ALMA as a prominence thermometer: First observations

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 Hα data from Wroc law-Bia lków 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 Hα 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 Hα 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 Hα 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.