$ K higher than in the surrounding plasma. The Joule heating flux $F_J$ exceeds $F_R$ by $\sim 4-34 \%$, the difference being balanced in the energy equation mainly by a negative compressive heating flux. Proton Pedersen current dissipation generates $\sim 62-77\%$ of the positive contribution to $F_J$. The remainder of this contribution is due to electron current dissipation near the neutral sheet where the plasma is weakly magnetized.