Coronal Bright Points

Nugget
Number:	462
1st Author:	Daniel Nóbrega-Siverio
2nd Author:
Published:	November 27, 2023
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Introduction

Understanding why the temperature of the solar corona is a few hundred times larger than that at the surface requires studying the heating mechanisms across different solar regions and phenomena. A particularly important case is that of the coronal bright points or CBPs. These are ubiquitous structures with projected sizes between 5 and 40 Mm composed of small-scale loops that stand out in the Sun by their strong X-ray and/or extreme-ultraviolet (EUV) emission for hours to days. In addition, CBPs can be the source of coronal jets and small-scale filament eruptions. So far, the existing CBP models have been very idealized, missing crucial aspects of the physics of the Sun, such as the energization of the magnetic structures by surface convection and the radiative transfer to explain the coldest solar atmospheric layers.

Numerical Models

In Ref. [1] we present a comprehensive 3D radiative-MHD model using the "Bifrost" code for a CBP based on a magnetic "null point" configuration, a common field-line topology related to this kind of phenomenon. We have shown that CBPs can be energized, for at least several hours, through the continuous action of stochastic photospheric convective motions, thus obviating the need for major, organized photospheric flows such as converging flows or large-scale surface flows. We have also detected continuous upflows with faint EUV signal related to the CBP, resembling observational dark coronal jets, and small-scale eruptions when Hα fibrils interact with a magnetic reconnection site (see Fig. 1).

Figure 1: Experiment overview. (a) 3D magnetic nullpoint topology (red lines) at t=111.67 minutes superimposed on a magnetic map at z=0. Blue-purple volume rendering corresponds to upflows with velocities from 45-75 km/s. The yellow isosurface at B=1 G harbors the null-point location at the center of the image. The red-green-blue coordinate system indicates the x-y-z axis orientation. (b) Temperature cut at y=16 Mm indicating the location of a hot collimated coronal jet and the cool erupting structure. (c) Synthetic response integrated along the y-axis to mimic a limb observation by Solar Orbiter/EUI-HRI at 174 Å.

Heating

Our investigation reveals that the CBP loops are heated by both Joule and viscous heating predominantly in the lower atmosphere, around the CBP's inner spine at a few megameters above the solar surface, with a secondary contribution at coronal heights close to the reconnection site (see Fig. 2). This is a major difference with 2D models, which can only capture the heating at the reconnection site itself. Important mechanisms, such as magnetic field-line braiding, are intrinsically three-dimensional. We have also found spatial and time correlation between the Joule and viscous heating within the CBP, which suggests that the heating results obtained may be widely independent of the value of the magnetic Prandtl number. This implies that, even though current numerical models are far from reproducing the Prandtl numbers of the solar atmosphere, our results may well be applicable to understand the heating of small-scale loops (CBPs) and their properties.

Figure 2: Heating in our CBP model, per unit mass, averaged over the 4.5 hours of the simulation for (a) the Joule term (electromagnetic) (b) the viscous heating (hydrodynamic) at z=[8,5,3,2] Mm.

Confronting the Modeling with Observations

To validate our theoretical model, we present simultaneous observations of a representative CBP from space (from SDO) and from the ground (from the SST). These observations are compared with observable diagnostics calculated from the numerical results for EUV wavelengths as well as for the Hα line (see Fig. 3). The similarities between the synthetic observables from the model and the observations are striking, reproducing the bright EUV rowel-like structure of the CBP, and chromospheric features such as the brightenings in the core of the Hα line, the dark and elongated Hα fibrils, as well as their magnetic photospheric connections.

Figure 3: Confronting the modeling with observations. Top: observations of a representative CBP on 2022 July 1 at 08:08:52 UT. (a), (b) Hot coronal response in EUV detected by SDO/AIA 193 Å and SDO/AIA 171 Å, respectively. (c) Cool chromospheric structure of the CBP in the Hα line core observed with SST/CRISP. (d) Photospheric line-of-sight magnetic field from SST/CRISP. (e) Hα profiles for different regions of the dark fibrils (cool colors) and core brightenings (warm colors) marked in panel (c). The average profile over the whole field-of-view is shown as a black dashed line. The intensity of the profiles has been normalized to the intensity value at 50 km/s. Bottom: forward modeling from our 3D CBP numerical experiment as observed on-disk. (a), (b) Synthetic EUV images for SDO/AIA 193 Å and Solar Orbiter/EUI-HRI 174 Å, respectively. (c) Synthetic Hα response in the core of the line as observed by SST/CRISP. (d) Vertical magnetic field at the solar surface. (e) Synthetic Hα profiles for different regions of the dark fibrils (cool colors) and core brightenings (warm colors) marked in panel (c).

References

[1] "Deciphering the solar coronal heating: Energizing small-scale loops through surface convection"