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

Abstract

By direct measurements of the gas temperature, the Atacama Large Millimeter/submillimeter Array (ALMA) has yielded a new diagnostic tool to study the solar chromosphere. Here, we present an overview of the brightness-temperature fluctuations from several high-quality and high-temporal-resolution (i.e. 1 and 2 s cadence) time series of images obtained during the first 2 years of solar observations with ALMA, in Band 3 and Band 6, centred at around 3 mm (100 GHz) and 1.25 mm (239 GHz), respectively. The various datasets represent solar regions with different levels of magnetic flux. We perform fast Fourier and Lomb–Scargle transforms to measure both the spatial structuring of dominant frequencies and the average global frequency distributions of the oscillations (i.e. averaged over the entire field of view). We find that the observed frequencies significantly vary from one dataset to another, which is discussed in terms of the solar regions captured by the observations (i.e. linked to their underlying magnetic topology). While the presence of enhanced power within the frequency range 3–5 mHz is found for the most magnetically quiescent datasets, lower frequencies dominate when there is significant influence from strong underlying magnetic field concentrations (present inside and/or in the immediate vicinity of the observed field of view). We discuss here a number of reasons which could possibly contribute to the power suppression at around 5.5 mHz in the ALMA observations. However, it remains unclear how other chromospheric diagnostics (with an exception of H α line-core intensity) are unaffected by similar effects, i.e. they show very pronounced 3-min oscillations dominating the dynamics of the chromosphere, whereas only a very small fraction of all the pixels in the 10 ALMA datasets analysed here show peak power near 5.5 mHz. This article is part of the Theo Murphy meeting issue ‘High-resolution wave dynamics in the lower solar atmosphere’.

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

“…Therefore, ALMA observations have been demonstrated to possess sufficient quality and temporal resolution to search for various long-lived and transient phenomena (within its current spatial-resolution limits), and likewise, to study MHD-wave signatures in the solar chromosphere. An overall study of temperature oscillations in the solar chromosphere with ALMA (using ten different datasets in both Band 3 and Band 6) has shown the dependency of the oscillation properties on the magnetic configurations in the mid-to-high chromosphere [44].…”

High-resolution wave dynamics in the lower solar atmosphere

“…Transitioning into radio observations of the solar atmosphere, Jafarzadeh et al [39] and Guevara Gómez et al [40] study pioneering ALMA observations to search for wave signatures in brightness temperature values that are coupled to the chromospheric plasma. Examining the brightest (and therefore hottest) locations of a plage/enhanced-network region observed by ALMA, Guevara Gómez et al [40] uncover significant oscillations in brightness temperature, size and horizontal velocity, which are believed to be combined signatures of fast sausage-mode and Alfvénic waves in these small structures.…”

“…Examining the brightest (and therefore hottest) locations of a plage/enhanced-network region observed by ALMA, Guevara Gómez et al [40] uncover significant oscillations in brightness temperature, size and horizontal velocity, which are believed to be combined signatures of fast sausage-mode and Alfvénic waves in these small structures. Turning attention towards ALMA observations demonstrating varying degrees of magnetic complexity, Jafarzadeh et al [39] highlight the potential challenges when interpreting radio observations. Notably, the authors find that the wave power spectra vary from dataset to dataset, which is further examined in terms of the underlying magnetic topology.…”

“…Notably, the authors find that the wave power spectra vary from dataset to dataset, which is further examined in terms of the underlying magnetic topology. Importantly, Jafarzadeh et al [39] propose that more theoretical and modelling work is required to understand why only a very small fraction of the ALMA data examined show peak power near 5.5 mHz, which is in contradiction to other chromospheric diagnostics where very pronounced oscillations are regularly found in this band.…”

Characterisation of shock wave signatures at millimetre wavelengths from Bifrost simulations

“…In order to detect the small scale dynamics in observational mmwavelength data, high spatial and temporal resolution is essential. In that regard, the Atacama Large millimetre/submillimetre 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][34][35][36][37][38][39]. Modelling has shown that propagating shock waves will cause variation in mm-wavelength observables [13,14].…”