High-frequency oscillations in small chromospheric bright features observed with Atacama Large Millimetre/Submillimetre Array

Abstract

We report detection of oscillations in brightness temperature, size and horizontal velocity of three small bright features in the chromosphere of a plage/enhanced-network region. The observations, which were taken with high temporal resolution (i.e. 2 s cadence) with the Atacama large millimetre/ submillimetre array (ALMA) in Band 3 (centred at 3 mm; 100 GHz), exhibit three small-scale features with oscillatory behaviour with different, but overlapping, distributions of period on the order of, on average, 90 ± 22 s, 110 ± 12 s and 66 ± 23 s, respectively. We find anti-correlations between perturbations in brightness, temperature and size of the three features, which suggest the presence of fast sausage-mode waves in these small structures. In addition, the detection of transverse oscillations (although with a larger uncertainty) may also suggest the presence of Alfvénic oscillations which are likely representative of kink waves. This work demonstrates the diagnostic potential of high-cadence observations with ALMA for detecting high-frequency magnetohydrodynamic waves in the solar chromosphere. Such waves can potentially channel a vast amount of energy into the outer atmosphere of the Sun. This article is part of the Theo Murphy meeting issue ‘High-resolution wave dynamics in the lower solar atmosphere’.

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

“…This observation (which is also used in Guevara Gómez et al [92]) samples a plage region, with strong magnetic-field concentrations covering the majority of the FOV (organized in an inverse c-shape) in the photosphere. As a result, the magnetic topology at chromospheric heights largely represents the vertical/near-vertical fields, but also more inclined/horizontal fields towards the edges of the FOV and within the areas between the centre and left edge of the FOV.…”

“…It is worth noting that high-frequency MHD waves (on the order of 9–17 mHz) have also been detected in small-scale structures in ALMA’s Band 3 observations (i.e. in the D2 dataset presented here [92]). Thus, many of the high-frequency oscillations (up to 9 mHz) we observed in small areas within the FOV of our datasets (see peak-frequency maps in figures 1–10), could also be due to various types of MHD waves which have vastly been observed in the mid-to-high chromosphere at small scales [42,43,103,104].…”

Characterization of shock wave signatures at millimetre wavelengths from Bifrost simulations

“…In that regard, the Atacama Large Millimeter/submillimeter Array (ALMA), which offers regular observations of the Sun since 2016, represents a major step forward in terms of resolution, and has already provided insights into the dynamics of mm-wavelength intensities, e.g. [33–39]. Modelling has shown that propagating shock waves will cause variation in mm-wavelength observables [13,14].…”

Magnetohydrodynamic Fast Sausage Waves in the Solar Corona

“…Given the prominence of the coronal heating problem in solar and astrophysics, one naturally asks how much energy is propagated in sausage modes (see also the review by Van Doorsselaere et al, this issue). This is particularly relevant for the lower solar atmosphere, where sausage modes have been observed to be sufficiently energetic in a considerable number of structures of various sizes, with sunspots (Dorotovič et al 2014), pores (Grant et al 2015), mottles (Morton et al 2012), and chromospheric brightenings associated with plages/enhanced-network regions (Guevara Gómez et al 2020) being examples. Theoretical examinations were therefore performed for sausage modes from the energetics perspective with photospheric and/or chromospheric applications in mind.…”