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ALMA will open a new chapter in the study of the Sun by providing a leap in spatial resolution and sensitivity compared to currently available mm wave- length observations. In preparation of ALMA, we have carried out a large number of observational tests and state-of-the-art radiation MHD simulations. Here we review the best available observations of the Sun at millimeter wavelengths.Using state of the art radiation MHD simulations of the solar atmosphere we demonstrate the huge potential of ALMA observations for uncovering the nature of the solar chromosphere. We show that ALMA will not only provide a reliable probe of the thermal structure and dynamics of the chromosphere, it will also open up a powerful new diagnostic of magnetic field at chromospheric heights, a fundamentally important, but so far poorly known parameter.
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We measured the center-to-limb variation of the brightness temperature, $T_b$, from ALMA full-disk images at two frequencies and inverted the solution of the transfer equation to obtain the electron temperature, $T_e$ as a function of optical depth, $tau$. The ALMA images are very similar to AIA images at 1600AA. The brightness temperature at the center of the disk is 6180 and 7250 K at 239 and 100 GHz respectively, with dispersions of 100 and 170 K. Plage regions stand out clearly in the 239/100 GHz intensity ratio, while faculae and filament lanes do not. The solar disk radius, reduced to 1 AU, is $961.1pm2.5$ arcsec and $964.1pm4.5$ arcsec at 239 and 100 GHz respectively. A slight but statistically significant limb brightening is observed at both frequencies. The inversion of the center-to-limb curves shows that $T_e$ varies linearly with the logarithm of optical depth for $0.34<tau_{100,GHz}<12$, with a slope $dln T_e/dtau=-608$ K. Our results are 5% lower than predicted by the average quiet sun model C of Fontenla et al. (1993), but do not confirm previous reports that the mm-$lambda$ solar spectrum is better fitted with models of the cell interior.
This document was created by the Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) in preparation of the first regular observations of the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA), which are anticipated to start in ALMA Cycle 4 in October 2016. The science cases presented here demonstrate that a large number of scientifically highly interesting observations could be made already with the still limited solar observing modes foreseen for Cycle 4 and that ALMA has the potential to make important contributions to answering long-standing scientific questions in solar physics. With the proposal deadline for ALMA Cycle 4 in April 2016 and the Commissioning and Science Verification campaign in December 2015 in sight, several of the SSALMON Expert Teams composed strategic documents in which they outlined potential solar observations that could be feasible given the anticipated technical capabilities in Cycle 4. These documents have been combined and supplemented with an analysis, resulting in recommendations for solar observing with ALMA in Cycle 4. In addition, the detailed science cases also demonstrate the scientific priorities of the solar physics community and which capabilities are wanted for the next observing cycles. The work on this White Paper effort was coordinated in close cooperation with the two international solar ALMA development studies led by T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be further updated until the beginning of Cycle 4 in October 2016. In particular, we plan to adjust the technical capabilities of the solar observing modes once finally decided and to further demonstrate the feasibility and scientific potential of the included science cases by means of numerical simulations of the solar atmosphere and corresponding simulated ALMA observations.
ALMA observations of the Sun at mm-$lambda$ offer a unique opportunity to investigate the temperature structure of the solar chromosphere. In this article we expand our previous work on modeling the chromospheric temperature of the quiet Sun, by including measurements of the brightness temperature in the network and cell interiors, from high resolution ALMA images at 3 mm (Band 3) and 1.26 mm (Band 6). We also examine the absolute calibration of ALMA full-disk images. We suggest that the brightness temperature at the center of the solar disk in Band 6 is $sim440$ K above the value recommended by White et al. (2017) and we give improved results for the electron temperature variation of the average quiet Sun with optical depth, as well as the derived spectrum at the center of the disk. We found that the electron temperature in the network is considerably lower than predicted by model F of Fontenla et al. (1993) and that of the cell interior considerably higher than predicted by model A. Depending upon the network/cell segregation scheme, the electron temperature difference between network and cell at $tau=1$ (100 GHz) is from $sim$660 to $sim$1550 K, compared to $sim$3280 K predicted by the models; similarly, the $T_e$ ratio is from $sim$1.10, to 1.24, against $sim$1.55 of the model prediction. We also found that the network/cell $T_e(tau)$ curves diverge as $tau$ decreases, indicating an increase of contrast with height and possibly a steeper temperature rise in the network than in the cell interior.
Using Atacama Large Millimeter/submillimeter Array (ALMA) observations of the quiet Sun at 1.26 and 3 mm, we study spatially resolved oscillations and transient brightenings, i.e. small, weak events of energy release. Both phenomena may have a bearing on the heating of the chromosphere. At 1.26 mm, in addition to power spectra of the original data, we degraded the images to the spatial resolution of the 3 mm images and used fields of view of equal area for both data sets. The detection of transient brightenings was made after the oscillations were removed. At both frequencies we detected p-mode oscillations in the range 3.6-4.4 mHz. In the corrected data sets, the oscillations at 1.26 and 3 mm showed brightness temperature fluctuations of ~1.7-1.8% with respect to the average quiet Sun, corresponding to 137 and 107 K, respectively. They represented a fraction of 0.55-0.68 of the full power spectrum and their energy density at 1.26 mm was 0.03 erg cm$^{-3}$. We detected 77 transient brightenings at 1.26 mm and 115 at 3 mm. Although the majority of the 1.26 mm events occurred in cell interior, their occurrence rate per unit area was higher than that of the 3 mm events. The computed low-end energy of the 1.26 mm transient brightenings ($1.8 times 10^{23}$ erg) is among the smallest ever reported, irrespective of the wavelength of observation. However, their power per unit area is smaller than that of the 3 mm events, probably due to the detection of many weak 1.26 mm events. We also found that ALMA bright network structures corresponded to dark mottles/spicules seen in broadband H$alpha$ images from the GONG network.
We present an initial study of one of the first ALMA Band 3 observations of the Sun with the aim to characterise the diagnostic potential of brightness temperatures measured with ALMA on the Sun. The observation covers 48min at a cadence of 2s targeting a Quiet Sun region at disk-centre. Corresponding time series of brightness temperature maps are constructed with the first version of the Solar ALMA Pipeline (SoAP) and compared to simultaneous SDO observations. The angular resolution of the observations is set by the synthesized beam (1.4x2.1as). The ALMA maps exhibit network patches, internetwork regions and also elongated thin features that are connected to large-scale magnetic loops as confirmed by a comparison with SDO maps. The ALMA Band 3 maps correlate best with the SDO/AIA 171, 131 and 304 channels in that they exhibit network features and, although very weak in the ALMA maps, imprints of large-scale loops. A group of compact magnetic loops is very clearly visible in ALMA Band 3. The brightness temperatures in the loop tops reach values of about 8000-9000K and in extreme moments up to 10 000K. ALMA Band 3 interferometric observations from early observing cycles already reveal temperature differences in the solar chromosphere. The weak imprint of magnetic loops and the correlation with the 171, 131, and 304 SDO channels suggests though that the radiation mapped in ALMA Band 3 might have contributions from a larger range of atmospheric heights than previously assumed but the exact formation height of Band 3 needs to be investigated in more detail. The absolute brightness temperature scale as set by Total Power measurements remains less certain and must be improved in the future. Despite these complications and the limited angular resolution, ALMA Band 3 observations have large potential for quantitative studies of the small-scale structure and dynamics of the solar chromosphere.
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