Abstract
Our aim is to test potential solar prominence plasma diagnostics as obtained with the new solar capability of the Atacama Large Millimeter/submillimeter Array (ALMA). We investigate the thermal and plasma diagnostic potential of ALMA for solar prominences through the computation of brightness temperatures at ALMA wavelengths. The brightness temperature, for a chosen line of sight, is calculated using the densities of electrons, hydrogen, and helium obtained from a radiative transfer code under non-local thermodynamic equilibrium (non-LTE) conditions, as well as the input internal parameters of the prominence model in consideration. Two distinct sets of prominence models were used: isothermal-isobaric fine-structure threads, and large-scale structures with radially increasing temperature distributions representing the prominence-to-corona transition region. We compute brightness temperatures over the range of wavelengths in which ALMA is capable of observing (0.32 – 9.6 mm), however, we particularly focus on the bands available to solar observers in ALMA cycles 4 and 5, namely 2.6 – 3.6 mm (Band 3) and 1.1 – 1.4 mm (Band 6). We show how the computed brightness temperatures and optical thicknesses in our models vary with the plasma parameters (temperature and pressure) and the wavelength of observation. We then study how ALMA observables such as the ratio of brightness temperatures at two frequencies can be used to estimate the optical thickness and the emission measure for isothermal and non-isothermal prominences. From this study we conclude that for both sets of models, ALMA presents a strong thermal diagnostic capability, provided that the interpretation of observations is supported by the use of non-LTE simulation results.
Highlights
The temperature structure of solar prominences remains an important question in solar physics
The importance of the prominence-to-corona transition region (PCTR) in prominence modelling has been discussed by Anzer and Heinzel (1999), and its effect on various spectral lines demonstrated by e.g. Heinzel et al (2001), Labrosse and Gouttebroze (2004), Labrosse et al (2010), Heinzel et al (2015b)
Non-Isothermal Large-Scale Structures In Figure 6 we show the variation in brightness temperature across the field of view (FOV) for a large-scale prominence structure with a radially increasing temperature and a pressure of 0.1 dyn cm−2 (Table 2) at several mm/sub-mm wavelengths
Summary
The temperature structure of solar prominences remains an important question in solar physics. From the material within the fine structure the hydrogen free-free extinction coefficient and the brightness temperature are calculated This model is used to visualise the brightness temperature and optical thicknesses of prominences on the limb and on-disk filaments at a range of ALMA wavelengths. The isothermal-isobaric fine-structure models correspond to individual threads of varying temperature or pressure, whilst the large-scale non-isothermal cases describe a prominence with a cool thread core surrounded by a sheath of increasingly hot PCTR material. For each of these cases, we investigate the temperature and plasma diagnostic capability of the brightness temperature measurements.
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