The Superflare SOL2017-09-06: from submm to mid-IR

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Number:	404
1st Author:	Guillermo GIMENEZ DE CASTRO
2nd Author:
Published:	15 March 2021
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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).

Figure 1: The Hale-type 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.

Figure 2: 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 Airy disk of the mid-IR camera.

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.

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).

Figure 4: 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 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 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]).