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Bastian et al. (2017) recently published a quantitative comparison between continuum observations of a solar active region at a wavelength of 1.25 mm by the Atacama Large Millimeter/submillimeter Array (ALMA) and observations of the Mg ii ultraviolet line emission made by the Interface Region Imaging Spectrograph (IRIS). The ALMA data were acquired as part of the commissioning and science verification effort. An error has been identified in the ALMA data calibration path that led to an error in the reported 1.25 mm brightness temperature distribution.
The ALMA 1.25 mm map included both interferometric and total power data. The former measured variations in the 1.25 mm brightness relative to the background brightness distribution, which is resolved out by the interferometer. To remedy this, a map of the large-scale brightness distribution of the Sun is made using fast-scanning total power measurements (White et al. 2017). This is combined with the interferometric observations using "feathering" techniques to produce a final map on an absolute brightness temperature scale (Shimojo et al. 2017). The overall calibration depends on correct scaling of the total power map. Following publication, an error was identified in the total power map used in the feathering process: it should have been scaled such that the mean quiet Sun brightness temperature was 5900 K. We have recomputed Figure 2 of Bastian et al. using the corrected total power map here, and have recalculated the quantities reported in the published article. The mean brightness temperature and the range of brightness temperatures in the ALMA 1.25 mm map are 6940 K and 4800–8345 K, respectively, rather than 7380 K and 5200–8700 K as originally reported. The linear fits to pixels in the sunspot umbra, "quiet" areas, and "plage" regions were also recomputed, yielding slightly different slopes. These are m = 0.38, 0.52, and 0.70 in the umbra, quiet, and plage areas compared with 0.39, 0.55, and 0.68, respectively, in the published article. We note that the difference between the mean 1.25 mm brightness temperature and the mean Mg II h line radiation temperature decreased from nearly 2000 K to 1500 K.
Figure 2. The main panel shows a scatter plot of IRIS Mg ii h line radiation temperatures vs. the corresponding ALMA 1.25 mm continuum brightness temperatures. Gray points represent all pixels in common to the two maps. Blue points mark pixels in the sunspot umbra; red for "quiet" areas; and yellow for "plage" regions. The corresponding histograms are shown for the ALMA data (top panel) and the IRIS data (right panel). The mean 1.25 mm brightness temperature of the sunspot, quiet, and plages are 6138 K, 6608 K, and 7264 K, whereas the mean Mg ii h line radiation temperatures are 5109 K, 5069 K, and 5672 K, respectively. Linear fits to pixels marking umbra, quiet, and plage pixels are shown as a solid, dashed, and dotted–dashed line, respectively.
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While the numerical values have changed, the conclusions detailed in Bastian et al. have not.
We thank Shahin Jafarzadeh and Mikolaj Szydlarski of the University of Oslo for identifying the scaling problem in the originally published ALMA data.