ALMA observations of the variability of the quiet Sun at millimeter wavelengths

Abstract

Aims. We address the variability of the quiet solar chromosphere at 1.26 mm and 3 mm with a focus on the study of spatially resolved oscillations and transient brightenings, which are small, weak events of energy release. Both phenomena may have a bearing on the heating of the chromosphere. Methods. We used Atacama Large Millimeter/submillimeter Array (ALMA) observations of the quiet Sun at 1.26 mm and 3 mm. The spatial and temporal resolution of the data were 1 − 2″ and 1 s, respectively. The concatenation of light curves from different scans yielded a frequency resolution in spectral power of 0.5−0.6 mHz. 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 that were equal in area for both data sets. The detection of transient brightenings was made after the effect of oscillations was removed. Results. At both frequencies, we detected p-mode oscillations in the range 3.6−4.4 mHz. The corrections for spatial resolution and field of view at 1.26 mm decreased the root mean square (rms) of the oscillations by a factor of 1.6 and 1.1, respectively. In the corrected data sets, the oscillations at 1.26 mm 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. We detected 77 transient brightenings at 1.26 mm and 115 at 3 mm. Although their majority occurred in the cell interior, the occurrence rate per unit area of the 1.26 mm events was higher than that of the 3 mm events; this conclusion does not change if we take into account differences in spatial resolution and noise levels. The energy associated with the transient brightenings ranged from 1.8 × 1023 to 1.1 × 1026 erg and from 7.2 × 1023 to 1.7 × 1026 erg for the 1.26 mm and 3 mm events, respectively. The corresponding power-law indices of the energy distribution were 1.64 and 1.73. We also found that ALMA bright network structures corresponded to dark mottles or spicules that can be seen in broadband Hα images from the GONG network. Conclusions. The fluctuations associated with the p-mode oscillations represent a fraction of 0.55−0.68 of the full power spectrum. Their energy density at 1.26 mm is 3 × 10−2 erg cm−3. The computed low-end energy of the 1.26 mm transient brightenings is among the smallest ever reported, irrespective of the wavelength of the observation. Although the occurrence rate per unit area of the 1.26 mm transient brightenings was higher than that of the 3 mm events, their power per unit area is smaller likely due to the detection of many weak 1.26 mm events.