Observations of solar chromospheric oscillations at 3 mm with ALMA

Abstract

Aims. To study spatially-resolved chromospheric oscillations of the quiet Sun in the mm-domain at a resolution of a few arcsec, typically 2.4 ×4.5 . Methods. We used Atacama Large millimeter and sub-millimeter Array (ALMA) time-series of interferometric observations of the quiet Sun obtained at 3 mm with a 2-s cadence and a spatial resolution of a few arcsec. The observations were performed on March 16, 2017 and seven 80 ×80 fields-of-view going from disk center to limb were covered, each one observed for 10 min, therefore limiting the frequency resolution of the power spectra to 1.7 mHz. For each field of view, masks for cell and network were derived, and the averaged power spectral densities (PSDs) for the entire field of view, cell and network were computed. The resulting power spectra were fitted with an analytical function in order to derive the frequency and the root-mean-square (rms) power associated with the peaks. The same analysis, over the same fields of view and for the same intervals, was performed for simultaneous Atmospheric Imaging Assembly (AIA) image sequences in 1600 Å. Results. Spatially-resolved chromospheric oscillations at 3 mm, with frequencies of 4.2 ± 1.7 mHz are observed in the quiet Sun, in both cell and network. The coherence length-scale of the oscillations is commensurate with the spatial resolution of our ALMA observations. Brightness-temperature fluctuations in individual pixels could reach up to a few hundred K, while the spatially averaged power spectral densities yield rms in the range ≈ 55-75 K, i.e., up to ≈ 1 % of the averaged brightness temperatures and exhibit a moderate increase towards the limb. For AIA 1600 Å the oscillation frequency is 3.7 ± 1.7 mHz. The relative rms is up to 6% of the background intensity, with a weak increase towards disk center (cell, average). ALMA 3 mm time-series lag AIA 1600 Å by ≈ 100 s, which corresponds to a formation-height difference of ≈ 1200 km, representing a novel determination of this important parameter. Conclusions. The ALMA oscillations that we detected exhibit higher amplitudes than those derived from the lower (≈ 10 ) resolution observations at 3.5 mm by White et al. (2006). Chromospheric oscillations are, therefore, not fully resolved at the length-scale of the chromospheric network, and possibly not even at the spatial resolution of our ALMA observations. Any study of transient brightenings in the mm-domain should take into account the oscillations.

An overall view of temperature oscillations in the solar chromosphere with ALMA

“…In both studies, the authors applied a high-pass filter to the data by subtracting a third-order polynomial fit from the time series. For short time series of just 10 min as that used by Patsourakos et al [109], this filters out oscillations below approximately 3 mHz, leaving a power peak at around 4 mHz. It has less of an effect for the 30 min long series studied by White et al [108].…”

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.…”

The Sun at millimeter wavelengths

“…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.…”