TY - JOUR
AU1 - Valle Silva, J, F
AU2 - Giménez de Castro, C, G
AU3 - Selhorst, C, L
AU4 - Raulin,, J-P
AU5 - Valio,, A
AB - ABSTRACT Active regions were observed with different instruments covering the spectral band from 17 to 405 GHz. The observations were made with the Nobeyama Radioheliograph (17 GHz), the Atacama Large Millimetre Array (107 and 238 GHz), and the Solar Submillimeter Telescope (212 and 405 GHz). A procedure was developed that allows the comparison between observations taken with telescopes of different operational characteristics and mainly of different spatial resolution. The brightness temperature and density flux spectra of several active regions corresponding to a different phase of its lifetime were obtained. The flux density invariably increases in all cases from 107 to 405 GHz and the mean spectral index is ∼2 showing that the dominant emission mechanism at submillimeter frequencies is still thermal. We show that Solar Submillimeter Telescope (SST) and Atacama Large Millimeter/submillimeter Array (ALMA) observations are compatible within the uncertainties, a result of great interest for future joint observations. methods: observational, Sun: chromosphere, Sun: faculae, plages, Sun: radio radiation, sunspots 1 INTRODUCTION Radio observations from submillimeter to centimetre wavelengths show enhanced brightness temperature regions over the quiet background. The size and location of these regions have a good coincidence with optical chromospheric features, and also with regions of intense magnetic field in the photosphere (Efanov, Kislyakov & Moiseev 1972; Righini-Cohen & Simon 1977; Silva et al. 2005). Constraints in models of the structure and dynamics of the lower solar atmosphere are imposed from observed brightness temperature at millimeter wavelengths. Although the emission mechanisms attributed to these sources are known, i.e. thermal free–free emission and thermal gyro-resonance (Dulk 1985), there are few examples that simultaneously show the spectral imprint of these mechanisms, and that help to understand how local gas temperature behaves at different heights of the solar atmosphere. Because the linear relationship between the brightness temperature and local electronic temperature, the non-LTE effects which often obscure a reliable diagnostic, are irrelevant in our work. Radio waves can map the chromosphere by measuring the central brightness temperature at different wavelengths, by centre-limb variation, by eclipse measures, and by detailed mapping. A continuum spectra can be assembled collecting radio observations from microwave until the submillimeter wavelengths without the effect of terrestrial atmospheric absorption irreversibly compromising the reliability of the measurements. During the 6th solar commissioning campaign (2015 December), ALMA observed the Sun with Band 3 (84–116 GHz) and Band 6 (211–275 GHz). White et al. (2017) describe the fast-scan single-dish (SD) mapping procedures with the Atacama Large Millimeter/submillimeter Array (ALMA) during that campaign. Shimojo et al. (2017) and Bastian et al. (2017) showed the importance of total-power (TP) measurements for calibrating interferometric maps of complex extended sources like the Sun. At the same period, two other solar dedicated instruments, the Nobeyama Radio Heliograph (NoRH; Nakajima et al. 1994) and the Solar Submillimeter Telescope (SST; Kaufmann et al. 2008), mapped the Sun extending the range from 17 up to 405 GHz. In this work, we combine NoRH, ALMA, and SST maps to analyse the spectral characteristics of the solar chromosphere over active regions and plages. Moreover, our aim is to present a method that allows the comparison of spatially resolved features over solar disc using observations from instruments with different characteristics. The main motivation of this work is to present the most complete spectral description of active regions adding the never included before 107-GHz frequency to make detailed spectra for ARs at different evolutionary stages. In doing this, we had to develop a method to reduce observations obtained with very different techniques and/or spatial resolutions and showing the great benefits that will be produced from coordinated observations with ALMA. High angular resolution is essential for observing the chromosphere, which shows rapidly evolving fine structures, but observe sources of different sizes and with high variability on very short time-scales is a difficult task to undertake. While these techniques are not fully developed we have to rely on average values obtained with the currently available resolutions. In Section 2, we briefly describe the instruments. In Section 3, we present the methodology of calibration and spectra construction. We discuss our results in Section 4 and we finally conclude in Section 5. 