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 Hα 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'.
Highlights
The solar chromosphere is historically known to be dominated by 3-min (≈5.5 mHz) oscillations of acoustic/magneto-acoustic waves [1,2,3,4,5]
We have studied temperature fluctuations in the solar chromosphere from Atacama Large Millimeter/submillimeter Array (ALMA), in a variety of solar regions with different magnetic field strengths and configurations
That classical fast Fourier transform (FFT) and Lomb–Scargle approaches offer similar power spectra at the detected frequencies
Summary
The solar chromosphere is historically known to be dominated by 3-min (≈5.5 mHz) oscillations of acoustic/magneto-acoustic waves [1,2,3,4,5]. In December 2016, the Atacama Large Millimeter/submillimeter Array (ALMA [55]) started regular observations of the solar chromosphere at millimetre wavelengths, providing new capabilities (for studying, e.g. waves and oscillations), namely, direct measurements of the gas temperature and stable observations at high temporal resolutions (i.e. 1–2 s cadences). Band 3 (centred near 3 mm) and Band 6 (centred at ≈1.25 mm), with 1 s and 2 s sampling cadences, have been provided for observing the Sun, both of which supposedly sample the mid-to-high chromosphere [62,63,64,65,66] Their precise formation heights are, not known to date and are predicted (from numerical simulations and solar models) to span a large range between the low chromosphere and the transition region [67]. The magnetic fluxes within the extended FOVs of D6 and D9 datasets are considerably (i.e. by 1–3 orders of magnitude) smaller compared to the other datasets (D5 has a relatively low magnetic flux)
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