The solar chromosphere at millimetre and ultraviolet wavelengths

Abstract

Solar observations with the Atacama Large Millimeter/submillimeter Array (ALMA) provide us with direct measurements of the brightness temperature in the solar chromosphere. We study the temperature distributions obtained with ALMA Band 6 (in four subbands at 1.21, 1.22, 1.29, and 1.3 mm) for various areas at, and in the vicinity of, a sunspot, comprising quasi-quiet and active regions with different amounts of underlying magnetic fields. We compare these temperatures with those obtained at near-and far-ultraviolet (UV) wavelengths (and with the line-core intensities of the optically-thin far-UV spectra), co-observed with the Interface Region Imaging Spectrograph (IRIS) explorer. These include the emission peaks and cores of the Mg ii k 279.6 nm and Mg ii h 280.4 nm lines as well as the line cores of C ii 133.4 nm, O i 135.6 nm, and Si iv 139.4 nm, sampling the mid-to-high chromosphere and the low transition region. Splitting the ALMA sub-bands resulted in an slight increase of spatial resolution in individual temperature maps, thus, resolving smaller-scale structures compared to those produced with the standard averaging routines. We find that the radiation temperatures have different, though somewhat overlapping, distributions in different wavelengths and in the various magnetic regions. Comparison of the ALMA temperatures with those of the UV diagnostics should, however, be interpreted with great caution, the former is formed under the local thermodynamic equilibrium (LTE) conditions, the latter under non-LTE. The mean radiation temperature of the ALMA Band 6 is similar to that extracted from the IRIS C ii line in all areas with exception of the sunspot and pores where the C ii poses higher radiation temperatures. In all magnetic regions, the Mg ii lines associate with the lowest mean radiation temperatures in our sample. These will provide constraints for future numerical models.

The Sun at millimeter wavelengths

“…The small size of the FOV and thus the peculiarities of the observed regions will produce variations in the statistical properties derived from different observations. These results should be compared to a corresponding analysis of mosaicking data that cover larger FOVs (see e.g., Bastian et al 2017;Jafarzadeh et al 2019). Furthermore, the test for different angular resolutions implies that more extended array configurations of ALMA, which might be offered in future observing cycles, are likely to lead to higher rms variations and thus more contrast in the reconstructed images.…”

“…While the first regular ALMA observations of the Sun were only offered in Cycle 4 with a first solar campaign in December 2016, earlier observations from Commissioning and Science Verification (CSV) campaigns have been made publicly available. Both regular and CSV data are already used in publications: Alissandrakis et al (2017), Bastian et al (2017), Shimojo et al (2017b), Brajša et al (2018), Nindos et al (2018), Yokoyama et al (2018), Jafarzadeh et al (2019), Loukitcheva et al (2019), Molnar et al (2019), Rodger et al (2019), Selhorst et al (2019), Patsourakos et al (2020), da Silva Santos et al (2020.…”

The multi-thermal chromosphere

“…According to the Proposer's Guide 3 for Cycle 4, the absolute calibration of the single-dish TP brightness temperatures has an uncertainty of between 10 and 15%, which translates to 590−885 K based on the reference value of 5900 K recommended by White et al (2017). However, the analysis of ALMA data implies that the uncertainty might be as low as 5% or even less (e.g., Rodger et al 2019;Jafarzadeh et al 2019). In addition to the uncertainty in the TP calibration, additional uncertainties in the interferometric brightness temperature differences have to be considered but they are expected to be smaller.…”

“…The Atacama Large Millimeter/Submillimeter Array (ALMA, Wootten & Thompson 2009) has overcome the spatial resolution limitations that observing at such long wavelengths entails. ALMA has shown that the landscape of the solar surface at 1 mm is marked by relatively cooler regions such as the QS and sunspot umbras, and hotter features such as plage regions (Loukitcheva et al 2017;Brajša et al 2018), and that the brightness of the mm continuum correlates with that of the Mg ii lines (Bastian et al 2017(Bastian et al , 2018Jafarzadeh et al 2019). However, the large scatter and offset between the two diagnostics is evidence for NLTE effects in the Mg ii lines that make them only partially sensitive to the chromospheric temperatures (e.g., Leenaarts et al 2013a), although there may also be systematic differences in their formation heights (e.g., da Silva Santos et al 2018).…”

Incoherent Solar Radio Emission

“…The first high-resolution images of the quiet Sun in the millimeter range were obtained by White et al (2006) and Loukitcheva et al (2006) who used the Berkeley-Illinois-Maryland Association Array (BIMA) to obtain ∼ 10 ′′ resolution. With the advent of ALMA a new generation of high-resolution millimeterwavelength images has been forming (e.g., Bastian et al, 2017;Shimojo et al, 2017a,b;Nindos et al, 2018;Yokoyama et al, 2018;Jafarzadeh et al, 2019;Loukitcheva et al, 2019;Molnar et al, 2019;Patsourakos et al, 2020;Wedemeyer et al, 2020) and an example is presented in Figure 2. The figure indicates that the chromospheric network, delineated in the AIA 1,600 Å image, is the dominant structure in the radio images.…”