2 INSTRUMENTATION The SST has been observing the Sun since 1999 and presently is the only radio telescope that makes daily maps at two simultaneous submillimetre wavelengths. Two receivers at λ = 0.74 mm (ν = 405 GHz), and four at λ = 1.41 mm (ν = 212 GHz) are placed in the Cassegrain focal plane of the 1.5-m parabolic antenna. The nominal Half Power Beam Width (HPBW) of its receivers are: 2 and 4 arcmin, for 405 and 212 GHz, respectively, and the absolute pointing accuracy is around 10 arcsec (Kaufmann et al. 2008). However, structural antenna problems produce deformations that change the expected size and form of the beams. Details and effects of the beams distortion on maps can be seen in Giménez de Castro et al. (2020). To minimize systematic errors, the overall procedure for map assembly was revisited and eliminated the most common errors that appear in the finished maps (Valle Silva 2016). SST maps are composed of an odd number of alternating raster scans shifted in the elevation direction. Typical values are 31 scans shifted by 2 arcmin (i.e. 212 GHz HPBW/2). The data acquisition system integrates the antenna temperature every 40 ms, and the antenna position corresponds to the mid-point of the integration interval. At typical SST scanning velocities for maps, between 0.02○ and 0.04○ per second, implies a pace of ∼5 arcsec in the azimuthal direction. It takes around 20 min for a map completion that covers a square field of 1○ by side in a frame of reference centred in the Sun. On the other hand, ALMA is an interferometer with capabilities to perform SD observations of astrophysical objects in millimeter and submillimetre bands. Solar observing mode was made available in 2016 (ALMA Cycle 4), initially in Band 3 and Band 6. The commissioning of solar observations in interferometric and SD observations with ALMA is described in detail by Shimojo et al. (2017) and White et al. (2017). Since ALMA is a general purpose astronomical instrument, solar observing fraction is small and not all the standard array configurations are optimum for solar observations. The absolute calibration of interferometric maps of complex extended sources like the Sun depends on SD maps of the solar disc. The instantaneous field of view (FOV) is determined by the primary beam of the single antenna and the chosen central frequency: for band 6, the HPBW = 23–30 arcsec and for band 3 HPBW = 54–75 arcsec (Bastian et al. 2017). NoRH is a solar-dedicated radio interferometer. Observations at 17 GHz started in 1992 April and since 1992 June, 8-hr daily observations have been carried out. The best spatial resolution of the NoRH is between 10 and 18 arcsec for 17 GHz. We are facing the challenge of making compatible instruments that use different techniques to compose a map of the solar disc. The NoRH maps were made using interferometric techniques, while the SST maps were made using SD techniques. ALMA is also an interferometer but in this work just the SD maps are used. 3 DATA SELECTION AND INTER-CALIBRATION Full-disc radio maps in total intensity and polarization were produced by the NoRH. The 17-GHz polarization maps are very important in the active regions studies, since high polarization degrees (≳30 per cent) indicate the presence of a gyro-resonance core above the sunspot (Vourlidas, Gary & Shibasaki 2006). The NoRH total intensity maps are in standard FITS format and were downloaded from https://solar.nro.nao.ac.jp/norh/images/daily/2015/12/ (accessed 10 May 2017). To avoid the beaming effect that usually affect the shape of the active regions during the Northern winter (Shimojo et al. 2006), the NoRH maps analsed here were produced with observations made during the local noon. Subsequently, the NoRH maps were degraded by the convolution with the SST beams. Intensity maps were obtained durin g the 6th commissioning campaign testing the ALMA solar capabilities. Observations were made in the highest central frequencies for each band: 107.0 GHz for Band 3 and 238.0 GHz for Band 6. Both files belong to the final product packages of two different observations and were downloaded from