ALMA as a prominence thermometer: First observations

TL;DR

This paper demonstrates the potential of ALMA Band 3 observations to serve as a thermometer for solar prominences by correlating brightness temperature with kinetic temperature, supported by modeling and inversion techniques.

Contribution

First observations of prominences with ALMA Band 3 are used to develop a method for deriving prominence temperatures, integrating ALMA data with H-alpha diagnostics and modeling.

Findings

ALMA brightness temperature correlates with prominence kinetic temperature in bright regions.

An inversion code can estimate realistic temperatures where optical thickness is sufficient.

The relation between brightness and temperature depends on prominence parameters and filling factor.

Abstract

We present first prominence observations obtained with ALMA in Band 3 at the wavelength of 3 mm. High-resolution observations have been coaligned with the MSDP H$\alpha$ data from Wroclaw-Bialk\'{o}w large coronagraph at similar spatial resolution. We analyze one particular co-temporal snapshot, first calibrating both ALMA and MSDP data and then demonstrating a reasonable correlation between both. In particular we can see quite similar fine-structure patterns in both ALMA brightness temperature maps and MSDP maps of H$\alpha$ intensities. Using ALMA we intend to derive the prominence kinetic temperatures. However, having current observations only in one band, we use an independent diagnostic constraint which is the H$\alpha$ line integrated intensity. We develop an inversion code and show that it can provide realistic temperatures for brighter parts of the prominence where one gets a unique solution, while within faint structures such inversion is ill conditioned. In brighter parts ALMA serves as a prominence thermometer, provided that the optical thickness in Band 3 is large enough. In order to find a relation between brightness and kinetic temperatures for a given observed H$\alpha$ intensity, we constructed an extended grid of non-LTE prominence models covering a broad range of prominence parameters. We also show the effect of the plane-of-sky filling factor on our results.