Heating of the solar chromosphere through current dissipation

Abstract

The solar chromosphere is heated to temperatures higher than predicted by radiative equilibrium. This excess heating is greater in active regions where the magnetic field is stronger. We aim to investigate the magnetic topology associated with an area of enhanced millimeter (mm) brightness temperatures in a solar active region mapped by the Atacama Large Millimeter/submillimeter Array (ALMA) using spectropolarimetric co-observations with the 1-m Swedish Solar Telescope (SST). We used Milne-Eddington inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions, and a magnetohydrostatic extrapolation to obtain constraints on the three-dimensional stratification of temperature, magnetic field, and radiative energy losses. We compared the observations to a snapshot of a magnetohydrodynamics simulation and investigate the formation of the thermal continuum at 3 mm using contribution functions. We find enhanced heating rates in the upper chromosphere of up to $\sim 5\mathrm{kW}{\mathrm{m}}^{-2}$, where small-scale emerging loops interact with the overlying magnetic canopy leading to current sheets as shown by the magnetic field extrapolation. Our estimates are about a factor of two higher than canonical values, but they are limited by the ALMA spatial resolution ($\sim 1.2^{\prime \prime }$). Band 3 brightness temperatures reach about $\sim 10^4$K in the region, and the transverse magnetic field strength inferred from the non-LTE inversions is on the order of $\sim 500$G in the chromosphere. We are able to quantitatively reproduce many of the observed features, including the integrated radiative losses in our numerical simulation. We conclude that the heating is caused by dissipation in current sheets. However, the simulation shows a complex stratification in the flux emergence region where distinct layers may contribute significantly to the emission in the mm continuum.

18 pages, 13 figures. Accepted for publication in A&A. Typos and bibtex corrected

References in corpus (25)

• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: SHARPs -- Space-weather HMI Active Region Patches
• Optimization code with weighting function for the reconstruction of coronal magnetic fields
• CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
• CRISPRED: A data pipeline for the CRISP imaging spectropolarimeter
• Coronal heating by MHD waves
• Extension of the MURaM radiative MHD code for coronal simulations
• Ellerman bombs and UV bursts: transient events in chromospheric current sheets
• Radiative diagnostics in the solar photosphere and chromosphere
• The effect of the relative orientation between the coronal field and new emerging flux: I Global Properties
• The usefulness of analytic response functions
• Millimeter radiation from a 3D model of the solar atmosphere I. Diagnosing chromospheric thermal structure
• Fine-scale explosive energy release at sites of prospective magnetic flux cancellation in the core of the solar active region observed by Hi-C 2.1, IRIS and SDO
• Compact solar UV burst triggered in a magnetic field with a fan-spine topology
• Ellerman bombs and UV bursts: reconnection at different atmospheric layers
• Clusters of small eruptive flares produced by magnetic reconnection in the sun
• An observationally-constrained model of strong magnetic reconnection in the solar chromosphere. Atmospheric stratification and estimates of heating rates
• Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes
• The inhomogeneous sub-millimeter atmosphere of Betelgeuse
• The influence of NLTE effects in Fe I lines on an inverted atmosphere I. 6301 A and 6302 A lines formed in 1D NLTE
• Reversed dynamo at small scales and large magnetic Prandtl number
• Observational study of chromospheric heating by acoustic waves
• High-frequency wave power observed in the solar chromosphere with IBIS and ALMA
• Heating of the solar chromosphere in a sunspot light bridge by electric currents
• Test magnetohydrostatic extrapolation with radiative MHD simulation of a solar flare
• Magnetohydrostatic modeling of AR11768 based on a SUNRISE/IMaX vector magnetogram

Cited by in corpus (2)

• Constraining the systematics of (acoustic) wave heating estimates in the solar chromosphere
• Calculated brightness temperatures of solar structures compared with ALMA and Metsähovi measurements