https://almascience.nrao.edu/alma-data/science-verification, accessed 10 May 2017. The ALMA maps obtained during the Science Verification campaign need to be after-imaging re-scaled based on the brightness temperature of the centre of the solar disc (White et al. 2017). The calculations are described in the casa guide (https://casaguides.nrao.edu/index.php?title=Sunspot_Band6_SingleDish_for_CASA_5.1). A procedure equivalent has been implemented in python programming language to re-scale the Band 3 and Band 6 maps. Table 1 shows the selected maps at the different frequencies and instruments with date and hour of observation and their calibration status. The selection was based on the time proximity of the observations. Table 1. The maps used in this work were acquired between 02:42 ut and 19:12 ut on 2015 December 17. Status . Instrument . Freq. . HPBW . Qty. . . . (GHz) . (arcsec) . . Calibrated NoRH 17 10 1 Calibrated ALMA 107 58 1 Calibrated ALMA 238 25 1 Non calib. SST 212 240 4 Non calib. SST 405 150 2 Status . Instrument . Freq. . HPBW . Qty. . . . (GHz) . (arcsec) . . Calibrated NoRH 17 10 1 Calibrated ALMA 107 58 1 Calibrated ALMA 238 25 1 Non calib. SST 212 240 4 Non calib. SST 405 150 2 Open in new tab Table 1. The maps used in this work were acquired between 02:42 ut and 19:12 ut on 2015 December 17. Status . Instrument . Freq. . HPBW . Qty. . . . (GHz) . (arcsec) . . Calibrated NoRH 17 10 1 Calibrated ALMA 107 58 1 Calibrated ALMA 238 25 1 Non calib. SST 212 240 4 Non calib. SST 405 150 2 Status . Instrument . Freq. . HPBW . Qty. . . . (GHz) . (arcsec) . . Calibrated NoRH 17 10 1 Calibrated ALMA 107 58 1 Calibrated ALMA 238 25 1 Non calib. SST 212 240 4 Non calib. SST 405 150 2 Open in new tab Solar differential rotation was applied to NoRH and ALMA solar maps to compensate different observation times with respect to SST, so that, in this work, active region locations correspond to SST observing time. NoRH and ALMA maps have been corrected using the drot_map.pro routine, within the solarsoft package (Freeland & Handy 1998) based on the solar differential rotation formula given by Howard, Harvey & Forgach (1990). In order to have a common temperature scale, we calibrated the SST with the ALMA maps: ALMA Band 6 (238 GHz) was used to calibrate the SST 212 GHz. The procedure step by step. Time correction: The lag time between ALMA and SST mid-time of map observation (16:36:39 ut) is approximately 105 min and the differential rotation correction produces minor changes over the disc. Re-scale ALMA to SST spatial resolution: We first resized and re-scaled the ALMA map from 800 × 800 pixel (i.e. 3.0 arcsec pixel−1) to a 675 × 675 pixel map (4.3 arcsec pixel−1). Line up and co-align ALMA with SST: This map was rotated counter clockwise ∼166○, corresponding to the parallactic angle in the mid-time of map observation. Afterward, we embedded it in the centre of a box-field of 4○ side. Degrade ALMA to SST Beams. We convolved the ALMA map with the SST holographically recovered beams (see Costa et al. 2002, for a description of the method). Fig. 1 shows ALMA Band 6 map after the described steps. Temperature calibration. 212 GHz: Using the scans of the interpolated 238 GHz ALMA map it is possible to obtain the brightness temperature over every pixel and transform the antenna temperature of SST map scans to brightness temperature in the ALMA scale. 405 GHz: The calibration of the 405-GHz data follows the same procedure, with the 238-GHz ALMA map multiplied by 0.86, i.e. the ratio of the quiet Sun brightness temperatures at 405 and 212 GHz, namely, 5100 and 5900 K, respectively (Silva et al. 2005). Finally, the calibrated SST maps were rotated clockwise 166.6○ to line up with the ALMA map original orientation. Figure 1. Open in new tabDownload slide (a) 800 × 800 pixel2 ALMA Band 6 map corrected by 1.7 h in differential rotation to match SST mid-time observation. (b) The same map resized and interpolated to 675 × 675 pixel2. (c) Rotated map by ∼166○ ccw, corresponding to the parallactic angle of SST mid-time observation. (d) ALMA Band 6 convolved map with SST holographically recovered beams (Costa et al. 2002). Figure 1. Open in new tabDownload slide (a) 800 × 800 pixel2 ALMA Band 6 map corrected by 1.7 h in differential rotation to match SST mid-time observation. (b) The same map resized and interpolated to 675 × 675 pixel2. (c) Rotated map by ∼166○ ccw, corresponding to the parallactic angle of SST mid-time observation. (d) ALMA