The Superflare SOL2017-09-06: from submm to mid-IR
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|publish_date = 15 March 2021 | |publish_date = 15 March 2021 | ||
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[[File:404f1.png|400px|thumb|center|<b>Figure 1:</b> | [[File:404f1.png|400px|thumb|center|<b>Figure 1:</b> | ||
- | The [https://en.wikipedia.org/wiki/George_Ellery_Hale Hale]-type coelostat installed at the rooftop of CRAAM, in the center of | + | The |
- | São | + | [https://en.wikipedia.org/wiki/George_Ellery_Hale Hale]-type |
- | the laboratory | + | coelostat installed at the rooftop of CRAAM, in the center of the city of |
+ | São Paul, to project the solar radiation into the 10 $\mu$m/30 THz telescope in | ||
+ | the laboratory, with imaging at H&alpha$ as well as the mid-IR. | ||
]] | ]] | ||
+ | |||
+ | With this unique facility, plus excellent sky conditions on the day of the flare, | ||
+ | we could obtain 10-minute movies at a 1-sec cadence. | ||
+ | These revealed dark spots, as expected from the pioneering work of Ref. [3], in | ||
+ | particular at the location of AR 12673. | ||
+ | The flare produced striking changes that we could characterize by wavelet transforms | ||
+ | and fitting to a 2D Gaussian emission profile, and thus obtaining the clean 10 μm | ||
+ | light curve that we compare, in Figure 2, with diverse other signatures. | ||
+ | |||
+ | [[File:404f2.png|600px|thumb|center|<b>Figure 2:</b> | ||
+ | Left: Intensity time profiles at selected wavelengths. Right: An | ||
+ | SDO/HMI 6173 Â image taken just before the flare. | ||
+ | The overplotted contours show the white-light flare emission at peak. | ||
+ | The dashed circle in the top left corner represents the | ||
+ | [https://en.wikipedia.org/wiki/Airy_disk Airy disk] of the mid-IR camera. | ||
+ | ]] | ||
+ | |||
+ | The Solar Submillimeter Telescope | ||
+ | [https://en.wikipedia.org/wiki/Solar_Submillimeter_Telescope (SST)], | ||
+ | at [https://en.wikipedia.org/wiki/Leoncito_Astronomical_Complex El Leoncito] | ||
+ | in the Andes, executed raster scans at 212 and 405 GHz during the flare, producing | ||
+ | the images shown in Figure 3. | ||
+ | |||
+ | [[File:404f3.png|600px|thumb|center|<b>Figure 3:</b> | ||
+ | SST maps starting at 11:53 UT: left panel is for 212 GHz, and right panel is for | ||
+ | 405 GHz. | ||
+ | The dashed circles in the bottom right corner represent the half-power beam widths | ||
+ | (HPBW) at the two frequencies. | ||
+ | |||
+ | To compare the variations across the different spectral bands, we plot | ||
+ | the normalized fluxes vs time in Figure 4 (left). | ||
+ | The white light and mid-IR fluxes start and peak together, although the former | ||
+ | decreases faster. | ||
+ | The brightness temperature at 10 μ (Figure 2, left) should be considered | ||
+ | as a lower bound since the spatial resolution of our camera is of the | ||
+ | same order of the emitting source size. | ||
+ | We cold assume that the mid-IR source is cospatial with the WL source, as | ||
+ | described in Ref. [4] for a different event. | ||
+ | But what we observe at 10 μ might be an average of dark and bright areas (see | ||
+ | Figure 3 right). | ||
+ | Note that we do not see an actual brightening but only the variations in the | ||
+ | darkness of the spot (Figure 2). | ||
+ | Adopting the WL emitting area for the mid-IR source size, we obtain a peak flux of | ||
+ | F(10μ) = 7,000 [https://en.wikipedia.org/wiki/Solar_flux_unit SFU] (about | ||
+ | 10<sup>-4</sup> W/m<sup>2</sup> total energy flux). | ||
+ | |||
+ | [[File:404f4.png|600px|thumb|center|<b>Figure 4:</b> | ||
+ | Left: normalized time profiles at selected wavelengths; right: spectrum of the | ||
+ | flare, from microwaves to mid-IR, at peak time 11:56:46 UT, expressed in | ||
+ | [https://en.wikipedia.org/wiki/Solar_flux_unit SFU]. | ||
+ | ]] | ||
+ | |||
+ | From the spectrum at peak time (Figure 4, right ) we cannot determine the | ||
