On the Visibility of Prominence Fine Structures at Radio Millimeter Wavelengths

Abstract

Prominence temperatures have so far mainly been determined by analyzing spectral line shapes, which is difficult when the spectral lines are optically thick. The radio spectra in the millimeter range offer a unique possibility to measure the kinetic temperature. However, studies in the past used data with insufficient spatial resolution to resolve the prominence fine structures. The aim of this article is to predict the visibility of prominence fine structures in the submillimeter/millimeter (SMM) domain, to estimate their brightness temperatures at various wavelengths, and to demonstrate the feasibility and usefulness of future high-resolution radio observations of solar prominences with ALMA (Atacama Large Millimeter-submillimeter Array). Our novel approach is the conversion of H$\upalpha$ coronagraphic images into microwave spectral images. We show that the spatial variations of the prominence brightness both in the H$\upalpha$ line and in the SMM domain predominantly depend on the line-of-sight emission measure of the cool plasma, which we derive from the integrated intensities of the observed H$\upalpha$ line. This relation also offers a new possibility to determine the SMM optical thickness from simultaneous H$\upalpha$ observations with high resolution. We also describe how we determine the prominence kinetic temperature from SMM spectral images. Finally, we apply the ALMA image-processing software Common Astronomy Software Applications (CASA) to our simulated images to assess what ALMA would detect at a resolution level that is similar to the coronagraphic H$\upalpha$ images used in this study. Our results can thus help in preparations of first ALMA prominence observations in the frame of science and technical verification tests.