Band 6 convolved map with SST holographically recovered beams (Costa et al. 2002). The same procedure was repeated with ALMA Band 3 and NoRH 17 GHz maps, but without the step 5 for temperature calibration. The final product is a set of scaled, aligned, and degraded until-SST-resolution maps covering from 17 to 405 GHz. Fig. 2 shows the excess brightness temperature maps aligned with the SST position, following the procedures described above and before the step (vi). Figure 2. Open in new tabDownload slide Excess brightness temperature maps aligned with the SST observation position.The procedure used to obtain these maps is described in Section 3. Figure 2. Open in new tabDownload slide Excess brightness temperature maps aligned with the SST observation position.The procedure used to obtain these maps is described in Section 3. 4 RESULTS AND DISCUSSION We used the Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline (Hoeksema et al. 2014) to recognize features of the Sun’s photosphere. Automatically identified HMI Active Region Patches (HARPs) track the location and shape of magnetic regions throughout their lifetime. The Space weather HMI Active Region Patch (SHARP) data (Bobra et al. 2014), encapsulates automatically identified active regions (AR) and other magnetic concentrations over the disc. With the cadence of 720 s and the spatial resolution of 1 arcsec, SHARP data are adequate to track a relatively long-term evolution (hours to days) of magnetic structures of the typical AR scale. HARPs are given an identifying number, HARPNUM, that can be associated with a NOAA AR. Since, the NOAA AR numbers are associated with the appearance of sunspots and ignores the spotless AR (Selhorst et al. 2014), there is not a one-to-one correspondence between both catalogues. A set of HARP patches inside their bounding boxes is shown in Fig. 4, resulting from active-region automatic detection algorithm applied to the data on 2015 December 17. NOAA AR numbers are labelled in yellow near the Equator, next to arrows indicating the hemisphere; the HARP number is indicated inside the rectangular bounding box at the upper right. Yellow ‘+’ symbols indicate coordinates that correspond to the reported centre of a NOAA active region. The temporal life of a HARP starts when it rotates on to the visible disc or 2 d before an emerging magnetic feature is first identified in the photosphere. The HARP expires 2 d after the feature decays or when it rotates completely off the disc HARP 6170 (upper left, in red) corresponds to two NOAA ARs, 12469 and 12470; while HARP 6174 (upper right, in orange) to a medium size plage, hereafter named TRAIL, and a more extensive size plage, hereafter named LEAD, both of them bright at submillimetre wavelengths. HARP 6172 (lower right, in blue) and 6189 (lower left, in blue) contain NOAA AR 12468 and NOAA AR 12471, respectively. Finally, HARP 6190 corresponds to another mid-size plage, hereafter named CLOSE, that emerged near to AR 12471 a few hours later. We chose four ARs for the present work: three containing sunspots (12468, 12469, and 12470); the fourth (12471), in the decaying phase, had no sunspot despite exhibiting enhanced bright regions at submmillimetre wavelengths. We also selected the plages LEAD, TRAIL (HARP 6174), and CLOSE (HARP 6190). Fig. 3 shows the time-line of each selected region, the vertical dashed line marks the day of the observations and the lateral labels show the correspondence between HARP number (left side) and NOAA AR number (right side) when it exists. Figure 3. Open in new tabDownload slide Timeline of the ARs and plages selected for this work. The vertical dashed line marks the day of the observations. Vertical axis shows the correspondence between HARP number (left side) and NOAA AR number (right side). Figure 3. Open in new tabDownload slide Timeline of the ARs and plages selected for this work. The vertical dashed line marks the day of the observations. Vertical axis shows the correspondence between HARP number (left side) and NOAA AR number (right side). Figure 4. Open in new tabDownload slide AR automatic detection algorithm applied to the data on 2015 December 17 at 17:00 TAI. NOAA AR numbers are labeled in yellow near the Equator, next to arrows indicating the hemisphere; the HARP number is indicated in the upper right corner of the rectangular bounding box. The HARP 6170 (upper left, in red) includes two NOAA ARs, 12469 and 12470. The HARP 6174 (upper right, in