The Sun at millimeter wavelengths

Abstract

Context. Solar observations with the Atacama Large Millimeter/sub-millimeter Array (ALMA) facilitate studies of the atmosphere of the Sun at chromospheric heights at high spatial and temporal resolution at millimeter wavelengths. Aims. ALMA intensity data at millimeter(mm)-wavelengths are used for a first detailed systematic assessment of the occurrence and properties of small-scale dynamical features in the quiet Sun. Methods. We analyzed ALMA Band 3 data (∼3 mm/100 GHz) with a spatial resolution of ∼1.4–2.1 arcsec and a duration of ∼40 min together with SDO/HMI magnetograms. The temporal evolution of the mm maps is studied to detect pronounced dynamical features, which then are connected to dynamical events via a k-means clustering algorithm. We studied the physical properties of the resulting events and explored whether or not they show properties consistent with propagating shock waves. For this purpose, we calculated observable shock wave signatures at mm wavelengths from one- and three-dimensional model atmospheres. Results. We detect 552 dynamical events with an excess in brightness temperature (ΔTb) of at least ≥400 K. The events show a large variety in size up to ∼9″, amplitude ΔTb up to ∼1200 K with typical values in the range ∼450–750 K, and lifetime at full width at half maximum of ΔTb of between ∼43 and 360 s, with typical values between ∼55 and 125 s. Furthermore, many of the events show signature properties suggesting that they are likely produced by propagating shock waves. Conclusions. There are a lot of small-scale dynamic structures detected in the Band 3 data, even though the spatial resolution sets limitations on the size of events that can be detected. The number of dynamic signatures in the ALMA mm data is very low in areas with photospheric footpoints with stronger magnetic fields, which is consistent with the expectation for propagating shock waves.

Characterization of shock wave signatures at millimetre wavelengths from Bifrost simulations

“…The local minimum with the highest temperature (here the post-shock minimum) is referred to as the ‘base temperature‘. The time between the two local minima is 146 s. Estimating the lifetime of the observable brightness temperature signature as the Full Width Half Maximum (FWHM) of the peak results in t FWHM = 41 s. The brightness temperature excess and lifetime shown here are in line with values derived from shock wave propagation in 1D simulations [13,14,43]. The resulting strong correlation between the gas temperature at z ( τ = 1.0) and the brightness temperature (figure 4 b ) is expected for mm-wavelength radiation (i.e.…”

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

“…Recent studies have shown the potential of ALMA to examine distinct phenomena both in quiet and active regions [31–40]. In particular, Eklund et al [41] found signatures of shock-wave events in Band 3 (2.8–3.3 mm) observations from December 2016, coming to the conclusion that there are numerous small-scale dynamic structures with lifetimes of 43–360 s present in the ALMA field of view (FOV), with excess temperatures of more than 400 K and a correlation between their occurrence and the magnetic field strength. Characterization of such events at millimetre wavelengths has also been studied in detail from numerical simulations [42].…”

The Solar ALMA Science Archive (SALSA)

“…Despite the technical challenges, an increasing number of studies based on ALMA observations of the Sun is being published (see, e.g. Bastian et al 2017;Shimojo et al 2017b;Yokoyama et al 2018;Brajša et al 2018;Jafarzadeh et al 2019;Rodger et al 2019;Selhorst et al 2019;Wedemeyer et al 2020;da Silva Santos et al 2020b;Eklund et al 2020;Nindos et al 2020;Patsourakos et al 2020;Brajša et al 2021;Chintzoglou et al 2021a;Guevara Gómez et al 2021;Jafarzadeh et al 2021;Nindos et al 2021;Eklund et al 2021a, and more). While scientific production using ALMA solar data is clearly picking up, interferometric imaging into stabilised, time-consistent data series in absolute temperature units remained difficult.…”