+ | gyrosynchrotron turnover frequency at microwaves, which depends the density | ||
+ | and magnetic field in the source. | ||
+ | On the other hand, the submillimeter emission seems to come from a different | ||
+ | mechanism and may not be co-spatial. | ||
+ | It is not possible to determine whether the submillimeter emission (212 and 405 GHz) | ||
+ | comes from a nonthermal source or not. | ||
+ | Recent radiative hydrodynamic simulations have demonstrated that the mid-IR | ||
+ | emission from solar flares may be accounted by optically thin thermal bremsstrahlung | ||
+ | from increased ionization in the chromosphere under non-LTE conditions. | ||
+ | But in general this first look at the broad mm-submm-IR spectral domain points | ||
+ | to rich possibilities of interpretation as observations improve. | ||
+ | |||
+ | == Conclusions == | ||
+ | |||
+ | "Superflares" such as this one bring new clues to better understand | ||
+ | different aspects of space-weather dynamics and perhaps the most | ||
+ | relevant, that is, the physical origins of flares. | ||
+ | The high-frequency radio spectrum and its connection with the heretofore | ||
+ | unobservable infrared emission, is still a very new aspect of solar flare studies. | ||
+ | This relation may also reveal the nature of the white-light emission | ||
+ | for which we do not have a clear explanation at the present time despite | ||
+ | [https://en.wikipedia.org/wiki/Carrington_Event 162 years of study]. | ||
+ | |||
+ | Observations at submillimeter to mid-IR frequencies are | ||
+ | scarce, and many frequency gaps must be filled. | ||
+ | We (at | ||
+ | [https://www.mackenzie.br/en/craam-center-for-radio-astronomy-and-astrophysics-at-mackenzie/ CRAAM]) | ||
+ | are about to deploy a new THz solar telescope, | ||
+ | the High Altitude THz Solar photometer (HATS, Figure 5). | ||
+ | This is based on a | ||
+ | [https://www.britannica.com/science/spectroscopy/Infrared-spectroscopy#ref620355 Golay cell] | ||
+ | detector and pass-band filters centered at about 15 THz (20 μ) that will be | ||
+ | installed, as soon as the COVID-19 pandemic allows, | ||
+ | at the | ||
+ | [https://en.wikipedia.org/wiki/Félix_Aguilar_Observator Felix Aguilar Observatory] | ||
+ | in Argentina, at 2300 m altitude (ref. [5]). | ||
+ | |||
+ | [[File:404f5.png|400px|thumb|center|<b>Figure 5:</b> | ||
+ | A projected 3D view of HATS inside the polypropylene radome in park position, pointing to South. | ||
+ | ]] | ||
+ | |||
== References == | == References == | ||
- | [1] | + | [1] [https://ui.adsabs.harvard.edu/abs/2013ApJ...768..134K "A Bright Impulsive Solar Burst Detected at 30 THz"] |
+ | |||
+ | [2] [https://ui.adsabs.harvard.edu/abs/2015SoPh..290.2373K "The New 30 THz Solar Telescope in S&atild;o Paulo, Brazil"] | ||
+ | |||
+ | [3] [http://adsabs.harvard.edu/abs/1970SoPh...14..112T "High resolution solar images at 10 microns: Sunspot details and photometry"] | ||
+ | |||
+ | [4] [http://adsabs.harvard.edu/abs/2016ApJ...819L..30P Spectral and Imaging Observations of a White-light Solar Flare in the Mid-infrared}Penn"] | ||
- | [ | + | [6] [https://ui.adsabs.harvard.edu/abs/2020SoPh..295...56G "HATS: A Ground-Based Telescope to Explore the THz Domain"] |
Revision as of 17:49, 18 March 2021
Nugget | |
---|---|
Number: | 404 |
1st Author: | Guillermo GIMENEZ DE CASTRO |
2nd Author: | |
Published: | 15 March 2021 |
Next Nugget: | Lyman Alpha |
Previous Nugget: | FLUKA as a tool for interpreting flare gamma-rays |
List all |
Contents |
Introduction
Solar flares famously emit radiation across all electromagnetic wave bands, but we first accessed the mid-infrared (10 μm or 30 THz) only recently (Ref. [1]). The M2-class flare SOL2012-03-13T17 showed up clearly in the mid-IR, and also as a white-light flare that could be interpreted as optically thin thermal emission from precipitating electrons, as described via numerical models. Since that time several other mid-IR flares have been reported, and now we describe the remarkable X9.3 "superflare" SOL2017-09-06T12.