orange) includes a mid-size plage (TRAIL) and a more extensive size plage (LEAD) that are bright at submillimetre frequencies. The HARP 6172 (lower right, in blue) and HARP 6189 (lower left, in blue) includes each one NOAA AR 12468 and NOAA AR 12471, respectively. HARP 6190 includes another mid-size plage (CLOSE) that emerged a few hours later and near to AR 12471. Yellow + symbols indicate coordinates that correspond to the reported center of an NOAA active region. Figure 4. Open in new tabDownload slide AR automatic detection algorithm applied to the data on 2015 December 17 at 17:00 TAI. NOAA AR numbers are labeled in yellow near the Equator, next to arrows indicating the hemisphere; the HARP number is indicated in the upper right corner of the rectangular bounding box. The HARP 6170 (upper left, in red) includes two NOAA ARs, 12469 and 12470. The HARP 6174 (upper right, in orange) includes a mid-size plage (TRAIL) and a more extensive size plage (LEAD) that are bright at submillimetre frequencies. The HARP 6172 (lower right, in blue) and HARP 6189 (lower left, in blue) includes each one NOAA AR 12468 and NOAA AR 12471, respectively. HARP 6190 includes another mid-size plage (CLOSE) that emerged a few hours later and near to AR 12471. Yellow + symbols indicate coordinates that correspond to the reported center of an NOAA active region. During the week from 14 to 20 December 2015, solar activity was at low levels. The period was dominated by low to mid-level C-class flare activity from a number of active regions, the largest of which was a C6/1F flare occurred on 10 December in AR 12468; which was the most productive on the visible disc throughout the period. An unnumbered region behind the north-east limb produced multiple mid-level C-class flares, though. Main characteristics of the selected ARs are listed in Table 2. Table 2. Main characteristics of ARs selected for this work. Region . SHARP . Magnetic . Area . Spots . Number . Number . Class. . (10−6) . Class/Count . 12468 6172 B 20 Cro/3 12469 6178 A 20 Hax/2 12470 6178 B 610 Eko/17 12471 6189 Region . SHARP . Magnetic . Area . Spots . Number . Number . Class. . (10−6) . Class/Count . 12468 6172 B 20 Cro/3 12469 6178 A 20 Hax/2 12470 6178 B 610 Eko/17 12471 6189 Open in new tab Table 2. Main characteristics of ARs selected for this work. Region . SHARP . Magnetic . Area . Spots . Number . Number . Class. . (10−6) . Class/Count . 12468 6172 B 20 Cro/3 12469 6178 A 20 Hax/2 12470 6178 B 610 Eko/17 12471 6189 Region . SHARP . Magnetic . Area . Spots . Number . Number . Class. . (10−6) . Class/Count . 12468 6172 B 20 Cro/3 12469 6178 A 20 Hax/2 12470 6178 B 610 Eko/17 12471 6189 Open in new tab The AR 12470 presented a high degree of polarization (∼100 per cent) at 17 GHz, which indicates the presence of gyro-resonance emission, and also explains the high observed brightness temperature. The other regions with spots, AR 12468 and AR 12469, have only a marginal polarization and minor brightness temperature enhancements. On the other hand, AR 12471, does not present sunspots or polarization on the day of observations. Since ALMA and NoRH maps were degraded to match SST resolution some features smaller than 2 arcmin are blunted by the effect of the convolution. We adopted the Silva et al. (2005) approach to obtain the flux density at both submillimetre wavelengths. The conversion from excess brightness temperature ΔTb = TAR − TQS to flux density uses the Rayleigh–Jeans approximation assuming a homogeneous source of known size $$\begin{eqnarray*} S_{\lambda } = \frac{2k_{B}}{\lambda ^{2}}\theta _{\mathrm{ pix}}^{2} \sum _{\mathrm{ box}} \Delta T_{\mathrm{ b}} , \end{eqnarray*}$$(1) where θpix is the angular size of the map pixel after interpolation, TAR and TQS are the brightness temperatures of the AR and the quiet Sun (QS), respectively. The summation is performed over all pixels within the box, boxes limits are defined where the brightness temperature equals to half the maximum of the region. After analysing all the AR defined, we found that all of them have very approximately the same size, around 3 arcmin, which is the typical size of magnetic features observed in the HARPs, therefore we adopted 3 arcmin square boxes, that can be seen over the full-disc HMI magnetogram (Schou et al. 2012) in Fig. 5. Extensive AR 12470 was separated in two boxes: 12470a (left) and 12470b (right), the same for AR 12469. The central box is the QS region, where we obtain the