A solar flare driven by thermal conduction observed in mid-infrared

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Nugget
Number: 425
1st Author: Guillermo GIMÉNEZ de CASTRO
2nd Author:
Published: 17 January 2021
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Contents

Introduction

The mid-infrared [https://en.wikipedia.org/wiki/Infrared (mid-IR) range has been mostly unexplored for the investigation of solar flares. It is only recently that new mid-IR flare observations have begun opening a new window into the response and evolution of the solar chromosphere. These new observations have been mostly performed by two telescopes that are operating in Argentina and Brazil, respectively (Ref. [1]), with one observation also at the venerable Kitt Peak McMath-Pierce telescope (Ref. [2]).

These studies have reported good temporal and/or spatial agreement between the novel mid-IR data and emissions in other wavelengths, such as microwaves, hard X-rays, and UV emission characteristic of the chromosphere. When compared to the flare emission in the submillimeter domain (much longer wavelengths) the mid-IR presents greater flux density (spectral irradiance) values. The study of flares in the mid-IR can provide very important information regarding the evolution of Hydrogen onisation in the chromosphere to help understand how the energy is deposited in the solar atmosphere during flares (Ref. [3]).

Observations

Here we analyze the C2 flare SOL2019-05-15T19:24 that occurred in active region (AR) NOAA 12741. It was observed at 30 THz (10 μm) with the mid-IR telescope AR30T, at the Estación de Altura Carlos U. Cesco of the Félix Aguilar Astronomical Observatory (OAFA) in the Argentine Andes at an altitude of 2500 m above sea level. The camera is was an uncooled microbolometer array of 640x480 pixels (17 μm) at the focus of the 20 cm Newtonian telescope with a diffraction limit of 12".6. In our analysis we also used magnetograms and continuum images from HMI/SDO, Hα data from the HASTA telescope at El Leoncito, observations in microwaves from EOVSA, and UV/EUV data from AIA/SDO.

Observations at 30 THz show two kernels with similar excess brightness temperatures and temporal evolution, spatially coincident with AIA 1600 Å sources. The overall mid-IR time profile perfectly matches the UV, peaking with Hα, but preceding the microwaves. Interestingly, the spectral analysis of microwaves, from 1 to 18 GHz shows no sign of gyrosynchrotron emission. The timeseries plots in Figure 1 summarize these timing relationships between these fundamentally new observations and other well-studied ones.

Figure 1: Timeseries, showing the context of our new 30 THz signature. From top to bottom, soft X-rays (the hot coronal plasma), 8 GHz (gyrosynchrotron radiation from trapped energetic electrons), the 30 THz flux compared with AIA 1700 &Aring: (the lower atmosphere), and the classic Hα.

The multiwavelength analysis suggests that, contrary to interpretations of the few previous events, the origin of the mid-IR emission here must be via energy transport from the corona by thermal conduction: a rough estimation of the emitted energy at mid-IR is order-of-magnitude similar to the energy flux transported by a conduction front from a hot coronal source located midway between loop footpoints.

Conclusion

For complete details of this remarkable event, with its rich promise for future diagnostic work in the mysterious flare chromosphere, please see Ref. [4].

References

[1] "Bright 30 THz impulsive solar bursts", "The 6 September 2017 X9 Super Flare Observed From Submillimeter to Mid-IR"

[2] "Spectral and Imaging Observations of a White-light Solar Flare in the Mid-infrared"

[3] "Formation of the thermal infrared continuum in solar flares"

[4] "A solar flare driven by thermal conduction observed in mid-infrared"

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