The superflare was one of a series of major events occurring in active region NOAA 12673, and the observations described here include mm-wave (212 and 405 GHz) observations as well as novel mid-IR imaging at 17 arc s resolution.
Observations
The mid-IR observations were obtained from a 15-cm telescope mounted on the roof of the CRAAM laboratory in the heart of São Paulo, Brazil (Ref. [2]). This Hale-type coelostat now feeds an uncooled microbolometer array with digital output at 320x240 pixels, matching the diffraction limit at 17 arc sec (Figure 1).
![](/~tohban/wiki/images/8/85/404f1.png)
With this unique facility, plus excellent sky conditions on the day of the flare, we could obtain 10-minute movies at a 1-sec cadence. These revealed dark spots, as expected from the pioneering work of Ref. [3], in particular at the location of AR 12673. The flare produced striking changes that we could characterize by wavelet transforms and fitting to a 2D Gaussian emission profile, and thus obtaining the clean 10 μm light curve that we compare, in Figure 2, with diverse other signatures.
![](/~tohban/wiki/images/thumb/3/3d/404f2.png/600px-404f2.png)
The Solar Submillimeter Telescope (SST), at El Leoncito in the Andes, executed raster scans at 212 and 405 GHz during the flare, producing the images shown in Figure 3.
[[File:404f3.png|600px|thumb|center|Figure 3: SST maps starting at 11:53 UT: left panel is for 212 GHz, and right panel is for 405 GHz. The dashed circles in the bottom right corner represent the half-power beam widths (HPBW) at the two frequencies.
To compare the variations across the different spectral bands, we plot the normalized fluxes vs time in Figure 4 (left). The white light and mid-IR fluxes start and peak together, although the former decreases faster. The brightness temperature at 10 μ (Figure 2, left) should be considered as a lower bound since the spatial resolution of our camera is of the same order of the emitting source size. We cold assume that the mid-IR source is cospatial with the WL source, as described in Ref. [4] for a different event. But what we observe at 10 μ might be an average of dark and bright areas (see Figure 3 right). Note that we do not see an actual brightening but only the variations in the darkness of the spot (Figure 2). Adopting the WL emitting area for the mid-IR source size, we obtain a peak flux of F(10μ) = 7,000 SFU (about 10-4 W/m2 total energy flux).
![](/~tohban/wiki/images/thumb/1/15/404f4.png/600px-404f4.png)
From the spectrum at peak time (Figure 4, right ) we cannot determine the gyrosynchrotron turnover frequency at microwaves, which depends the density and magnetic field in the source. On the other hand, the submillimeter emission seems to come from a different mechanism and may not be co-spatial. It is not possible to determine whether the submillimeter emission (212 and 405 GHz) comes from a nonthermal source or not. Recent radiative hydrodynamic simulations have demonstrated that the mid-IR emission from solar flares may be accounted by optically thin thermal bremsstrahlung from increased ionization in the chromosphere under non-LTE conditions. But in general this first look at the broad mm-submm-IR spectral domain points to rich possibilities of interpretation as observations improve.
Conclusions
"Superflares" such as this one bring new clues to better understand different aspects of space-weather dynamics and perhaps the most relevant, that is, the physical origins of flares. The high-frequency radio spectrum and its connection with the heretofore unobservable infrared emission, is still a very new aspect of solar flare studies. This relation may also reveal the nature of the white-light emission for which we do not have a clear explanation at the present time despite 162 years of study.
Observations at submillimeter to mid-IR frequencies are scarce, and many frequency gaps must be filled. We (at CRAAM) are about to deploy a new THz solar telescope, the High Altitude THz Solar photometer (HATS, Figure 5). This is based on a Golay cell detector and pass-band filters centered at about 15 THz (20 μ) that will be installed, as soon as the COVID-19 pandemic allows, at the Felix Aguilar Observatory in Argentina, at 2300 m altitude (ref. [5]).
References
[1] "A Bright Impulsive Solar Burst Detected at 30 THz"
[2] "The New 30 THz Solar Telescope in S&atild;o Paulo, Brazil"
[3] "High resolution solar images at 10 microns: Sunspot details and photometry"
[4] Spectral and Imaging Observations of a White-light Solar Flare in the Mid-infrared}Penn"
[6] "HATS: A Ground-Based Telescope to Explore the THz Domain"
RHESSI Nugget Date | 15 March 2021 + |
RHESSI Nugget First Author | Guillermo GIMENEZ DE CASTRO + |
RHESSI Nugget Index | 404 + |