background brightness temperature. We opted for this alternative because after the convolution, the maps show a diminished centre-to-limb variation and because we exclude the ARs close to the limb and select those that are towards the centre (within 80 per cent of solar radius). Figure 5. Open in new tabDownload slide Regions selected for this analysis are indicated with red boxes of 3 arcsec by side. The central box labelled QS is the region used to define the background (quiet Sun) brightness temperature. The background image is a full-disc HMI magnetogram (Schou et al. 2012) of 2015 December 17. Figure 5. Open in new tabDownload slide Regions selected for this analysis are indicated with red boxes of 3 arcsec by side. The central box labelled QS is the region used to define the background (quiet Sun) brightness temperature. The background image is a full-disc HMI magnetogram (Schou et al. 2012) of 2015 December 17. Fig. 6 presents the brightness temperature spectra. The black and red error bars represent the obtained AR and quiet Sun brightness temperatures Tb, respectively. Based on the analysis of solar brightness temperature measurements from White et al. (2017), we have conservatively introduced uncertainties of the order of 10 per cent of the value of the average temperature of each analysed region of the Band 3 and Band 6 maps. Following the ALMA-SST inter-calibration procedure, we applied to the SST temperatures the same 10 per cent criterion for error bars we note that ALMA 238 GHz and SST 212 GHz are consistent within the uncertainties. Figure 6. Open in new tabDownload slide Brightness temperature Tb spectra for each region. Black error bars represent the AR Tb while the red error bars are the quiet Sun TB. Figure 6. Open in new tabDownload slide Brightness temperature Tb spectra for each region. Black error bars represent the AR Tb while the red error bars are the quiet Sun TB. On the day of the observation, ARs with sunspots that have not completed the emergence phase of their lifetime, i.e. AR 12470a,b and AR 12469a show higher brightness temperature excesses relative to QS value at 17 GHz, which is also in agreement with the gyro-resonant emission mechanism. When the temperature excess difference is smaller, as in the AR 12469b and AR 12468 cases, the region is already in the decay phase of its lifetime or simply no longer has sunspots as shown in AR 12471. The CLOSE plage also shows this behaviour, probably as a consequence of the proximity to the magnetic field of AR 12471. The LEAD and TRAIL plages have the minimal brightness temperatures excesses at 17 GHz. At submillimetre, the excess brightness temperatures remained between 200 and 400 K for all regions with exception of LEAD and TRAIL where the excesses were below 300 K. Fig. 7 shows the flux density spectra obtained using equation (1). We note that flux density increases with frequency. Since brightness temperature excesses at ν > 100 GHz are of the same order, the submillimetre emission spectrum is shaped by the Raleigh-Jeans dependence with λ−2. We also note that spectra above 100 GHz are very similar. We fitted a power law F ∝ να to the spectra between 17 and 405 GHz, the spectral index α is written in the bottom right of every panel of Fig. 7 in red. An average spectral index can be derived from our observations |$\overline{\alpha _{17}} = 2.19 \pm 0.18$|⁠. The spectral index above 100 GHz (blue number in the bottom right of every panel), is consistently smaller than the previous one and yields a smaller mean but with a greater dispersion, |$\overline{\alpha _{107}} = 2.11 \pm 0.55$|⁠. In a recent work, Pereira, Giménez de Castro & Valle Silva (2018) have used an Artificial Intelligence (AI) procedure to analyse around 3000 AR observed at 212 and 405 GHz with the SST between 2002 and 2017, and they report an spectral index |$\overline{\alpha _{212}} = 1.58$|⁠, showing that the index tends to reduce when frequencies near 405 GHz are included. Moreover, Giménez de Castro et al. (2020) have shown that it might be anticorrelated with the solar cycle. Figure 7. Open in new tabDownload slide Density flux spectra of active regions analysed in this work. The fitted indexes are written in the bottom right of every panel, in red for frequencies between 17 and 405 GHz and in blue for frequencies above 100 GHz. Figure 7. Open in new tabDownload slide Density flux spectra of active regions analysed in this work. The fitted indexes are written in the bottom right of every panel, in red for frequencies between 17 and 405 GHz and in blue for frequencies above 100 GHz. Although very similar, the ARs show differences between them: AR12471 and AR12468 have the highest 405 GHz fluxes, whereas AR12470a and b, have the highest 17 flux. It is also interesting to note that AR12470 is the biggest in size and has the largest number of spots, although its submilimetre emission is not the highest. In contrast AR12471 is in the decaying phase without magnetic classification and spots. Loukitcheva et al. (2017) have shown that the umbra is cooler than the penumbra at ν = 230 GHz (λ = 1.3 mm), while it is brighter at ν = 100 GHz (λ = 3.0 mm). Since we are integrating the emission coming from different spots in the AR, the larger the number of spots the weaker the integrated flux; this can explain what is observed here. 5 CONCLUSIONS Wide band solar radio-spectra have been made with the concurrence of several instruments observing at the same time, while observations of full disc on a daily basis are still scarce. The combination of instruments has allowed us to glimpse how the solar observation should be in high frequencies with recent facilities like ALMA and the new ones to come in the next future. Chromosphere emission detected at high frequencies is a challenge for this new instrumentation but the particular characteristics of SST, with low resolution but with the highest radio frequency for a solar-dedicated instrument, has proved useful for this purpose. In this work, we developed a procedure to compare observations taken with telescopes with very different spatial resolutions. Once applied, the comparison of 238 GHz (ALMA) and 212 GHz (SST) AR fluxes are very consistent, validating the method. Some active regions were observed at different stages of their lifetime, their brightness temperatures and corresponding flux densities were obtained assuming an AR average size of 3 arcmin × 3 arcmin. The regions studied maintained a low activity, B an C-class flares during their lifetime and no flares during the observing day. Therefore, the contribution to the excesses of temperature is only attributable to gyro-resonance emission in the cm part and thermal emission in the submillimetre part of the spectrum. As it was first observed by the SST (Silva et al. 2005), the flux increases towards submillimeter frequencies, the inclusion of the intermediate ν = 107 GHz observation, further confirms the behaviour of the emission at these frequencies which is completely different from that at 17 GHz. The derived spectral submillimeter index α is consistent with optically thick thermal emission. The spectral index is expected to be less than 2 because at higher frequencies we are looking deeper into the atmosphere where the temperature is expected to be slightly lower, e.g. the quiet Sun brightness temperature at disc centre decreases from 7300 K at 100 GHz to 5900 K at 230 GHz (White et al. 2017), and since the flux Sλ∝Tb/λ2, the weak frequency dependence of Tb lowers the spectral index, as it is shown when |$\overline{\alpha _{107}} \lt \overline{\alpha _{17}}$|⁠. Something similar happens in active regions as long as the temperature is decreasing with decreasing height. Above 200 GHz, the temperature decreases much more slowly with frequency so the spectral index should be closer to 2. ACKNOWLEDGEMENTS This work is based on data acquired at Complejo Astronómico El Leoncito, operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba and San Juán. This paper makes use of the ALMA data: ADS/JAO.ALMA#2011.0.000020.SV. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The research was partially financed by the Brazilian Agency FAPESP through grant 2013/24155-3, CAPES grant 88881.310386/2018-01, and by the U.S. Air Force Office for Scientific Research FA9550-16-1-0072, JFVS is thankful to MackPesquisa and PNPD-CAPES grant at Universidade Presbiteriana Mackenzie, CGGC and JPR are thankful to CNPq by the support through grants 305203/2016-9 and 312066/2016-3, respectively. 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TI - Spectral signature of solar active region in millimetre and submillimetre wavelengths
JF - Monthly Notices of the Royal Astronomical Society
DO - 10.1093/mnras/staa3354
DA - 2020-11-21
UR - https://www.deepdyve.com/lp/oxford-university-press/spectral-signature-of-solar-active-region-in-millimetre-and-FDwhSJ0clT
SP - 1964
EP - 1969
VL - 500
IS - 2
DP - DeepDyve
ER -